Car Is Having Dead Spots While Driving....

[RandySC400]

hey guys im once again asking for your help with my car it has dead spots during excelleration (lunging) and is driving me crazy!! i have replaced both coils and plugs and wires...caps and rotors are also new...i'm thinking it might be the MAF, IAC or the TPS... all which are expensive to replace..any suggestions on how to test each before i spend my money on it??? thank you again for your input!! There may be topics already on this but i cant figure out how to define my searches... thanks guys!!

[python]

u own an sc400? wrong forum if so...no cel on? does it misfire at idle?

***Moved to proper forum.*** Smooth 1

[1990LS400]

It can be far less expensive in the long run to take a car to a professional mechanic who has the appropriate diagnostic equipment instead of just diving in and replacing stuff. This aren't 1950's or the 1960's cars -- these care are very complex. Save your self any more grief, bite the bullet and take your car to someone who specializes in repairing Lexus vehicles. Ask for a diagnosis -- you don't necessarily have to have the mechanic do the repair.

[RandySC400]

okay so i posted this in the LS400 column and it got moved here because of my name RandySC400....i do own 2 1992 SC400s and a 1993 LS400...thats why the name...anyway i need this to be moved back to the LS400 section so i can get some info from the other LS400 owners even thou the motors are the same... as far as taking my car to a mechanic...that won't happen ever... especially owning a lexus!! Most mechanics are over paid and i am capable of doing the work myself once i know what the problem is. i have replaced those parts because they needed to be replaced...not out of guessing...my lexus has 230,000 on it and some parts are starting to go on it... PLEASE MOVE ME BACK TO THE LS400 SECTION.... THANK YOU...

No need for attitude. There wasn't anything in your original post that said it was in the correct forum either, like: "My 93 LS400 is having some issues." So now it's back in the LS forum's for your enjoyment. So have a seat and no touching or picture taking during the shows. THANK YOU!

[eatingupblacktop]

hey guys im once again asking for your help with my car it has dead spots during excelleration (lunging) and is driving me crazy!! i have replaced both coils and plugs and wires...caps and rotors are also new...i'm thinking it might be the MAF, IAC or the TPS... all which are expensive to replace..any suggestions on how to test each before i spend my money on it??? thank you again for your input!! There may be topics already on this but i cant figure out how to define my searches... thanks guys!!

If you don't want to pay for a tech, hook up a diagnostic EDR (event data recorder) to your OBD port. While driving, push the event button every time you experience the "dead spots". Read the diagnostics on your computer. Check the data immediately before and after your marks and compare. You should be able to narrow down the prob.

[lexfourcam]

ive noticed i have those dead spots too, an no matter hard i push the pedal down sometimes in 1st gear it wont go above 3k rpm. it will shift into second. sometimes it'll go past but it seems like its just dead until it gets to about 4 and it seems to have its *BLEEP* together.

[VBdenny]

If you haven't done so you, take your car over to Autozone or Advance and they will lend you an OBDII for free. Bring something to take notes with as it gives codes and descriptions of any information entered. I have used this on several occasions. Free, free, free, that is the key. Your problem almost sounds like a fuel issue? If you use bottom tier gasoline on a routine basis your throttle body could be not ideal. I would add 2 cans of Valoline Complete Fuel System cleaner (about $10) to your tank. I have had a lot of success with that. Since I began usuing only Top Tier fuels several years ago, I no longer have fuel system issues. Next, have you (or your wife) ever run the car out of gas? Very very bad for your fuels pump to let gas tanks get low, especially in the summer. Those fuel pumps work hard and they heat up. The gasoline in your tank is what cools them and when you run out, they can be compromised. I never let my tank get lower than 1/4 although my wife is another issue. She ran a Chevy Caprice I had out of fuel a couple times and a year later while on the Jersey Turnpike, late on a Sunday night, I go the same symptoms you described which got worse and work until I was parked and towed. No fun, not a bit. The bill for the fuel pump replacement wasn't any fun either, around $600.

