Engine Shudder & Check Engine Light With Moderate To Heavy Acceler

[NCGuy]

Several weeks ago I took the advice of a friend and began using Plus rather than Premium; it was about two weeks after this that the problems began happening.....

If I accelerate in anything other than a timid manner, the engine shudders as if it is stalling and the check engine light comes on. If I remove my foot from the accelerator, the check engine light goes off after a few seconds and shuddering stops.

Since the problem began I have switched back to Premium and have added a couple of carb/injector cleaner additives to the gas. Despite this, the problem continues.

I have read a few posts of others experiencing similar problems and have read a lot about the possible problems might be, but haven't seen anybody identify what the final solution was.

I have not obtained the code, but will be doing that later this week. I am guessing that something has gotten gummed up. I am planning on switching out plugs and air filter in hopes this will help. However, I would enjoy hearing from anybody that can shed some light on the situation and what I may have done to resolve this issue.

Thanks in advance for any help!

[maxed_out]

do yourself a favor and do the the plugs and wires. Use OEM stuff then post results . You might want to do the rotors and caps too. We'll help you out.

[Ross W.]

If I accelerate in anything other than a timid manner, the engine shudders as if it is stalling and the check engine light comes on. If I remove my foot from the accelerator, the check engine light goes off after a few seconds and shuddering stops.

Thanks in advance for any help!

It's quite possible that your switch in fuel octane and your problem is coincedental. If the CEL is coming on, I'd bet that it's a sensor malfunction. Poke around under the hood and visually check each connector to eliminate the possibility of a dirty or loose connection, and do the same with the O2 sensors. My sister's car (a '93) behaved somewhat like what you're describing, and it turned out to be the MAF sensor. But by all means do the plugs, wires and caps/rotors. Depending on your car's mileage, do the O2 sensors as well. Oh, and check the TPS switch. It's on the throttle assembly, and on these cars, very sensitive.

Good luck!

[AussiLS400]

Sounds to me like a lack of fuel. Could be the fuel pump not working properly. The new fuel could have done something to this.....On the other hand you might have got contaminated fuel, maybe water or something.

[monarch]

If I accelerate in anything other than a timid manner, the engine shudders as if it is stalling and the check engine light comes on. If I remove my foot from the accelerator, the check engine light goes off after a few seconds and shuddering stops.

How many miles are on the spark plugs, plug wires, distributor caps, distributor rotors, oxygen sensor, and air filter? Have the throttle plates ever been scrubbed clean?

[Ross W.]

Sounds to me like a lack of fuel. Could be the fuel pump not working properly. The new fuel could have done something to this.....On the other hand you might have got contaminated fuel, maybe water or something.

I second that. I was considering that last night, and I agree. You could have picked up a bad batch of fuel with rust or sand in it, and it's plugged the fuel filter or the screen on the fuel pump inlet. Replace the filter and check out the pump.

[LScott400]

My '94 began doing this also a couple of months ago - exact same symptoms, but always using premium. Haven't had an opportunity to track it down, but new O2's, removing & cleaning TB, changing vac lines & fuel filter, and new TPS have made no difference. Mine throws a code 41 (lean O2). My son (18 - go figure...) found that if you manually downshift to 2nd you can accelerate more aggressively.

I've had some capacitor leakage in ECU's on Stealth/3000GT's so I was thinking of pulling and inspecting that next.

272,000 miles.

Edit 10-11-2006 : The lean O2 is code 25 which I incorrectly stated above as 41. (Sorry about that Gumart1)

[Gumart1]

Oh, and check the TPS switch. It's on the throttle assembly, and on these cars, very sensitive.

Good luck!

Ross - Exactly where is the TPS switch? I think I remember you mentioned that there is more to check or replace than just the sensors. Do you know how to check the wiring from the connector back to the ECU, or can the problem be the ECU itself? I am throwing a code 41 like LScott400 and was sorry to hear that the replacement TPS did not solve his problem.

