Sporadic Misfires - Any Insight?

[blk_on_blk]

I've got a '99 RX300 with ~134K on the odo and it has recently started to have random sporadic misfires. It first showed up on an hour long trip back from the Salem area last week. The CEL light would flash, then the engine would smooth out and the CEL would turn off, then do it again. When I got home I pulled the codes. It was P0300 (random misfire) and P0303 (misfire in cylinder #3)... I went straight to a faulty coil pack because of the miles and the known failure of the coil packs. I changed out the #3 coil pack and pulled the battery cable to let the ECU reset. Once done, I took it out for a test drive and it ran smooth... for a while.

The next day the engine ran smooth half way in on the drive to work (about 11 miles), but then it started to misfire again... again, I got a flashing CEL... then the CEL stayed on. I thought I might have gotten some bad gas, so I filled up at a trusted station, threw in some injector cleaner, and it drove smoothly... for a while. Later in the day the CEL started flashing again. When I pulled the code, I again got the P0300, P0303, but also the P171 (bank 1 lean). So, I've done some searching and it seems it could be one of a few things (assuming the new coil pack isn't also faulty):

Faulty fuel injector

MAF

Oil Control Valve (but this seems to trigger a whole side of misfire codes, not just one)

Bad Spark Plug

I went to the easiest first (seeing that the #3 spark-plug is a buggar to get to) and got some spray automotive electrical cleaner and cleaned the MAF. It was dirty, but cleaned up nicely. I put it back in and started the engine... it started smoothly, and as I took it out for a drive, I could feel a faint misfire sporadically... it would run smooth, then the faint misfire, then run smooth. It wasn't enough to trigger the CEL, but it probably will if I drive it more.

Here's what I'm thinking on some of this. I don't think it's the Oil Control Valve, since it would show up as a full side of the engine with misfires (to my understanding). It could be a bad MAF, and cleaning it just reduced the problem. It could be a faulty fuel injector, but how to test for that? It could be a bad sparkplug, but why would cleaning the MAF help reduce the misfire if it was a spark plug... if it truly has reduced it.

So, at this point I'm not sure what I should go after next... any suggestions? I guess it's to where I should just start replacing things, since it's not even enough to trigger the CEL, but it's still misfiring.

Here's an overview on misfires that I pulled off the web: "A random misfire code can be set on newer vehicles with OBD II onboard diagnostics when multiple misfires occur randomly in multiple cylinders. The cause is typically a vacuum leak in the intake manifold, throttle body or vacuum plumbing, a defective Exhaust Gas Recirculation (EGR) valve that is leaking exhasut into the intake manifold, or even bad gasoline. Less common causes include bad spark plug wires, worn or fouled spark plugs, a weak ignition coil, dirty fuel injectors, low fuel pressure, or weak valve springs. If a misfire is occuring in only one or two cylinders, you will usually find a misfire code for that specific cylinder rather than a random misfire code."

Any input/suggestions are welcome... thanks in advance.

[Grumpa72]

When I had a misfire in my Acura, I checked the manual for the injector. The manual said to measure the resistance across the contacts on the injector and if should fall into a specific range. I will assume that Lexus injectors are similar and if someone has that resistance value then it is a very simple and quick check.

[smooth1]

Hmmm, if you know which cylinder is having the misfire, then you might just want to move the fuel injector to a different cylinder, and see if the misfire follows to that cylinder, if it does, then obviously you'll need to replace the fuel injector. If not, then it's something else. Lexus uses a high a high impedance fuel injector that should measure between 13.4-14.2 on your multimeter. But to check the fuel injector's performance, you'll need to connect an Oscilloscope to #10, #60 and E01 in the ECM while the car is idling. Then look at the injector signal waveform to see if it is performing correctly. You should see a 100 msec./division at idle. Your specifically looking at the injection duration which should be 1 msec./division at ground.