[1990LS400]

Randy, do I understand correctly? You own three Lexus cars with identical V8 engines, you won't use a professional mechanic and you won't pop for a cheap set of repair manuals that have detailed diagnostic procedures for the MAF, IAC, TPS and most everything else? Is this correct?

[Quack11]

If you haven't done so you, take your car over to Autozone or Advance and they will lend you an OBDII for free. Bring something to take notes with as it gives codes and descriptions of any information entered. I have used this on several occasions. Free, free, free, that is the key. Your problem almost sounds like a fuel issue? If you use bottom tier gasoline on a routine basis your throttle body could be not ideal. I would add 2 cans of Valoline Complete Fuel System cleaner (about $10) to your tank. I have had a lot of success with that. Since I began usuing only Top Tier fuels several years ago, I no longer have fuel system issues. Next, have you (or your wife) ever run the car out of gas? Very very bad for your fuels pump to let gas tanks get low, especially in the summer. Those fuel pumps work hard and they heat up. The gasoline in your tank is what cools them and when you run out, they can be compromised. I never let my tank get lower than 1/4 although my wife is another issue. She ran a Chevy Caprice I had out of fuel a couple times and a year later while on the Jersey Turnpike, late on a Sunday night, I go the same symptoms you described which got worse and work until I was parked and towed. No fun, not a bit. The bill for the fuel pump replacement wasn't any fun either, around $600.

Very common issue and i'm currently having the same problem with my 91 LS400. I too have refused to take my car to any mechanic and have invested in an OBD-I&II scanner for future use due to my hobby of fixing the cars that I can handle. I've done extensive and indepth troubleshooting, fault isolation and test/check of this problem up to today and all problem is pointing to the fuel pump. Ordered one two hours ago and will post the results after installation. Currently I have two of everything that will cause that problem and nothing seem to work so I got a used fuel pump and the car drove fine for a day until after I 1/2 fill the tank with premium gas, then the issue came back. No CEL and nothing stored in the ECM, i've tricked the system to trip the CEL and it came on and was picked up by the scanner tool. Every sensor, wire, hose, etc that I have access to was thoroughly inspected. Mounting surface where EGR valve connects to intake was badly clogged but did not affect functionality, not sure why. Osolate both engine banks and individual cylinder and the operational difference was felt, not to mention the electrical shock from the plug wire. What I did here was verify that ignition/spark was not a factor or contributor to this problem and I totally ruled it out. T/S fuel flow system to include, pressure, relay, regulator and nothing was changed. swapped ECM and results are the same so I went a bit further because this problem will not conquer me one bit. I removed the rear seat and open the fuel tank and start the engine. With a constant 12V going to the pump it was surging and the engine would react to the high/low fuel delivery pressure. This tells me that the voltage is correct and consistent but the fuel pump is either cavitating or worn internally. So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and is hoping a new bug will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. I hooked up a fuel line from my SUV onto the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache. I think this is the problem and I will provide an update after installation and a few operational miles. Sorry for the long write up but I hope it will help.

[Quack11]

If you haven't done so you, take your car over to Autozone or Advance and they will lend you an OBDII for free. Bring something to take notes with as it gives codes and descriptions of any information entered. I have used this on several occasions. Free, free, free, that is the key. Your problem almost sounds like a fuel issue? If you use bottom tier gasoline on a routine basis your throttle body could be not ideal. I would add 2 cans of Valoline Complete Fuel System cleaner (about $10) to your tank. I have had a lot of success with that. Since I began usuing only Top Tier fuels several years ago, I no longer have fuel system issues. Next, have you (or your wife) ever run the car out of gas? Very very bad for your fuels pump to let gas tanks get low, especially in the summer. Those fuel pumps work hard and they heat up. The gasoline in your tank is what cools them and when you run out, they can be compromised. I never let my tank get lower than 1/4 although my wife is another issue. She ran a Chevy Caprice I had out of fuel a couple times and a year later while on the Jersey Turnpike, late on a Sunday night, I go the same symptoms you described which got worse and work until I was parked and towed. No fun, not a bit. The bill for the fuel pump replacement wasn't any fun either, around $600.

