My 94 sc400 idles rough, engine shakes and then it will stall out. First I referred to the Lexus shop manual for items to check if the engine has a rough idle and then there is a separate column of items to check if the engine stalls.

Previously, I had an issue with the car stalling out after ten minutes of driving from a cold start. The engine would just cut off on the highway. But now after changing out quite a few parts (including a tune-up) the engine has a rough idle and will stall within a minute from a cold start. So either one of the parts I installed is bad, or the existing problem got worse.

The very first time I changed the spark plug wires on this car about 8 years ago one of the plug wires was improperly routed and interfered with the cam signal. Not sure how that's possible, but that's what Toyota said. The engine had a rough idle just like now, but I can't remember if it actually stalled out. But Toyota fixed it just by re-routing one of the plug wires. I've checked the plug wires and they all appear to be routed correctly and it doesn't look like any of them are crossing over another wire incorrectly. Does that make any sense?

I dont have any of the wires mismatched from the cylinders to the distributor caps. They are all correct. Just not sure whats causing the misfire.

Pulling off a single plug wire one at a time with the engine running isn't going to help me pinpoint where the problem is correct?

If I check for spark, do I just need to check to make sure there is a spark coming off each of the two coil plug wires, or should I check for spark for all ten plug wires?

Also, if I had a bad idle air control valve, I should be able to keep the car running by pressing on the accelerator right? If I can't keep the car running by pressing the gas pedal that should mean its not the iacv, correct? According to the lexus shop manual under rough idle is said it could be the idle air control valve as one of the causes for both rough idle and stalling issue. I understand how it could cause the stalling issue, just dont see how it could cause the engine to misfire causing the rough idle. I'm 90% sure I don't have a vacuum leak as well.

Also, the plug wires are only 3 years old (passed resistance test), new plugs, distributor caps & rotors, fuel filter, pcv valve.