[smooth1]

And btw, I know you have dismissed the oil control valve, but........it could be stuck or sticking. As the oil control valve also is connected to the VVTI, which would affect timing and could also be a cause. Although I don't think that is the case here, I just wanted to point out that a semi sticking or stuck valve may not throw a hole bank of codes. I'm not really sure about it being a coil issue also. As the coil packs charge two cylinders, and you would be having an issue in both of those, not just one. Have you inspected the injector seal?

[RX&ES300]

I seem to be having similar problems to blk-on-blk's.

My '99 RX300 AWD with 110k miles started idling rough and misfiring last week resulting in the CEL light to come on. I dug through the various posting (many thanks goes to LOC for such a super forum) and cleaned out the MAF, psuedo cleaned out the IACV (meaning that I only took off the black IACV magnet cover on the throttle body and found that it moved freely enough so I only sprayed the cleaner into to it vs take off and clean the whole TB) and cleared the CEL by removing the EFI fuse. While this made the car idle and run better, the idle still was rough and misfiring and the CEL came on but this time only blinking.

So off to Autozone where they read a P0303 code - UGH!!! I bought the coil from them which actually was an OE (Denso) part at almost half the cost ($59) of all other parts shops which were selling 3rd party coils. Since I figured I was already going to be tearing apart the back side of the engine, I hopped over to Lexus and bought a set of Denso Iridium plugs from them (got a deal for $11 each).

Installing the replacement coil became somewhat of a challenge. I found that the original part (90919-02234) was superceded with a 90080-19016 (which is what I bought from A/Z) according to some of the postings. However, it appears that it is not a 100% replacement as the "rings" on the tube of the coil are more "pronounced" than the original allowing it to only slide part way in (up to the 1st ring). From postings, it appears others have also run into this same problem and explained that supposedly Toyota made "minor" engine changes mid model year but that most parts stores don't know anything about. From everything I read on the web, it sounded like electronically the parts were interchangeable but that the "rings" prevented them from fitting. So I lightly sanded down the rings with extra fine sand paper and made it fit perfectly.

The car ran great for 10 or so miles on a couple of short trips around town: no CEL's, no misfires, no rough idle, great acceleration. Then it started again...same rough idle, etc. Went to Autozone and read the codes...this time: P0300, P0302 and P0303. On the way back home, the transmission wouldn't kick-in to O/D even though it changed properly on the way to A/Z. Turned the engine on and off which didn't help any. So I reset the CEL (pulled out EFI fuse) and low and behold, the O/D worked again!!! I got home and verified that all the vacuum hose, connections, etc were all connected and seated properly (which they were).

So I'm back to dealing with the P0300, P0302 & P0303 codes -- ANY ONE HAVE ANY SUGGESTIONS?

(Sorry for the long posting but thought that something that I did might be important and trigger a solution).

Thanks in advance for your help.

[smooth1]

I seem to be having similar problems to blk-on-blk's.

My '99 RX300 AWD with 110k miles started idling rough and misfiring last week resulting in the CEL light to come on. I dug through the various posting (many thanks goes to LOC for such a super forum) and cleaned out the MAF, psuedo cleaned out the IACV (meaning that I only took off the black IACV magnet cover on the throttle body and found that it moved freely enough so I only sprayed the cleaner into to it vs take off and clean the whole TB) and cleared the CEL by removing the EFI fuse. While this made the car idle and run better, the idle still was rough and misfiring and the CEL came on but this time only blinking.

So off to Autozone where they read a P0303 code - UGH!!! I bought the coil from them which actually was an OE (Denso) part at almost half the cost ($59) of all other parts shops which were selling 3rd party coils. Since I figured I was already going to be tearing apart the back side of the engine, I hopped over to Lexus and bought a set of Denso Iridium plugs from them (got a deal for $11 each).

Installing the replacement coil became somewhat of a challenge. I found that the original part (90919-02234) was superceded with a 90080-19016 (which is what I bought from A/Z) according to some of the postings. However, it appears that it is not a 100% replacement as the "rings" on the tube of the coil are more "pronounced" than the original allowing it to only slide part way in (up to the 1st ring). From postings, it appears others have also run into this same problem and explained that supposedly Toyota made "minor" engine changes mid model year but that most parts stores don't know anything about. From everything I read on the web, it sounded like electronically the parts were interchangeable but that the "rings" prevented them from fitting. So I lightly sanded down the rings with extra fine sand paper and made it fit perfectly.