Very common issue and i'm currently having the same problem with my 91 LS400. I too have refused to take my car to any mechanic and have invested in an OBD-I&II scanner for future use due to my hobby of fixing the cars that I can handle. I've done extensive and indepth troubleshooting, fault isolation and test/check of this problem up to today and all problem is pointing to the fuel pump. Ordered one two hours ago and will post the results after installation. Currently I have two of everything that will cause that problem and nothing seem to work so I got a used fuel pump and the car drove fine for a day until after I 1/2 fill the tank with premium gas, then the issue came back. No CEL and nothing stored in the ECM, i've tricked the system to trip the CEL and it came on and was picked up by the scanner tool. Every sensor, wire, hose, etc that I have access to was thoroughly inspected. Mounting surface where EGR valve connects to intake was badly clogged but did not affect functionality, not sure why. Osolate both engine banks and individual cylinder and the operational difference was felt, not to mention the electrical shock from the plug wire. What I did here was verify that ignition/spark was not a factor or contributor to this problem and I totally ruled it out. T/S fuel flow system to include, pressure, relay, regulator and nothing was changed. swapped ECM and results are the same so I went a bit further because this problem will not conquer me one bit. I removed the rear seat and open the fuel tank and start the engine. With a constant 12V going to the pump it was surging and the engine would react to the high/low fuel delivery pressure. This tells me that the voltage is correct and consistent but the fuel pump is either cavitating or worn internally. So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and is hoping a new bug will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. I hooked up a fuel line from my SUV onto the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache. I think this is the problem and I will provide an update after installation and a few operational miles. Sorry for the long write up but I hope it will help.

[SRK]

[ So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and is hoping a new bug will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. I hooked up a fuel line from my SUV onto the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache.

A professional would simply do a fuel system pressure check and scan the pump for current draw and rpm.......

[Quack11]

[ So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and is hoping a new bug will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. I hooked up a fuel line from my SUV onto the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache.

A professional would simply do a fuel system pressure check and scan the pump for current draw and rpm.......

100% agree. Professionals also have all the required tools and a price chart ready to make hard times harder.

[RandySC400]

Thanks for all the advice guys...this is why this site is so valuable...i cant afford to take my car to a mechanic...because that is taking money away from making it the way i want... yes i should be able to do most of all the work myself,and save money...so i ask for you guys for your past experiences and knowledge to help...and i do have 3 lexus' but i havent had to pay over $250.00 for any of them...it was people who just couldnt afford to keep paying those over priced lexus mechanics, and one was hit hard in the front...but repairable...keeping costs down doing the work myself... but maybe i will invest in the manuals...i love lexus'. Anyway guys thanks for the advice i will let you know how i make out...THANKS AGAIN

[lexfourcam]

this problem has created such a headache for me and other people who have worked on it. my car runs great when it first fires up, but then after it warms up it just doesnt go at all. has dead spots, sputtering, and very abrupt shifts.

[oldskewel]

Thanks for all the advice guys...this is why this site is so valuable...i cant afford to take my car to a mechanic...because that is taking money away from making it the way i want... yes i should be able to do most of all the work myself,and save money...so i ask for you guys for your past experiences and knowledge to help...and i do have 3 lexus' but i havent had to pay over $250.00 for any of them...it was people who just couldnt afford to keep paying those over priced lexus mechanics, and one was hit hard in the front...but repairable...keeping costs down doing the work myself... but maybe i will invest in the manuals...i love lexus'. Anyway guys thanks for the advice i will let you know how i make out...THANKS AGAIN

I admire the attitude. Good luck in fixing this.

