I need some help figuring out what's wrong with my 92 sc400.
I've been working on it since I purchased it.
Before I cleaned the ISC valve, when idling in parking, it felt as if it was missing (not too bad though). When cruising between 40-45 mph it sputters from time to time and when stopped at a light it sputters and dies from time to time.
So far I have replaced the following:
1- Maf sensor
2-Cam sensor
3- Crank sensor
4- Spark plugs and cables
5- Air filter
6- Both upstream o2 sensors
7- Performed the fuel pump ecu mod and no change.
8- poured seafom though vacuum line
9- Inspected the ecu and there's no evidence of damaged capacitors; in fact, it looks very good...
10- cleaned and tested the isc valve. I checked the resistance as per the shop manual and the numbers are all within range but when I put power to it as explained in the shop manual it clicks and moves a tiny bit. Is it supposed to move up and down significantly or is it just that tiny click??
Anyways, when I installed it the idle speed went up to approximately 1200. I removed it again and turned it by hand, reinstalled it and the rpms went down to 800 or so but it appears that my original issue persists... Could it be the ISC valve??? Is there a way to troubleshoot the ecu or should I check something else?
Any help is greatly appreciated..

 

Looks like you've tried everything except the likely culprit. (Hey, I went through much the same process.) Check or replace the ignition coils. The driver's side is supposedly the more likely one to be bad.

 

I disconnected the driver's side coil and the car won't start. Disconnected the top one and it started but ran terrible...

 

That's why I said check or replace. The tests were inconclusive for me, but replacing them solved all the problems.

 

Is there a way to test them other than disconnecting them?

 

IG-9 in the factory service manual has some tests you can run on them.

 

I'll Google it and see if I find it.

 

T2D2 AND Osanyin, I have been trying to track down this same issue. I have noticed that the symptons are greater when the ac is on. Did you experience this?

 

I don't remember AC making any obvious difference. Then again, I had a leaky AC system back then and early on had the knock sensor issue (actually an ECU issue) that flared up more frequently with the AC running, so it's possible I didn't thoroughly test the combination.

 

No, the ac doesn't have any effect in my case.
I was able to ruled out the idle valve today because it did exactly what is supposed to do. When the car was cold it brought up the idle speed and brought it back down when it warmed up.
I just ordered 2 new coil packs. As soon as I install them I'll report back.

 

It sounds like your ignition coils are fine. Peace of mind, I suppose. Throw a couple injector cleaners in the tank for more peace of mind.

 

The tank is loaded with sea foam and I also sea foamed the car through a vacuum line