The car ran great for 10 or so miles on a couple of short trips around town: no CEL's, no misfires, no rough idle, great acceleration. Then it started again...same rough idle, etc. Went to Autozone and read the codes...this time: P0300, P0302 and P0303. On the way back home, the transmission wouldn't kick-in to O/D even though it changed properly on the way to A/Z. Turned the engine on and off which didn't help any. So I reset the CEL (pulled out EFI fuse) and low and behold, the O/D worked again!!! I got home and verified that all the vacuum hose, connections, etc were all connected and seated properly (which they were).

So I'm back to dealing with the P0300, P0302 & P0303 codes -- ANY ONE HAVE ANY SUGGESTIONS?

(Sorry for the long posting but thought that something that I did might be important and trigger a solution).

Thanks in advance for your help.

Hmmm, I'm not really sure your having a coilpack or spark plug issue. You may be having a fuel injector issue. And as for the transmission thing, I think the car just went into a limp mode or a safety protocal due to the cel being thrown. I have some questions if you could answer them would probably isolate the issue better:

1) If you reset the cel, and start the car, does the cel come on while idling? Or do you have to drive it?

2) Did you notice if you stayed under say like 3K rpms the car runs fine, but if you go over XXrpm then the cel comes up?

3) What gap did you use for the new plugs?

4) Have you checked the coils to make sure they are all still connected right and everything looks good?

5) have you run low on coolant or overheated the car recently?

6) Have you checked your fuel pressure and fuel pump?

[RX&ES300]

Smooth 1 - thanks for your reply.

Good questions and to answer especially 1 & 2, I went out and tested it:

1. The CEL came on after 4 mins, once the engine warmed up (it hasn't been run today) and RPM dropped from ~1300 to ~ 1000.

2. I reset the CEL and drove the car keeping the RPM's under 2k. The CEL came on within 1 min.

FYI - When I was driving aroung yesterday, it seemed to be running smoother/less misfires at higher RPM's (3k & over when the O/D did not engage) than at lower ones but then again, it may it's just less noticeable at higher RPMs.

3. I didn't touch the gap -- the dealer told me that they were all preset and each plug was secured in a collar. But in retrospect, I should have double checked to ensure that each was correctly gapped. DUH!

4. I triple checked them before "closing shop" on the project on Friday but haven't check them since. I'll have to pull the cover off and wait until the engine cools off before attempting to snake my hand in under the intake manifold

5. Nope - the coolant has been at steady state for well over a year and the car has never overheated (we're the original owners). However, when my wife initially mentioned it was running rough, I did the typical checks and found the oil to be two quarts down (well, two quarts brought it to mid-point between the two "bumps" on the dipstick so maybe it was a little more). I hope that being low didn't suck more "crud" of the bottom of the oil pan than usual especially since it's overdue for an oil change by 1k miles.

6. Fuel Pressure & Pump - nope, haven't look at that. Any suggestions on how best to do that and the recommended PSI range?

The guy at Autozone mentioned checking the fuel injectors but also recommended to first run BG44K on a full tank of gas to clean out the intake and injectors. That was one the next things I was planning on doing before changing out a bunch more sensors or coils. But I'm game to try anything else that you may suggest.

Thanks for your insight and help.

[smooth1]

Yeah, you have a fuel injector issue or a fuel pump/pressure issue. I don't think your coils are bad. Considering the mileage and age of the motor, I would suggest just replacing all of them and the seals. You can get a full set of 6 Denso fuel injectors for less than $150.00 retail, and I would just get a new fuel filter and replace that at the same time. I test the pressure of the fuel pump off the fuel rails with a small gauge, but I don't know how you would be able to that. I have a feeling if you replaced the injectors and the filter, you should be good to go afterwards.

the reason it seems to run better at higher rpm's is becuase fuel has more pressure and is squeeking past the bad injectors, or the fuel filter is still able to allow enouph fuel by it when it has more force. Which is why once the rpm's fell from cold idle to warm idle it started to have problems again.