I haven't had this problem on my '91, but here's a theory in case the fuel pump issue does not fix it:

there may be a problem with the air flow sensor - PN 22250-50010

http://www.toyodiy.com/parts/p_U_1991_LEXU...EPGKA_1703.html

I believe this unit is almost identical to the air flow sensor on my other car, a '85 Porsche 911, which I also keep away from professional mechanics. On that car, I would have seemingly intermittent dead spots, and would sometimes have trouble getting over 5000 rpm. Somehow I traced it down to this air flow sensor. Basically the sensor is a flapper that opens wider with more air flow. The opening flapper moves wipers along a resistive surface, just like a potentiometer. Over time, grooves had been worn in this surface, leading to dead spots and spurious air flow readings. With bad air flow readings, not enough fuel will be injected, limiting the rpm, and in general causing driveability problems.

An easy test is to connect an ohmmeter to the right two connectors (not sure which) on the air flow sensor. Power can be off, nothing has to be plugged in. Move the flapper with your hand and see if the resistance changes continuously and monotonically (no dropouts, jumps, or going down after going up). It should, if not, your unit needs repair or replacement. Since it is a $1k+ part (as for the Porsche - they really may be identical), repair may be better. If you narrow things down to this, I'll pass on a trick I used to repair mine on the 911. It drove like a brand new car after I made that change (and still does).

[SRK]

I believe this unit is almost identical to the air flow sensor on my other car, a '85 Porsche 911, which I also keep away from professional mechanics. On that car, I would have seemingly intermittent dead spots, and would sometimes have trouble getting over 5000 rpm. Somehow I traced it down to this air flow sensor. Basically the sensor is a flapper that opens wider with more air flow. The opening flapper moves wipers along a resistive surface, just like a potentiometer. Over time, grooves had been worn in this surface, leading to dead spots and spurious air flow readings. With bad air flow readings, not enough fuel will be injected, limiting the rpm, and in general causing driveability problems.

An easy test is to connect an ohmmeter to the right two connectors (not sure which) on the air flow sensor. Power can be off, nothing has to be plugged in. Move the flapper with your hand and see if the resistance changes continuously and monotonically (no dropouts, jumps, or going down after going up). It should, if not, your unit needs repair or replacement. Since it is a $1k+ part (as for the Porsche - they really may be identical), repair may be better. If you narrow things down to this, I'll pass on a trick I used to repair mine on the 911. It drove like a brand new car after I made that change (and still does).

Your Porsche has a Bosch L-jetronic fuel system, using a swing valve air flow sensor. The Lexus uses either an optical air mass meter, or in the later cars, a hot wire air mass meter. In either case they are quite unlike the primitive (by today's standards) L-jetronic of the Porsche, or other Bosch equipped engines of the 1980's.

[curiousB]

A professional would simply do a fuel system pressure check and scan the pump for current draw and rpm.......

100% agree. Professionals also have all the required tools and a price chart ready to make hard times harder.

A fuel pressure gauge is probably a good investment given this is becoming a hobby that you plan to continue for some time.

The difficulty with these systems is that they are “closed loop” systems. That is the end to end performance is due to the individual elements along the chain (tank, fuel filter, fuel pump, fuel pressure regulator, injectors, ECU, O2 sensors, MAF sensor, IACV, temp sensor, throttle position sensor, and so on) but ALSO the fact they’re connected in a feedback loop (under ECU control). The good new is this feedback loop allows the system to be stable as it compensates for individual factors. For instance if the engine is cold the IACV will open a little more until the engine warms up to hold the idle constant). The difficulty in diagnosing these systems is because every element in the chain can affect performance in not always obvious ways. It can lead to false conclusions of which element is corrupting the feedback loop performance. You’ve felt the impact of some of this with your problem.

Anyway I’d encourage a $20 fuel pressure test set as you can connect it into the fuel manifold and monitor pressure when engine starts up cold, after warmed up, when running at idle, perhaps even while driving (maybe can rig gauge to a wiper blade so you can see it through windshield while driving a test run. Given the way closed loop systems all move together as one, it’s important to be able to test elements individually to isolate the problem area which is blurred by the feedback nature of the design. Hopefully you’ll see poor running tied to low pressure (or less likely but maybe too high pressure). Then you can have high confidence in your new fuel pump being the solution.

