Surging idle

[rhpete]

I've got a 1992 Wagon that intermittently surges at idle when first started. I don't think there's a real pattern of whether or not it's when it's hot or cold (engine and/or weather). We're talking anywhere from stall out r.p.m. to 1500. Maybe you could call it a oscillation or galloping.
The computer sets a # 45 Diagnostic Code. The factory shop manual suggests the following as possible causes: Fuel Vapor Pressure Control Valve, Fuel Vapor Cannister Purge Solenoid, Map Sensor, Coolant Temperature Sensor or Throttle Positioner Sensor.
Car runs great otherwise. Just a spooky problem at times (especially when trying to exit a Ferry Boat). Problem will go away if you turn the car off and then restart it.
Has anyone else seen this? Anyone have any suggestions as how to narrow down the possibilities (other than trial and error $$) ??

THX,

rhpete

[jcwren]

My GM code reader manual (1983 to 1995) says that a #45 is "Rich exhaust indicated (Left side on dual oxgygen models)". I'd tend to trust a shop manual more, though.

Mine does something sort of similiar, without the error code. Sometimes at startup, it will barely hold idle, or surges up and down between near stall and what the idle speed for the current engine temperature should be. I believe it is the IAC value (Input Air Control) sticking. According to a GM mechanic I cornered, he said they replaced hundreds of these things under warranty when the vehicles were new, and that'd be his first bet. Apparently, they're not difficult to clean or replace.

--jc

[rhpete]

To: jcwren

That's a pretty juicy tidbit of information. I'll try it. I'm at work
right now and don't have the manual with me but I'll find that gadget.
Yup, thinking back I think my idle probably surged to as high as the
temperature/computer called for as yours did. (The rest was probably
just some overshoot due to the surge effect)

Thank you,

rhpete

[jcwren]

I finally got around to replacing the IACV the other day. $28 from some local auto parts store. Engine now has what appears to be a correct fast idle when cold, settles down after it warms up. I haven't put a calibrated tach on it, but the dash tach says 900 RPM park, 750 RPM in drive. I'm been trying to locate what the idle RPM should be, but that feels approximately right for a V8.

Change out took all of 10 minutes, although I did break the retaining clip that keeps the connector in the IACV. Be very careful, that aged hot plastic gets brittle. To make the matter worse, the clip is facing down (at least on mine, but I imagine that's pretty consistent), and I snapped it as I was feeling around for it. It's not going to fall out without the clip, but I may wrap a small strip of aluminum tape around it to be sure.

--jc