I would also pull those plugs and check em. I know it's a PIA, but I have never seen a plug come out of the box that was correctly gapped for the Lexus motor. Pregapped plugs are almost allways .026-.028's. Lexus uses a .043 gap, or 1.1 mm. Which is huge!! I usually gap to around .040 though. So maybe pull the easiest one to pull, and check that one.........I almost bet they aren't right.

[RX&ES300]

Thanks for the feedback.

I checked the gap on the even cylindered plugs and all three were slightly greater than .040" which leads me to believe that the other bad boys will be too. So buying them from Lexus may have saved me the effort of having to go pull and regap the 1-3-5's.

I'm heading out to the parts store to pick up the filter but for the sake of not sounding too dumb, I'm assuming that it's the little black canister hanging just prior to the fuel injector manifold on the driver's side, more or less between the oil filler cap and the throttle body?

And to change all 6 injectors, it appears that I'll need to remove the air intake manifold unless there's another way. This looks somewhat ugly.

Thanks

[smooth1]

Thanks for the feedback.

I checked the gap on the even cylindered plugs and all three were slightly greater than .040" which leads me to believe that the other bad boys will be too. So buying them from Lexus may have saved me the effort of having to go pull and regap the 1-3-5's.

I'm heading out to the parts store to pick up the filter but for the sake of not sounding too dumb, I'm assuming that it's the little black canister hanging just prior to the fuel injector manifold on the driver's side, more or less between the oil filler cap and the throttle body?

And to change all 6 injectors, it appears that I'll need to remove the air intake manifold unless there's another way. This looks somewhat ugly.

Thanks

They must gap them or have them specially gapped for Lexus at the dealerships then. ( I've never bought them from there before.) Hmm, I'll have to check into that....

As for the fuel filter, you'll have to take out the back seat and fold the rear section of the carpet forward. On the driver's side floor is a round metal cover with a wire harness running through it. Pry off the cover (it's glued on) to reveal the pump top. Remove the wire connector. Carefully remove the orange snap-lock from the fuel line, then pry the fuel line up and off. Remove the bolts and flange holding the pump in place, then pry the pump loose and lift straight up and out. The filter is on the pump.

And don't smoke while your doing this! LOL!!

[RX&ES300]

Boy - I really missed the boat on where the filter is. So what the heck is that small black (filter-looking) canister that feeds the fuel into the injection rack?

Also, where can I get the 6 Denso injectors for $150? Every where I looked, $150 is the price for 1.

[smooth1]

I think your looking at brand new ones. I would get rebuilt ones. Try here:

Mr. Fuel Injector

I would just call em and tell then what year and model you have. I don't think they keep thier website up very well. I've bought other injectors from them that weren't even listed, and some were less then what the site said they would be. But you might want to just call around locally also. And ask if they come with the seals. They usually do, but I have gotten some that didn't.

[RX&ES300]

So you're cool with using rebuilt ones? But I think I'll run a tank with BG44K through first before I go down the route of changing injectors.

I think I figured out little black canister - it's the vacuum surge tank feeding off the VSV assembly.

[smooth1]

So you're cool with using rebuilt ones? But I think I'll run a tank with BG44K through first before I go down the route of changing injectors.

I think I figured out little black canister - it's the vacuum surge tank feeding off the VSV assembly.

Yeah, I think I've only ever had 1 bad one. And I've have used a bunch. ( All for different cars though.)

I have never been a chemical tool user. Trying to fix stuff with some magic potion just never appeals to me. Not to say that some of those things don't work. But just to note, the BG44K won't fix any bad injector seals, or if the electromagnetic plunger just above the atomizer is bad.( Which is what I think.) It will only help if the injector is clogged or blocked, and even then it may not unclog it enouph to help. So if it doesn't work, or works temporarily, you may still have the same issues as before. But it's a cheap alternative. So what the hey right? Start there and maybe keep me up on how it's going and if the problem is resolved, what the issue ended up being? I generally try not to get to much into diagnostics over the website like this, as I don't have the motor in front me, and I can't run any scans and do some datalogging to see the problem for myself. And then go from there. It's generally alot harder to do it this way.