In doing other improvements not related to the code 41, I have already changed the plugs, wires, caps, rotors, fuel filter, cleaned the TB, air filter, and main O2 sensors. Though I can't pinpoint which had the greatest effect, I can definitely say that the car starts faster and has more power than ever before. Throttle response is excellent. I am stunned at how well it runs at 253k miles. I have a slight vibration at idle that I suspect is one or both motor mounts which I am attacking next.

I throw a code immediately after resetting the ECU or disconnecting the battery. 41 is the only code. In addition, I have two TPS sensors, apparently because I have traction control. Is one of these a "sub-sensor" and the other the main, and exactly what is the purpose of the second sensor?

Finally, I tried to correctly test the sensors following the shop manual/lexls, even used the correct thickness gauges under the throttle stop lever to take my readings, but I could not attach my multimeter to the TPS, even with micro alligator clips. The angle seems impossible. Any tips? Anyone?

[LScott400]

Sounds to me like a lack of fuel. Could be the fuel pump not working properly. The new fuel could have done something to this.....On the other hand you might have got contaminated fuel, maybe water or something.

I second that. I was considering that last night, and I agree. You could have picked up a bad batch of fuel with rust or sand in it, and it's plugged the fuel filter or the screen on the fuel pump inlet. Replace the filter and check out the pump.

An experience that supports these suggestions:

Yesterday at noon my '98 Chev van died after a quick run into Sam's Club. I didn't get out of the parking lot. After much fussing, kicking and cajoling I got it the 25 miles home many hours later. By then it had thrown codes that showed too lean. Today I waited until Sears opens and buy a fuel pressure test gauge. It shows 15 psi instead of the 60 it should be. I change the fuel filter and no change. I siphon down the tank to about 5 gallons and what came out had all kinds of floaties in it. I drop the tank and my 6 month old fuel pump has a pickup filter full of dark stuff. I sloshed the remainder in the tank and amazingly almost no floaties. I replaced the pump and now the gauge shows 60 psi and it runs like a champ. So, I guess the next step for the LS is to see if my gauge will fit or adapt to it.

P.S. I had filled up at a non-name brand station late Thursday because I could save 10 cents a gallon. What a bargain!

[Ross W.]

Ross - Exactly where is the TPS switch?

The TPS is on the throttle assembly housing, where the throttle shaft is, facing the front of the car on the front side of the housing. I don't recall if there are 3 or 4 wires going into it. To get at the conductors in the connector, use small wire guage brads, then connect to them with alligator clips. The "mechanic" I tried first with my problems simply pierced the insulation ahead of the connector, a definite no-no, because that will allow moisture into the wire. There may even be a special piggyback adapter through the service industry to get your reading without damage.

Hope this helps!

P.S. --- what is code 41, anyway? Lean condition?

[Threadcutter]

Oh, and check the TPS switch. It's on the throttle assembly, and on these cars, very sensitive.

Good luck!

Ross - Exactly where is the TPS switch? I think I remember you mentioned that there is more to check or replace than just the sensors. Do you know how to check the wiring from the connector back to the ECU, or can the problem be the ECU itself? I am throwing a code 41 like LScott400 and was sorry to hear that the replacement TPS did not solve his problem.

In doing other improvements not related to the code 41, I have already changed the plugs, wires, caps, rotors, fuel filter, cleaned the TB, air filter, and main O2 sensors. Though I can't pinpoint which had the greatest effect, I can definitely say that the car starts faster and has more power than ever before. Throttle response is excellent. I am stunned at how well it runs at 253k miles. I have a slight vibration at idle that I suspect is one or both motor mounts which I am attacking next.

I throw a code immediately after resetting the ECU or disconnecting the battery. 41 is the only code. In addition, I have two TPS sensors, apparently because I have traction control. Is one of these a "sub-sensor" and the other the main, and exactly what is the purpose of the second sensor?

Finally, I tried to correctly test the sensors following the shop manual/lexls, even used the correct thickness gauges under the throttle stop lever to take my readings, but I could not attach my multimeter to the TPS, even with micro alligator clips. The angle seems impossible. Any tips? Anyone?

