Good Morning to most and Good Evening to some (holla night shift!):

I'm getting a high idle issue after my truck has been driven around for a while. It doesn't happen only after highway cruising as noted in the TSB - sometimes it will happen just running errands around town - but the TSB describes the same type of problem which is why I'm referencing it.

In the TSB, they basically have you check to make sure the cables & throttle body aren't binding or sticking, and as long as they're moving freely, they tell you to replace the IAC valve.

Does anyone know why they jump straight to IAC replacement and don't have you pursue circuit and component testing (assuming it's not just a case of bad service info)?

I'm curious because the test we all rely on - unplugging the IAC with the engine running - is a pass. When I unplug it (at operating temp), the engine starts stumbling, and when I plug it back in, the engine smooths out.

Additionally, if I shut the truck off and start it back up, the idle returns to normal, furthering my suspicions that it's an electronic issue instead of a mechanical one.

I'm having trouble finding more specific information, but the wiring diagram (per Alldata) shows the ECM provides both power and ground for the IAC. To me this suggests that, whether it's on the power or ground side, the ECM should be (for lack of a better term) pulsing the voltage to control the IAC's position. I'm not saying that it's using pulse width modulation - I just can't think of a better term for it. If that's the case, then I should expect to see this pulsing using a test light, correct? (Either replace the IAC in the circuit with my test light or splice into the circuit with my test light connected in parallel.)

Now, the part you've all been waiting for - the codes.

KOEO - 111 system pass
KOEO (CM) - 172, 452
KOER - 172, 411 & 412

The 411 & 412 codes are another reason I'm suspecting it's an electronic issue specifically related to the IAC circuit.

The O2 sensor code I haven't yet investigated, but I can see why it might have an effect on idle speed.

I'm also curious to know - which inputs does the ECM use to determine idle speed? Some of it's obvious, such as the TPS and ECT, but what are the other inputs it's using to set and adjust idle speed? I want to test said inputs as well to see if there's an issue there.

And finally, recent work that might have an effect on this. I can't give you exact details because I don't keep very good track of receipts, but here's a basic rundown:

-Approx 2 years ago replaced IAC b/c it failed the unplugging test.
-Approx 6 months ago I found 2 leaking caps on the ECM and had it professionally repaired at an electronics shop.
-Approx 3 months ago I replaced the ignition module. (It died w/out warning and testing showed no spark control from the module - pulled it off and had it tested at Oh-Really's and their test confirmed it was bad.) I made sure to set the base ignition timing since I had to rotate distributor to replace the module.

That's all I can tell you for now until I can get more testing done. Not sure when that will be as my time is very limited.

Thanks.

 

The power for the IAC valve solenoid does not come from the PCM Computer. It comes from the PCM Power relay.

The engine RPM sensor for the PCM is the PIP train from the distributor.

Most TSB's assume you have a brand new truck with all good wiring so they are missing leading for the most part. That is why we have sites like this one.

A test light is not fast enough to check pulse width modulation or on and off at different spacing. You need a scope for that. If the test light was fast enough your eyes are not.

 

Yeah, that's my bad, subford - I was trying to recall the diagram from memory this morning - the power feed from the relay splices off to the PCM & the IAC, which is why I thought both power and ground were fed from the PCM.

I think I'm going to rig up my meter to read the TPS while I'm driving. Then, the next time it starts revving, I can see for sure whether or not the throttle cables/body is binding or sticking. That will at least narrow down the possible issues.

Thanks.