[RX&ES300]

Smooth1 - appreciate all your help and suggestions. I'm off to check things out and will keep you updated on how things go.

[RX&ES300]

I may have found the problem and it may be with the Intake Air Temp (IAT) sensor housed on the MAF. To test it, the manual says that you can read the resistance but to use different terminals/pins depending whether its for a RX300 or ES300. The reason why I ask is because the readings using the ES terminals (left most two when looking directly at the connector) are correctly measuring 2100 ohms at 72deg F while the RX terminals (right most two) are reading over 17 million ohms.

FYI - the readings in both the ES and RX configurations changed when I changed the temperatures (I placed it the freezer and then outside it the sun, and the resistance changed as the MAF warmed up and cooled down) and the ES terminal tracked correctly while the RX terminals continued to be way off.

What I would like to confirm is whether the manual is correct that the RX300 uses the right most two terminals/pins for the sensor?

[artbuc]

I may have found the problem and it may be with the Intake Air Temp (IAT) sensor housed on the MAF. To test it, the manual says that you can read the resistance but to use different terminals/pins depending whether its for a RX300 or ES300. The reason why I ask is because the readings using the ES terminals (left most two when looking directly at the connector) are correctly measuring 2100 ohms at 72deg F while the RX terminals (right most two) are reading over 17 million ohms.

FYI - the readings in both the ES and RX configurations changed when I changed the temperatures (I placed it the freezer and then outside it the sun, and the resistance changed as the MAF warmed up and cooled down) and the ES terminal tracked correctly while the RX terminals continued to be way off.

What I would like to confirm is whether the manual is correct that the RX300 uses the right most two terminals/pins for the sensor?

Looks like your IAT sensor is ok. You should be checking resistance across the two pins on the left. The one all the way to the left is E2 (ground) and the one just to the right of E2 is THA. This is a little tricky because the Lexus FSM RM626U2 incorrectly labels the MAF pins on page SF-32 in Vol 2. The resistance across E2-THA should be 2210--2690 ohms at 68 degress F. You can check the wiring harness by making sure you have 5vdc being supplied to the IAT.

Unfortunately, the Lexus FSM, although quite expensive, has a number of errors. Eg, the electrical check-out described for the IACV is entirely wrong.

[RX&ES300]

artbuc - thanks for the insight...it's not very comforting to hear that the FSM can have such fundamental errors for their own product. Any suggestions on which 3rd party manual to get?

[blk_on_blk]

Thanks for all the input and advice. I went after the MAF first... I pulled it, cleaned it with automotive electrical cleaner, and then reinserted it. It started decently with no CEL on. I drove the vehicle for about 5 miles and it ran well, but after the 5 mile mark I could feel a very faint sputter/studder/misfire at cruising speeds... but no CEL.

I wasn't able to drive it again for a few days due to a family emergency, but today I took it out. It started a little rough, but then smoothed out. I drove it for about 15 miles and it drove nicely... then, while sitting idle in Drive at a light, I could feel the engine kind of shudder very slightly, then smooth out, then a slight shudder, then smooth out again. When accelerating the engine was smooth. Other times when at a stop light, the engine didn't shudder, but it was just slightly rough. At one point the CEL came on and flashed, but then went off again within seconds. Finally, on the last bit of the drive home the CEL popped on and stayed on. I pulled the code when I got home and it was P1133 - Air/Fuel Ratio Sensor Circuit Response Malfunction. So, now it's something different.

Could this all be related to the MAF? I mean, since I cleaned it, it has really reduced in the studdering/shuddering/misfires. Could it be the MAF is faulty, and cleaning it just reduced the impact (but it's still bad)? Can a faulty MAF have that much of an effect/impact on the engine's operation? Or would it just make it run rich, rough overall, and sluggish?

