Starting a new thread as I still have not solved the surging idle issue on my 93 ls400, 125k miles. I have checked for vacuum leaks with starting fluid and can find none. On startup idles smooth but a little fast,900 rpm, as it warms up the idle increases to 1100 rpm in drive. When hot the idle surges when in park between 1100 and 1800 rpm. At 1800 the idle is a little rough but smooth at 1100. I assume the surge is caused by the computer telling the IACV to adjust the idle. Does this also mean that the IACV is probably not the fault because it is moving back and forth, which is causing the surge? The IACV is a stepper motor that receives a signal from the computer and can adjust the idle speed in increments up or down. Is their a defect in the IACV that could cause it to move back and forth contrary to correct input from the computer? What sensor informs the computer of the idle speed? If that sensor is defective would that cause the computer to ask the IACV to constantly change the idle speed? Other people on this site have stated this is a common problem that defies solution. All problems have a solution and I am trying to eliminate things that are probably not the cause especially things that can be checked without throwing parts at it.

 

Your logic sounds good to me.

Would simply disconnecting the electrical connector to the IACV at some point (like at a particular phase during warm up) be a good test? Once disconnected, the ECU should not be able to command changes in valve position, so any fluctuations in idle after that would need to be explained by something else. Helpful in debugging?

Here's a counterpoint example to those who think this might defy a solution:

Looks pretty brilliant to me, but I can't hardly understand a word he's saying, and I'm originally from England.

 

I disconnected the battery for a few minutes, restarted the car and the idle seems ok. It had been idling high until it started to surge when hot. Now it seems fine. I'll drive it some and see if it behaves.

 

I don't see any way to disconnect the IACV without removing it.

 

Ran perfect for a few miles then returned to high idle and surging. While idling (high) I disconnected the AF connector and the engine stalled. I restarted the engine and it ran with a low idle, 500 rpm, reconnected the AF connector and the idle returned to normal.

 

AF ? and my message is too short.

 

I think Air Flow or Air Mass Sensor

 

Picked up some wrecking yard parts, AFM, and ECU, no way of testing them but when I tried them in the car nothing changed, idle was still high and surging. I then removed the large air supply hose from the IACV (3/4 inch hose) and the idle slowed down. Why would that happen? If the IACV hose is removed the only difference is that the IACV is getting unfiltered air and the AFM is not involved. I then squirted throttlebody cleaner into the IACV and ran it at 2500 rpm for a minute. When I reconnected the IACV hose the idle slowed down to normal. I guess next step is to take it for a test drive.