Gum:

A little perspective;

TPS

It's not conclusive, nor 100% accurate to your vehicle. However, the point being that the TPS is "just" a potentiometer. It's probably 5,000 Ohms (5KOhms), but they come in many different values. You can unplug the connector & check it with an Ohmmeter. If it's bad, go to Rat Shack, spend $10.00 & replace it.

[Gumart1]

Ross and Threadcutter - Thanks for your input. Here is a post from member Obergc (Curt) that I found helpful:

"A code 41 for a 93 indicates a bad throttle position sensor CIRCUIT which includes the throttle position sensor, part of the ECM since it supplies excitation voltage for the sensor and all of the interconnecting wiring between the throttle position sensor and the ECM. This should be an easy problem to troubleshoot if you have a digital voltmeter. Your 94 might be different from the 93 but for a 93, the throttle position sensor has 4 wires connected to it. With the Ignition Switch in the Run position, the blue/red wire should have a constant +5V amd the brown wire should show a good ground connection. The yellow wire should have a voltage between 0-5V and it should change smoothly when you move the throttle linkage. The green wire should also show a good ground connection with the throttle at idle position."

Now my shop manual only shows testing the actual TPS and not the wiring harness that connects to it. Also the shop manual doesn't mention testing the sub-sensor but I assume the test is the same. So if I understand correctly, to accurately pinpoint the problem I need to test the actual contact leads on the TPS for the correct resistance as well as the the wires from the ECU for the proper voltage, all tests to be made with the ignition on but not running. Ross, are you saying that you can use the test brads to get to the wires from the open end of the harness where it connects to the TPS?

Threadcutter - Is the Radio Shack part you are referring to the TPS or the wiring harness? Sorry I am confused here. Do you have a part #? Thanks again for your input.

[Threadcutter]

Ross and Threadcutter - Thanks for your input. Here is a post from member Obergc (Curt) that I found helpful:

"A code 41 for a 93 indicates a bad throttle position sensor CIRCUIT which includes the throttle position sensor, part of the ECM since it supplies excitation voltage for the sensor and all of the interconnecting wiring between the throttle position sensor and the ECM. This should be an easy problem to troubleshoot if you have a digital voltmeter. Your 94 might be different from the 93 but for a 93, the throttle position sensor has 4 wires connected to it. With the Ignition Switch in the Run position, the blue/red wire should have a constant +5V amd the brown wire should show a good ground connection. The yellow wire should have a voltage between 0-5V and it should change smoothly when you move the throttle linkage. The green wire should also show a good ground connection with the throttle at idle position."

Now my shop manual only shows testing the actual TPS and not the wiring harness that connects to it. Also the shop manual doesn't mention testing the sub-sensor but I assume the test is the same. So if I understand correctly, to accurately pinpoint the problem I need to test the actual contact leads on the TPS for the correct resistance as well as the the wires from the ECU for the proper voltage, all tests to be made with the ignition on but not running. Ross, are you saying that you can use the test brads to get to the wires from the open end of the harness where it connects to the TPS?

Threadcutter - Is the Radio Shack part you are referring to the TPS or the wiring harness? Sorry I am confused here. Do you have a part #? Thanks again for your input.

Gum:

I haven't removed mine to know what the value is, but I bet it is 5K Ohms (5000 Ohms). I was looking at LexLS's site the other day & it appears that the actual throttle position sensor (that contains the potentiometer) is perhaps a little smaller than what could adequately accomodate a "standard" size potentiometer. I think he says that the value should be 5K Ohms, but it should be double checked for sure.

Anyway, yes, there is a simple potentiometer inside of the black body that is referred to as the "TPS" or "Throttle Position Sensor". It may be that an "off the shelf" Radio Shack potentiometer won't physically fit inside the black body. But, potentiometers come in lots of different sizes & shapes. The point I was really trying to make is that there is absolutely nothing magical about the "innards" of a TPS............it's just a pot, that's all. So, when the stealer tries to charge someone hundreds of dollars for this part, they're getting ripped off.

BTW, your write-up of the testing of the TPS makes perfect sense. If a person were to do these tests, then it would be easy to isolate either the TPS, wiring, or ECU as being problematic. I would tend to think that the TPS itself would be the point of failure in about 90% of the cases.