What gets me is that at times it will run whisper-smooth, but then out of the blue it will give a slight shudder/hesitation, then smooth out again. I would think if it's a spark plug failure, it should be pretty pronounced and consistent, same with a fuel injector or coil pack. It sounds more like a sensor somewhere acting sporadically... but why the multiple and different codes?

Any additional input is welcome.

[RX&ES300]

Thanks for all the input and advice. I went after the MAF first... I pulled it, cleaned it with automotive electrical cleaner, and then reinserted it. It started decently with no CEL on. I drove the vehicle for about 5 miles and it ran well, but after the 5 mile mark I could feel a very faint sputter/studder/misfire at cruising speeds... but no CEL.

I wasn't able to drive it again for a few days due to a family emergency, but today I took it out. It started a little rough, but then smoothed out. I drove it for about 15 miles and it drove nicely... then, while sitting idle in Drive at a light, I could feel the engine kind of shudder very slightly, then smooth out, then a slight shudder, then smooth out again. When accelerating the engine was smooth. Other times when at a stop light, the engine didn't shudder, but it was just slightly rough. At one point the CEL came on and flashed, but then went off again within seconds. Finally, on the last bit of the drive home the CEL popped on and stayed on. I pulled the code when I got home and it was P1133 - Air/Fuel Ratio Sensor Circuit Response Malfunction. So, now it's something different.

Could this all be related to the MAF? I mean, since I cleaned it, it has really reduced in the studdering/shuddering/misfires. Could it be the MAF is faulty, and cleaning it just reduced the impact (but it's still bad)? Can a faulty MAF have that much of an effect/impact on the engine's operation? Or would it just make it run rich, rough overall, and sluggish?

What gets me is that at times it will run whisper-smooth, but then out of the blue it will give a slight shudder/hesitation, then smooth out again. I would think if it's a spark plug failure, it should be pretty pronounced and consistent, same with a fuel injector or coil pack. It sounds more like a sensor somewhere acting sporadically... but why the multiple and different codes?

Any additional input is welcome.

Sounds similar to what I was having with the exception that I did not get a P1133, but I did want to check out if the MAF was working correctly.

Actually, there are two sensors housed together - the MAF sensor and the IAT (intake air temp) sensor. To the test the IAT sensor, you are looking for a specific ohm (resistance) readings across two pins on the MAF at given temperatures while for the MAF sensor, you are looking for a fluctuation of voltage across two other pins. As you may see from the above threads, the issue has been around the accuracy of the Lexus factory service manual when it comes to which pins to use for the IAT test which artbuc pointed out. Actually, I hope he's right which would mean that my MAF is good but for the sake of your testing, I'll share the pins that I used and the ones the FSM specifies.

For purposes of these tests, let's first identify the pins by position in the MAF housing. Looking at the MAF's connector in it's normal orientation ("plunger" down), I'll refer to the left most pin as #1 with the right most pin being #5. You will need a VOM meter and compressed air (in a can will do). I had done my testing after I had cleaned the MAF and IAT sensors.

IAT Sensor Resistance Test

Using a VOM meter - place it into OHM mode: the FSM's instructions for the RX300 place the positive (red) leads on the 4th pin and the negative lead (black) on the 5th pin. For the ES300, the FSM places the positive lead on pin 2 and the negative on pin 1. (Once again, the reason why I give you both configurations is because I found that when using the RX pins, my readings were out of this world and artbuc pointed out that I should be using the ES pins...but you may want to test both for yourself).

Note the resistance reading at your room temp. For my testing, I went a bit further by placing the MAF into my freezer for 15 min, did another reading, and then placed it outside in the sun and did another reading when it heated up (it's been in the mid 90's in Denver lately). This gave me enough of a swing to see if the values are changing/tracking which would most likley indicate a properly functioning IAT sensor. If you want to take the guessing out, measure the temps in the freezer and the outside and keep the meter attached as it warms up.

