1991 Ls400 Miss-Firing

[hughes369]

I bought 1991 LS400 with 279,000 plus miles on it. TPS sensor appears to have been messed with on the throttle body. RPM gauge has needle sitting below the Zero, and only comes above the Zero after Revving really high. Temp guage would not move either. Replaced the sender for the temp gauge(single blade wire transmitter) now that works. Oil Switch was leaking and illuminating the warning light, replaced this switch, now no leaks or warning light. Transmission had pieces in the pan, plastic thrust washer found sucked into the Trans filter. This problem caused the car to jerk and "burn rubber" with the tires when putting into gear from park. Check Engine Light Bulb was burned out. Replaced the bulb and got a code 28 flashing. Right Bank O2 sensor. So replaced the O2 sensor with one from Advanced Autoparts for about $100.00.

Plugged the vacuum hoses going from the intake to the Power Steering Pump, the common ATF in intake problem.

Check Engine Light now off.

Tranmission was rebuilt and reinstalled with Rebuilt Torque Converter, new Rear Oil seal on Engine, and Brazed/welded up the hole in the EGR Tube to fix the common exhaust leak there, also replaced a few broke exhaust studs and nuts and a new Rear Transmission Mount.

Transmission works fine.

Replaced spark plugs and gapped to .045 NGK BKR6EGP and used anti-seize compound.

Tried to diagnose the engine missfire at RPM, revving above idle, the car misses. Tried several test parts from a known good 1991 LS400, ignition coil, tried it on the left and right coil location, Air Flow Meter. Still no improvement when revving up the engine, still misses. Tried a one year old fuel pump off a 1992 LS400, along with the other parts from the 1991, still no improvement when revving up the engine, still misses. Tried removing the coil wire off each bank to verify that the coils are contributing to combustion. Engine died down each time i pulled the coil wire off left and right Distributor Caps.

Compression Test on all cylinders with uncalibrated Compression testor were all 240 to 260 PSI with 7 Revolutions for each cylinder test.

Fuel Injector Resistance measured on Left Bank front to back are as follows: 15.5 15.5 15.9 and 16.0

Fuel Injector Resistance measured on Right Bank front to back are as follows: front cylinder-19.0 rear cylinder 17.2. did not check the two under the throttle body.

Old Fuel pump voltage was 8.30 volts at idle before I swapped with the good pump.

(Borrowed Good Fuel pump) voltage was 9.30 volts at idle running on the suspect car.

10.30 volts running on the known to be good car(the 1992 donor car).

Engine also idles up and down up and down on its own, as if someone is tapping the gas pedal. I squeeze the Idle air hose shut between the front of the intake manifold and the Air Intake hose before the throttle body and car idles down and if squeezed hard enough, engine shuts off. If squeezed part way it idles normal and does not go up and down on its own. WTF......

Have not replaced the Fuel Filter Yet.

Have not tried using another ECU.

Have not replaced plug wires or cap and rotors. I have cleaned the cap and rotors of corrosion and replaced 2 of the plug wires that were suspect from a few old wires from my 1992.

Do not have a shop manual, only use Lextreme.com and LexLS.com for technical data.

Fuel pump voltage is a concern for me and so is injector resistance. What are the OEM values for these paramaters and does my car fall into them.

What am i not getting right? what am I missing on this Engine Miss? I have built several Nissan and Toyota Lexus engines, and need some input.

Thanks for any ideas.

[hughes369]

Also, car starts up fine, just when revved up it misses.

[hughes369]

Also, car starts up fine, just when revved up it misses.

16 views and still no advice. Come on, does anyone on here have a shop manual open or have any ideas?

[curiousB]

Missing sounds like ignition to me. I wouldn't be overly concerned about DC resistance readings of injectors. They are inductive devices so DC really only tells you if coil is shorted or open. Trying to discern an exact resistance measurement is not likely meaningful and unless you nulled your meter for the test leads it probably isn't a super accurate reading anyway.

I would not get excited about fuel pump voltage either. I would put more stock in fuel rail pressure reading than an electrical reading anyway. Also if fuel pump is working at all it is probably fine for idle anyway where it works the least (lowest flow/least mechanical load). A marginal fuel pump will starve the engine at higher engine loads. If fuel pump is a problem at idle it probably seized and not pumping anything which doesn't sound like your symptom.

So I'd look for arcing of HV wires to chassis as a possible problem (run engine when dark and look for arcing). Bad wires. Dirty or carbon deposits inside dist cap. Intermittent/faulty coil. Dirty plugs (arcing across ceramics spacer instead of gap). Or maybe misaligned or damaged crank position sensor. Bad/loose connector on primary side of coils.

The churning idle is likely a combination of dirty throttle plate and IACV. Take them apart and clean up with solvent for this purpose. Good general maintenance item to do anyway.

