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'92 LS400, Maybe it's haunted ?


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1992 LS400, 180k no traction control

I thought I had this fixed, but I don't. At this point, I suspect a demonic possession. Here goes.

Start car cold it runs fine. Take a trip of 10 miles or less and it runs fine. Then, let it sit on idle for 20-30 minutes and the possession begins. Idle begins to cycle from 800-900 down to 100 and back. Stepping on gas has no effect. Pumping gas pretty much has no effect. Turn car off and restart it with gas pedal half way to floor and it starts right up running 3000-4000 rpm. Let it drop back to idle and you lose control of the engine again and it just starts cycling. No codes.

Unplugged MAF sensor during engine cycling, engine dies, throws a code, so coding seems to be working..

What I have done so far.

Started simple – replaced air cleaner, it just needed it.

Replaced one suspected coil, same one that failed last time I had an issue but it was NOT like this, but what the heck, I had a spare. – no change at all

Unplugged IACV electrically and plugged air hose to it – This is what I thought fixed it but seems I did not run the car long enough.  First long trip and it started again.

Plugged vac line to EGR valve – no change.

Put timing light on plugs (both coils) and watched during these episodes – appears to be good spark.

3 – 4 cans of dry gas in gas tank – no change.

Replaced ECU computer under dash on passenger side with an identical one from wreck at junk yard – no difference at all.

Next step – replace throttle position sensor.

Any other thoughts would be greatly appreciated.

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The caps in your ecu might be bad and the junkyard one could also be bad. This seems to be a common issue with the early 400s as they age. Check this thread out and determine if this is the road you want to go down.


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Thanks.  I understand and after reading the text I agree it is very probable that the capacitors are the issue.  But I have to point out that I have disassembled both ECU's and visually, there is not even one blown capacitor.  I'm not sure if it was mentioned in the other thread but the larger capacitors not only leak, they will bubble the scored tops, many times before they leak.  However, I trust you guys a lot so I have ordered all the capacitors and will replace them on one of the 2 ECUs and let you know what happens.  Thanks again.

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Ah...I'm all done soldering and I have 2 extra capacitors ????????  Is the list for an LS400 '92 possibly the max I might have needed?

And now the bad news.  It's still cycling with the new Throttle sensor and all the capacitors changed and now the AC compressor or clutch seems to have locked up.   Not good.


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Thanks, I;ll tell u where I am now.  I completely removed the IACV from the picture by plugging the air line to it and electrically disconnecting it and resetting the idle screw to 1000 rpm.  Still does it.  I reread the original capacitor post top to bottom and saw something that might be a problem,  someone mentioned these are multilayer boards and should be soldered from both sides.  I did not do that.  So I pulled it apart again and resoldered with significantly more solder top and bottom.  My plan is to test it again tomorrow.  I;ll post here what happens.  As a side question, are there any posts on how to rebuild a locked up AC compressor?  This is number 4 and it's really getting old 😞

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  • 3 weeks later...

Ok, sorry it took so long to get back to you but a lot of things have happened.  First, I bought a second set of capacitors and soldered them on my junk yard ECU board.  Now that I knew what I was doing this job came out much better.  Sure enough, my stranger things are gone.  Unfortunately, my AC compressor clutch decided to lock up.  Smoked the fan belt.  Replaced it and now i'm losing freon some place. 2 cans overnight.   Maybe the clutch destroyed the seal in the front of the compressor ?  I hope not.  Know of any good AC stop leak?  I really don't want to be replacing the compressor again.  This would be the 4th  one.

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