The resistance readings should fall within the following ranges...you may need to do some interpolating based on your temps (for instance, at 72 deg, you'll be closer to 2000 ohms than at 68 deg):

Ambient Temp in F..........Ohms

....-4.......................13,600 - 18,400

....68.........................2210 - 2690

...140..........................493 - 667

If your readings don't fall close at the temp ranges, chances are that your IAT sensor is malfunctioning.

MAF Sensor Test

Attach the MAF sensor connector back on to the MAF but keep it out of the air filter housing.

Using the VOM, place it into DC voltage mode: for the RX300 - use pin 1 for the negative (black) lead and pin 3 for the positive (red). For the ES - use pin 3 for the negative and pin 4 for the positive.

I gently slid my probe in between the rubber around the wire and the actual connector housing until it made contact (bottomed out). Make sure that you don't push too hard as you don't want to damage the connector.

Turn ignition ON (but don't start the car). You should see a voltage reading. Now apply compressed air into the MAF chamber and you should see the voltage fluctuate. If it doesn't, replace the MAF sensor.

Let me know what your findings are if you chose to do the tests.

[blk_on_blk]

Thanks for the quick reply, RX&ES300 (and thanks for the detailed input and link, Smooth). I'll probably check the MAF as you describe, but these would be 'end-state' static checks, right? a point-checked with the MAF at different conditions. I need to check it, but I'm going to be cautious of the results because of the behavior of the engine. What I mean is there are times when the engine runs smooth (drove it all the way into to work today, 23 miles, and no CEL and just a slight few hiccups of studdering), but then times when it doesn't... plus it always starts up smooth. So, at most times everything is working properly, but the it will act up. So, if I test a componenent statically, then the reading may be good, but I'll miss that it sporadically fails. Ah, these are the toughest failures to find, because they are semi-failures. I still think it's in the MAF, since cleaning it has made such a pronounced positive impact on it's operation.

Just a few things to note for reference, when your CEL is blinking, that means there is a problem with the engine that is 'critical' and reducing the operation of the engine and it's power. That will then kick in the transmission to not go into overdrive. If you are already in overdrive when this happens, it will apparently stay in overdrive (it did with me).

When the CEL is on and stays on, then depending on the code, it will trigger the O/D disable to protect the transmission (like Smooth said). Once you reset the ECU and clear the code, the transmission with shift into O/D again until another code pops up. For me, I've had the ECU come on and blink, but then go off and the transmission will still shift into all gears.

I did notice something else, and it could just be me, but when the CEL would come on and stay on, my engine seemed to run better with less misfires/studders. Don't know if the CEL coming on changes the way the ECU behaves or not, but it seemed noticable.

At this point, I drove all the way into work with no CEL coming on and just very faint studdering out of the engine at only a few moments. I still wonder if I got some bad gas that's working it's way through the system (each time I fill up I just dilute the residual contaminants, and that's why the symptoms are progressively less and less with time). I too ran some injector cleaner (Berryman), but I'm with Smooth in that I never really rely on a chemical fix, unless it's just mechanism that is gummed up, and not worn.

I'm going to keep any eye on my RX over the next couple days and I'll report in any new developments, codes, or lack of problems. Hopefully it's a relatively easy and painless (financial and labor wise) fix and we'll be back to running flawlessly here soon.

Other input is always welcome!

[RX&ES300]

Just to update, I finally got my RX to run great (knock on wood - Knock! Knock!).

Initially, which started this whole project, I had a P0303 error and ended up replacing the #3 cylinder coil with a new one from Autozone (and later verified that indeed the original one was faulty). After replacing it and 6 new spark plugs, things went from bad to worse as I ended up with P0300, P0303, & P0304 codes. I went back and totally cleaned out the throttle body and the IACV (already did the MAF/IAT sensors before) and took the opportunity to recheck that all the plugs were gapped properly (which they were) and that the coils were firing.

While all coils were firing properly, because of the P0303 & P0304 codes, I decided to swap the new coil which was on the #3 cylinder with the one on #2, and swapped #4 with #6 thinking that if there were more errors and by chance were P0302 or P0306, I had isolated my problem to the coils.