[Gumart1]

I am guessing it is the throttle position sensor itself but wanted to share my readings. I just finished testing the resistance on the sensor and I have to say it is not easy to get to. I had to remove the sensor first (marking the location of the housing). Then I attached micro alligator clips to the correct terminals. After reinstalling the housing, I slipped some flexible electrical wire into the alligator clip ends, and then attached my ohmmeter leads to the wires. This was the only way to clearly see the terminals to make sure I had it right.

There are 5 different test for resistance between 4 different terminals. 3 of my tests were perfectly within specs. However the two tests that had me connect terminals IDL and E2 gave me NO reading at all. I repeated the test to make sure I had the contacts in place. I also had the correct thickness gauges to insert between the throttle stop screw and stop lever.

I am surprised because I expected to have too little resistance, telling me the sensor was bad but no reading? Do you think that one terminal is bad or somehow broken? Is this a possibility or did I do something wrong? I'd love to conclude that I indeed have a bad TPS from this test. What do you think?

Threadcutter, just from looking at the diagram of the innards of the TPS I can see now what you have been saying, that it is just a simple potentiometer. In that case I'm betting that my one terminal (IDL) is somehow broken inside or not reacting. Can the housing be removed to access this? I don't see that it is removeable.

[Threadcutter]

I am guessing it is the throttle position sensor itself but wanted to share my readings. I just finished testing the resistance on the sensor and I have to say it is not easy to get to. I had to remove the sensor first (marking the location of the housing). Then I attached micro alligator clips to the correct terminals. After reinstalling the housing, I slipped some flexible electrical wire into the alligator clip ends, and then attached my ohmmeter leads to the wires. This was the only way to clearly see the terminals to make sure I had it right.

There are 5 different test for resistance between 4 different terminals. 3 of my tests were perfectly within specs. However the two tests that had me connect terminals IDL and E2 gave me NO reading at all. I repeated the test to make sure I had the contacts in place. I also had the correct thickness gauges to insert between the throttle stop screw and stop lever.

I am surprised because I expected to have too little resistance, telling me the sensor was bad but no reading? Do you think that one terminal is bad or somehow broken? Is this a possibility or did I do something wrong? I'd love to conclude that I indeed have a bad TPS from this test. What do you think?

Threadcutter, just from looking at the diagram of the innards of the TPS I can see now what you have been saying, that it is just a simple potentiometer. In that case I'm betting that my one terminal (IDL) is somehow broken inside or not reacting. Can the housing be removed to access this? I don't see that it is removeable.

Gum:

Awww...........for all the !Removed! around one would probably have to do, maybe the easiest thing in the long run is to just buy a new TPS from PPL;

PPL

throttle position sensor, LS400, Nippondenso 1995 $90.31 $0.00 $74.96

Electrical - Powertrain control - throttle position sensor

throttle position sensor, LS400, Toyota 1995 $140.64 $0.00 $116.73

Electrical - Powertrain control - throttle position sensor

I don't know how much they'd charge for a 1991 but a 1995 genuine Toyota is $116.73. Not terrible...........

I have had pretty good luck with PPL.............

[SKperformance]

Save the guessing ,get it read and know exatly what the code is. Until then it is just a best guess.

[Gumart1]

I did have the codes read as I mentioned in my earlier posts and I have a code 41. According to prior posts this points to the TPS itself or the wiring to the TPS or the ECU. That is why I set out to troubleshoot instead of throwing parts at it.

Thanks Threadcutter and others for the help - I plan to order the part today it is about $110 shipped for my '91. Not sure if I should replace the sub-sensor as well ($75) because it is just as old as the main TPS, even if it doesn't throw a code? Does anyone here know exactly what the sub-sensor does? All I know is that it is related to the Traction Control as I understand the sub-sensor only exists on models with this option. I'll report if the problem is fixed. Besides my CEL with code 41 the car starts, idles, and runs great.

[Threadcutter]

I did have the codes read as I mentioned in my earlier posts and I have a code 41. According to prior posts this points to the TPS itself or the wiring to the TPS or the ECU. That is why I set out to troubleshoot instead of throwing parts at it.

