Yamae

The only thing I can tell you is that the emission control system was changed in August 92.

Your current problem may be caused by the clogged throttle body including the IACV. I'd clean them first before I try others. Also I would change all the QAS capacitors as a preventive measure.

davespall

Thanks for the reply. I have already got my throttle body cleaned and i cleaned the IACV myself but still having low idle issues. I recently got the tune up done as well excelt the changing the timing belt and water pump i pretty much replaced all the stuff. I read alot forums talking abt fuel pump resistor and relay as well. So i am kind of thinking first replacing the ECU and then working on fuel pump.

fast325is

that was a very good write up hade same problem 92 ls400 dies at idle runs bad change out capasiters now runs very good thanks so much for the infor.

Yamae

Thanks for the report, fast325is. Another happy owner.
It would be helpful for those who are thinking of the capacitor change to inform them what were your problems other than the low idle and the bad running. Didn't you have any of following problems?

● Engine won't rev out/rev limits at 2000 or less rpm
● Hard to start
● Transmission shift problem
● Transmission jerks from 1st to 2nd gear, and kinda slips from 2nd to 3rd
● No A/C compressor operation or A/C going into reset with flashing front defog light (front defog light typically flashing 10 times)
● Speedometer not working reliably
● Cruise control not working
● Tachometer not working sometimes
● Air bag light flickering intermittently
● Check engine light on sometimes, but no codes stored, often in concert with ECT light
● ECU not communicating with scanners or not displaying codes with jumper installed
● Bad sulphur exhaust smell in concert with not running right above

davespall

Can anyone here provide me a video link to replace the ECU in a 1990 ls400? I would really appreciate it. I have tried almost eveything to fix the low idle issue but no luck.

davespall

the strange this is there is no check engine light and dats y i cnt figure it out. 😒 really in trouble guys help me out

89OLDMAX

First thank you for this thread, it has opened my eyes to a whole new level of auto repair. I have recapped nine computers so far 8 successful and 1 not. There are 2 capacitors I am having trouble finding a low esr replacement for on a 1992 Nissan maxima. One is a 1uf 50 volt and the other is a 10uf 63 volt.
The 1uf 50 volt I bought has an esr of 1.1 and I cannot find a 10uf 63 volt low esr. I have an esr meter and the 1uf is the actual measurement. P/N EEU-HD1H1ROB
Any help finding good replacements would great, thanks

davespall

hey guys, so after going through tuff time in diagonising my low idle and stalling issue i finally fixed it. First i went through replacing alot of stuff to fix the low idle, stalling, jumpy idle needle, jerk after driving 50mph in which includes tune up with new spark plugs, ignition wires, valve cover gasket, throttle body cleaning etc but got no luck so at the end i bought a used ECU from a 1992 and i drive a 1990 year. So i installed the ECU myself and i felt no jumpy idle needle, idle is stable at 650 +-50. No signs of stalling till now, no check engine light and i am assuming that idle low idle issue is gone. For IACV i cleaned it really good and one thing u gotta make sure is that the IACV should work smoothly and that you can make sure by moving the needle inside the IACV forward and backward by taking out the bearings, make sure they are moving freely as well. If its not working properly u will feel its not smooth and it will get stuck which will cause an engine to stall. If its working smoothly thn for sure problem is gone. Install it and check for engine codes if there is any. Also i noticed a chane in gas mileage. First while taking turns and going upgill my gas needle goes to go down like its taking a huge load but today the needle barely moved. I AM SO FAR HAPPY WITH THE PERFORMANCE AND IT WAS WORTH IT. If anybody have any questions please let me know.

Yamae

So the thread is read by non-Lexus/Toyota owners too. That may be another reason why the view count is counting up very fast these days.

Instead of using a 10uf 63 volt, I would use a 10uF 100V which can be purchased at e-bay below.
http://www.digikey.com/product-searc...C2A100&stock=1

Regarding a 1uf 50 volt, I don't think a 1992 Nissan maxima requires a low ESR one. Judging from the capacitance, it may not be used to reduce the ripple but to couple the signal. For that purpose a regular one can be used, I bet.