The car started and while it idled much much better, it was still rough and misfired during acceleration. Within minutes, I got a CEL and went to Autozone to read it...sure enough it was P0302, the newly replaced coil. I pulled the coil out in their parking lot, and upon close inspection, saw a some small amount of carbon deposit (from arcing) and traced it to a hair line fracture in the tube. Luckly enough, Autozone had another one in their stock, and replaced it free of charge, and LO and BEHOLD, this time it was the original Denso part number and not the replacement one. Dropped it in, screwed it down and now have driven 30+ miles without even a studder. Hopefully, this has solved my problem.

Two notes:

1) Prior to going back to Autozone to read the codes, I checked out the fuel injectors and they all were "clicking" with the exception of #3 which I couldn't get to. Given this, I have decided not to run the BG44K as who knows what internal damage it may do to the fuel system while cleaning it. Everything seems to be good so I'm leaving it "status quo".

2) I'm still not sure if my IAT sensor is OK or malfunctioning, depending on which pins should be used for the resistance testing. But I tend to lean towards the "OK" given that I'm not seeing the CEL and the car is running smooth. But I will change out the MAF if I get some fuel/air related error codes, just to be on the safe side.

Thanks all for your insight and help.

[blk_on_blk]

My update isn't so good, but I think I'm on the right track. I drove the RX to work today, and as I was getting on the interstate I could feel the slight jerks from the power drop/misfires... and it triggered the CEL to flash. I was in the middle of morning work traffic, and the sporadic misfiring wasn't that bad, so I finished the drive to work. When I got there, I pulled the codes: P0300, P0303, P0305, and P0171. So, with that, I kind of locked on my vehcile's problem being the oil control valve. I did a little more research on it, and there is one OCV on each side of the engine for each bank of cams. This valve adjusts the amount of oil for the vvt-i, and if it has problems, it can result in anything from sporadic misfires to the run lean bank one code. So, on this last round of codes I'm getting everything for the rear cam bank except for the code for the OCV failure itself. That may just mean the OCV electronics are trying to work, but it's just not mechanically working properly (sure hope that doesn't lead to a sludge problem, but I don't think so... I'm pretty religous about checking and changing my oil, plus I live in a mild climate... although the vehicle was from Phoenix, AZ). I left work early today and stopped by Lexus of Portland to pick up the part. It's Toyota/Lexus part number 15330-0A010, which is for the 'right', or the rear bank of cams. It's $91 at the Lexus dealer and $85 at the Toyota dealer, but the Lexus dealer had one in stock.

Strangely, once I got it and started to drive home, the vehicle drove beautifully... made the long drive in rotten rush hour traffic without a hiccup. Strange. It turned pretty hot today, so maybe that had something to do with it... I don't really know at this point. I haven't put in the part, but I will see if I can get to it tomorrow.

On another note, I've read where people have tried diagnosing a faulty MAF (which I thought it might be), and they suggested pulling the wire harness of the MAF and if it runs better, then there is something faulty with the MAF. From that I'm guessing the MAF doesn't have enough of an impact on engine performance to cause the hesitation/stammering/studdering that I'm getting. It seems more that MAF related problems are with sluggish throttle response, reduced gas mileage, and muted all-over performance. I'm not having any of that... in fact I just filled up with gas and checked my mileage performance: right at 20mpg for mixed driving.

I'm feeling confident that it's the OCV (and I'll have to give Smooth kudos for noting to not rule it out), for my symptoms are now all pointing toward it (other than the actual code for it itself). I'll report in the findings when I can get this thing replaced.

[blk_on_blk]

After I picked up the part from the dealership, the vehicle ran perfectly for the rest of the day without even installing the part. It also drove great yesterday, but today started studdering slightly and showing codes, so I installed the part tonight. It was an easy install, and it started up and ran perfectly afterwards. I'll give an update after driving it a few days. Here are a couple pictures of the part and location.

[blk_on_blk]

Drove the vehicle to work today... more misfires. This time I got codes P0300, P0301, P0303, P0305, P0171, and P1133. Sheesh! Looks like it wasn't the OCV (there goes $90). Now it's on to the fuel injectors.