[oldskewel]

I believe this unit is almost identical to the air flow sensor on my other car, a '85 Porsche 911, ...

Your Porsche has a Bosch L-jetronic fuel system, using a swing valve air flow sensor. The Lexus uses either an optical air mass meter, or in the later cars, a hot wire air mass meter. In either case they are quite unlike the primitive (by today's standards) L-jetronic of the Porsche, or other Bosch equipped engines of the 1980's.

You're right. I should have looked it up first. My '91 LS400 has an optical-based meter (page FI-47 of the 1991 manual). For some reason, I recalled looking up the part for my 911 and seeing a Toyota on the compatibility list (a Supra, I believe). When I got the Lexus, they looked similar enough that I thought it could be the same.

Porsche used that flapper-based sensor on the 911s from 84-94, so it was not obsolete back when our cars were made, even though it may be now.

Regarding getting the right tools - yes. As soon as I get any car, first thing I buy is the best service manual available. You save enough $$ by doing the work yourself that buying the right tools is often justified.

[RandySC400]

thanks again for the advice....all of, i do appreciate!! my lexus is a 1993 ls400 and i went to autozone and their tester oboII cant be used for my year car.. i don't have a check engine light on and i've jumped and read the codes and nothing is coming up...i will now try your other suggestions...thanks again.

[lexfourcam]

well tonight when i was driving, i tried to take off fast and wow she hauled some !Removed!! i look down and my check engine light is off. i did fix this hose on my intake, but it came back on when i started the car after i reset the ecu, and now its off. if only it would stay gone....

[Quack11]

well tonight when i was driving, i tried to take off fast and wow she hauled some !Removed!! i look down and my check engine light is off. i did fix this hose on my intake, but it came back on when i started the car after i reset the ecu, and now its off. if only it would stay gone....

I can relate to your misery or a CuriousB calls it, "Murphy". Continue to isolate the many possibilities and I am sure your problem will soon be identified and fix. Good luck.

[lexfourcam]

thanks, im gettin the exhaust re-done pretty soon, x pipe and some borla mufflers. took a video with my phone of my old setup but the mic just sounds terrible.

[lexfourcam]

so today i unplugged my maf, and holy *BLEEP* did she run amazing. i !Removed! raped this guy in my neighborhood and he was like i'll try not to dust you next time because we'd raced one time when my car was off time and had 3 or 4 other problems. an i was like you wont because my car will be fixed by then. hahaha i bet he went home and cried...still dont kno if its the maf itself or the connector. the connector is very cracked up gonna test it with a volt meter, but wow guys it runs like it should now. if it is the connector though how in the hell would i replace it?

[curiousB]

so today i unplugged my maf, and holy *BLEEP* did she run amazing. i !Removed! raped this guy in my neighborhood and he was like i'll try not to dust you next time because we'd raced one time when my car was off time and had 3 or 4 other problems. an i was like you wont because my car will be fixed by then. hahaha i bet he went home and cried...still dont kno if its the maf itself or the connector. the connector is very cracked up gonna test it with a volt meter, but wow guys it runs like it should now. if it is the connector though how in the hell would i replace it?

You mean you ran with MAF unplugged??!! Or did you put a new one in?? Not sure how an engine could run with any sensor attached as the engine would revert to limp mode if it could even work at all without a sensor connected. I know when the ECU is running open loop it is biased to run rich vs lean so that might explain some of this. You'd fail an emissions test but maybe thats not an issue in your case.

btw this is a luxury sedan not exactly a 1/4 mile candidate.

[lexfourcam]

yea i went outside and i was just as stumped as you, i was like this is not going to work, but the car runs fine without the maf hooked up. i do know that its probably gonna run rich because it doesnt have a sensor to tell it how much air is coming in. i remember when this sensor first went bad my car would stumble, but it runs fine doesnt stumble or idle rough.