Thanks Threadcutter and others for the help - I plan to order the part today it is about $110 shipped for my '91. Not sure if I should replace the sub-sensor as well ($75) because it is just as old as the main TPS, even if it doesn't throw a code? Does anyone here know exactly what the sub-sensor does? All I know is that it is related to the Traction Control as I understand the sub-sensor only exists on models with this option. I'll report if the problem is fixed. Besides my CEL with code 41 the car starts, idles, and runs great.

Gum:

I don't know if you've already seen this, but maybe it'll help;

TPS LexLS

I really doubt your wiring has "gone bad"...........Could be, but I think chances are high that it's your TPS. I've got to admit that I don't know anything about the sub-sensor, so I shouldn't do any speculating on that. In any electronics/electrical circuit, it will usually be the "active" components that fail/are the wear items. It happens, but it's very unusual for wire or connectors to "go bad".

Oh, & By the way..............Break open (after everything is back together & works) the old TPS & let us know (picutres would be nice) what's inside. I'm guessing (strongly) that it's a 5K pot with an end of rotation switch (hence 5 terminals). It may be that the potentiometer body is smaller than the usual "run of the mill" Rat Shack specials.

[LScott400]

Cracked open the ECM today and found a leaking capacitor (C108 if I remember correctly). Removed the old one, cleaned the residue from the leak off some traces, and soldered in a replacement from Radio Shack. The acceleration is back!

Apparently from having had the ECM disconnected for awhile, the codes were gone (41 & 25). Drove it awhile and only Code 25 came back. So next will be tracking that one down. Had noticed that there was not good ground at E2 of ECM. After ECM service, ground was significantly better (maybe affected by cleaning shorted traces due to leakage?) but still not right. Also, having trouble locating engine ground for E1 of ECM.

[LScott400]

Code 25 Adventure:

Wild day! Followed the manual procedure for checking voltage at Diag Connector of main O2's. Warm engine held at 2500 RPM watch for voltmeter fluctuation between 0 and 5 volts looking for 8 times every 10 seconds. Mine stayed at 5V nonstop. Research on web determines 5 would be rich and 0 would be lean. So I've got O2's seeing rich and a trouble code that says lean. No doubt the ECM is going rich to correct the lean it thinks it sees.

Talk to 2 different techs with extensive Lexus dealer experience. Both say bad O2's. I say I changed them for maintenance back in February. They both tell me I should have bought OE sensors and speak poorly of the universal Bosch's I installed.

I jack up the LS, pull the connectors and meter the heater resistance. Spec is 5.1 - 6.3 ohms @ 68 degrees F. I have 9 ohms. It still doesn't add up for me, but I go to parts store. They would have to order OE style, so I get another set of universals. Get home and put the meter to them. 9 ohms resistance. Well, it drove fine before, so no way am I going to change them out.

Back to troubleshooting procedure for codes 25 & 26. 5V go to step 7. #1 check Fuel system - Good. #2 check engine coolant temp sensor. Cheated and read it off terminals at ECM - Good. #3 check intake air temp sensor - Good. #4 check volume air flow meter - Good. Go to step 8. Check compression - Unnecessary. Got to step 9. Does malfunction disappear when a good main heated O2 sensor is installed? Well, they are the same as the new ones in the boxes. Final step - Replace ECM. That would make sense as the O2's read rich, but the ECM sees them as lean.

Drive to local ECM rebuilder and exchange ECM. Get it home. Now the car won't even start. Continued cranking causes a noise like the old carbureted engines used to make when they would backfire through the carb. Jump E1 and TE1 and won't even go into diagnostic mode.

Look at receipt. Rebuilders part number and then a description with my original ECM number. The exchange ECM has the same number as the original except for the last digit. Search Forums. Thank you 95LS400Bob!!!!!!!!!!!!!!!!!! Posting (#123) with 93 and 94 ECM numbers. My original number is indeed for a 1994 non-California model. They have given me a 1993 non-California model.

Call rebuilder. He says their computer shows 50140, 50141 and 50142 all work in a '94. I know that's not quite right. 50141 is for a California model. I say my car won't even start now. He asks if it's showing any codes. I say it won't even go into diagnostic mode. He says they'll set up another and I can return this one.