89OLDMAX

I have been working on my own cars and motorcycles as a hobby for 46 years. But since I retired 8 years ago I have been maintaining all the family vehicles and a few friends cars for a total of about 20 cars. Of these cars two are 1990 ls400's, one 1994 ls400, one 1995 sc400, one 1991 camry v6, one 1990 4runner v6 and seven 1989 to 1994 sohc nissan maximas. The rest are random other makes and models. As you can see I like 1980's and 1990's vehicles because they are not too complicated and I can keep them on the road. The lexus ls models brought me to this site and I really appreciate the knowledge you and lsCowboy have shared.
The 1uf 50 volt is in the maxima transmission computer and that explains that. The 10uf 63volt is in the maxima ecu. Toyota/Lexus evidently does not use that value as any of those that I have recapped only use 10uf 50volt, so thanks for the recommended replacement.
The one unsuccessful recapped computer I did was for a 94 ls400. I purchased a used computer on ebay from California. It had 7 of the 8 caps leaking and a lot of corrosion. 600mv ac ripple. I think it has traces that are corroded away as the capping did not help. After that experience though I agree with you that 100% of the QAS capacitors will leak and ruin the computers over time if not changed. I would have never believed that my car repair would include changing ecu capacitors. Thanks again for all the information.

Yamae

Congratulations on your success to fix 8 ECUs out of 9, 89OLDMAX.
So you have many Lexus/Toyata cars as well as Maximas. My daughter used to drive a 92 Maxima when we were in USA in the late 90s and early 2000s. So I know the car some.

It's my pleasure to help you to see some different new world. Modern cars have electronics circuits a lot and these require DIYers totally different skills. We definitely need to get used to those to fix cars these days. One of the major issues is to know that the electrolytic capacitors fail someday.

Electronic circuits use electrolytic capacitors and those will loose the capacitance in accordance to the Arrhenius equation. A standard 85 degrees C capacitor lasts only 2000 hours if used under 75 degrees C circumstances.

In addition to that, QAS capacitors tend to increase the strength of Alkali as aged and even the specially coated leads are corroded leaking the liquid. We must know the fact seriously.

Bill Dowd

Since I am unable to do these repairs I've found a company in Santa Ana CA Auto Electric Rebuilders that rebuild the ECU units for our Lexus and Toyota's at a very reasonable price starting about $200.00 their phone # 714-554-1574 ask for Lee. They also do fan motors for the front of our Lexus window motors etc. The dealer wants something like $1800.00 and above for these units the guarantee their work.

erroldo

Are the caps being replaced in the ECU part of filter(low/band pass) filters? or are they being used for just for CR discharge. I am replacing a 94 ECU caps and looking into solid state possibility.
I noticed Yamae mentioned bode response as critical, hence I am assuming these are all in filter circuits. Not expecting to see the ecu schematics as that would be Toyota's trade secret.

Nice details on the caps needed and links to buy them by LsCowboy. Very time saving for the rest of us.
Just purchased a 94 for US$450 with the engine disassembled but the body in perfect condition with no cracks in the interior. Figured I would make a hobby to see if I could bring the car back to life. After 2 weeks it started! rough idle and dead instrument display, so I started on the electrical system now. This site was very helpful. Replaced the LCD and my temp and time back up and running....

Yamae

Welcome, erroldo. You must be one of few people who understand the Bode plot. Most of those QAS capacitors in the ECU are used at voltage regulator circuits aiming to reduce the noise and ripples. Voltage regulators have a feed back loop and the gain with the phase rotation affects the stability. You must use capacitors that can reduce the noise and ripples well but not to cause any oscillation.

Those QAS capacitors in your 94 LS400 are too old to be good and simply replace them and you will be happy.

erroldo

I see. If they are in VR circuits, then u want as low parasitic resistance as possible(low ESR). The digi-keys are on their way next week. I can't wait to see how my tinkering finally turns out.
as for Bode charts, i did those up to graduate school. Now I write firmware for NAND devices.
Fun to be dealing with some analog circuitry again.
I have a 91 ls400 since 1993 and it never give any electrical problems. I may just replace the caps anyways, since they will likely start fail soon.