SPFI pulsing idle speed, code 24

[msteel]

My SPFI has recently started doing this thing again, and I didn't find anything too helpful in my searches. Here's what it does:

 

* Idle speed will go back and forth between about 1000 and 1250 about every second.

* This has happened occasionally for a long time, but recently it happens more often, maybe once in every 15 or 30 trips.

* If I don't shut it off, it will continue to pulse. The CEL will eventually come on, but even before the CEL comes on the ECU flashes a code 24 (air control valve).

* The code 24 is not stored, just displayed while the engine is running.

* If I start the engine back up it is fine.

* This usually happens at the end of a short trip when I'm ready to shut the car off.

* Most often the short trip started with a cold engine.

 

* I swapped in an IAC/solenoid assembly that were last known good, tested good, and didn't look too dirty. No change (except it hesitated really bad until I reconnected the PCV hose to the intake boot...)

 

Is the Code 24 really pointing me the way, or is the ECU just guessing about what the problem really is?

[Gloyale]

Look for vacuum leaks. If there is one, the ECU may not be able to get the idle to steady using the IAC. This could throw the code.

 

Altough my first thought i just check all the connectors and wiring, and test the IAC. Hoofbeats ussually mean horses, not Zebras.