[hughes369]

Missing sounds like ignition to me. I wouldn't be overly concerned about DC resistance readings of injectors. They are inductive devices so DC really only tells you if coil is shorted or open. Trying to discern an exact resistance measurement is not likely meaningful and unless you nulled your meter for the test leads it probably isn't a super accurate reading anyway.

I would not get excited about fuel pump voltage either. I would put more stock in fuel rail pressure reading than an electrical reading anyway. Also if fuel pump is working at all it is probably fine for idle anyway where it works the least (lowest flow/least mechanical load). A marginal fuel pump will starve the engine at higher engine loads. If fuel pump is a problem at idle it probably seized and not pumping anything which doesn't sound like your symptom.

So I'd look for arcing of HV wires to chassis as a possible problem (run engine when dark and look for arcing). Bad wires. Dirty or carbon deposits inside dist cap. Intermittent/faulty coil. Dirty plugs (arcing across ceramics spacer instead of gap). Or maybe misaligned or damaged crank position sensor. Bad/loose connector on primary side of coils.

The churning idle is likely a combination of dirty throttle plate and IACV. Take them apart and clean up with solvent for this purpose. Good general maintenance item to do anyway.

Engine seems fine at idle, so maybe pump is ok. Plugs are new NGK, and i gapped to .045. Im gonna look at the engine while Dark, for arcing, and change the Fuel Filter Friday Night. Will post findings afterwards. iacv also seems like easy fix, since I have read all the posts on lexls and lextreme.

[fsuguy]

hughes369,

What specification data are you looking for from a manual? I have a '92 and both the manuals, so some of what you may need should probably be common, and I can get you that if you want it. Just let me know.

Also, you have not mentioned (or I missed it) whether you did anything with the "messed with" TPS. I had been having some acceleration issues with my '92, including cruise control not working, and most of them went away once I replaced with a new one.

BTW, your notes on what you did to fix your car are very informative - thanks for posting.

[hughes369]

hughes369,

What specification data are you looking for from a manual? I have a '92 and both the manuals, so some of what you may need should probably be common, and I can get you that if you want it. Just let me know.

Also, you have not mentioned (or I missed it) whether you did anything with the "messed with" TPS. I had been having some acceleration issues with my '92, including cruise control not working, and most of them went away once I replaced with a new one.

BTW, your notes on what you did to fix your car are very informative - thanks for posting.

I am about to try the Lexls.com tutorial on TPS adjustment. I just now finished with the Fuel Filter install, and about to put the dash cluster back in place since i just put the TACH needle back at Zero. I read on Lextreme how to fix the needle, and now can't find that thread again on exactly how to fix that needle. I fixed it from memory, should have saved that page as a book mark. I could have seen it on this forum, but it was late last night,and i can't remember. Also i have the IACV off the car, and taking it apart now. In an hour or so, I will be starting it up with this round of tricks being tried. Im crossing my fingers. I don't think the fuel filter was an issue, but its changed now, so that can be ruled out. But I am still running on the pump that came with this car and the coils as well. I had to give back all those parts to my daily driver (1992) and my Dad's 1991 LS 400...Ok, I will post results a little later tonight.

[Jerjer]

I had a very similar problem on my 91 I checked everything turned out to be a couple of bad plug wires.

Installed new NGK plugs and wires and all was as it should be.

[hughes369]

Idle is now fixed, larger bearing in AICV was frozen, used AEROKROIL/ (WD-40) type stuff to free it. Lubed the shaft with Petroleum jelly before putting the bearings and magnet back onto the jack-screw. ALSO ADJUSTED THE TPS SENSOR WITH THE HELP FROM LEXLS.COM. (I used a standard injector connector from an old Nissan to plug into the two bottom terminals on the TPS sensor. I had to cut off all the outer part of the injector connector to expose just the two female pins housed in the immediate surrounding plastic of the connector. Then was able to attach test leads to the modified injector connector for resistance measurements and continuity tests for the adjustment procedure. Later I went to a junk-yard and cut a TPS harness off a 1991 LS400, eventually will modify that harness for a more specialized TPS TEST HARNESS.) RPM Gauge now working, but may have bumped into the temp gauge needle some, so now its showing higher on temp by a 1/4 inch towards the Hot side. Engine still misses. Looking at the Wires tonight in the dark, but pretty much gonna buy some new wires, Iknow they are not good, from pulling them apart trying to change the spark plugs. So Sewell Lexus will get some more business from me. More posts to follow by the weekend.

[hughes369]

The 1991 LS400 is now fixed. I put a set of old OEM plug wires on it from my 1992 LS400, and now it has more power and drives alot better. Thanks for all the posts and I hope this post shows that cheapy plug wires that come apart when pulled on to change the spark plugs are not usuable on these cars, incase anyone ever buys a car with these crappy wires installed... Use OEM wires, even old OEM wires are better than the cheap ones.

Peace Out.