I get the call to come get the 2nd one. Happy to see it is a 50142 like the original. Take it home and plug it in. Runs like a champ. Don't know if the other one was bad, but would have worked or if the '94 part number is required.

All I can figure is that the leaking capacitor did additional circuit damage to the original ECM. Careful cleaning after removing the old cap revealed that the leakage appeared to have eaten through the varnish/shellac that covers the printed circuits.

Thanks again to all who both run and participate in this Forum. You have been invaluable!

[maniek_LS]

Cracked open the ECM today and found a leaking capacitor (C108 if I remember correctly). Removed the old one, cleaned the residue from the leak off some traces, and soldered in a replacement from Radio Shack. The acceleration is back!

Did you made a photo of open ECM and bad capacitor here ? Can you post it please ? How did you find this damaged part ?

[LScott400]

I failed to take pictures at the time. The same problems are common to vehicles years 1989-1994:

MITSUBISHI/MAZDA/HONDA/SUZUKI/LEXUS/TOYOTA/GEO and most Japanese manufactured

automotive computers.

This is a how-to with great pics for a Mitsubishi.

This is a 6-page document with details on capacitor failure and the problems associated with it. Also, many pics.

Due to the corrosive nature of these older design capacitors it is recommended they be replaced BEFORE they began leaking to avoid ECM damage. In my case I was too late.

Hope this helps!

[93ls400walt]

What type of service or repair did the ECU rebuilder do to your new ECU? Also, if you don't mind. What did this cost? I have a 93 Ls with similur acceleration problems. But the Cel light does not come on. I do have code41. And have replaced the TPS. If you could provide a contact number for this shop. That would be great.

[maniek_LS]

I failed to take pictures at the time. The same problems are common to vehicles years 1989-1994:

Hope this helps!

Yes,

thanks. But I have one more question to you. How did you brainstormed to open ECU ? You go via whole check service matrix ? Or what ? I'm very impressed !

[LScott400]

Walt-

I'm not sure what the procedure is that the rebuilder uses. It was a simple exchange for the original unit. $300.00 plus tax with one year warranty. AutoComp Technologies, 8515 North Fwy, Houston, TX 77037. 888-217-4072

I do know that their facility is huge and they have many salvage engine/electrical mock-ups that they use to test ECM's. Quite an operation. (I saw a shelf with mass air flow sensors - excuse me, volume air flow meters - so they may repair those as well.)

Their weakness (and this may apply to all of these kind of places) is that they don't adhere to a same part number exchange. I had a '96 Dodge Stealth on which a Mitsu dealer diagnosed a faulty ECM (hard to find a Dodge dealer with a tech that has experience with these cars). AutoComp showed a different part number than the one I brought in. I called the Mitsu dealer and he ran the VIN and came up with the same part number as AutoComp. The car wouldn't hardly start with it installed. They gave me another one and same thing. I talked to a Dodge dealer parts guy and found out that number was for a non-turbo, but meant for a double overhead cam. Apparently Mitsu only offered those 2 engines that year. '96 was the last year for the Stealth and Dodge had a 3rd single overhead cam engine (mine). Went back to AutoComp and got a hold of a higher up who spent some time on his computer and found they actually had 3 of the part number I originally had, but the application was unknown to them (Dodge only sold 318 of that model in that last year and it was the first year of OBDII). Anyway, once they gave me one of that part number the car was good to go. I got it inspected and sold it quickly since parts for that year are so hard to find.

Maniek-

After reading about the techinfo.lexus.com website on this forum I paid my $10 and downloaded all of the service manual portions. And, yeah, I actually went through the manual procedures. You wouldn't have been so impressed if you had seen me sweating and scratching my head.

The reason I suspected the ECM early on was that I had a similar problem on a '92 Mitsubishi 3000GT. That's why my links were to Mitsu ECM repair pages. I was fortunate enough on that car to resolve my problems by changing out the capacitors after cleaning the circuit board. Not so on the Lexus. That is why I included the bit about changing the old style caps BEFORE they start leaking.