2001 - Silverado 2500 - high RPM

[6872xtc]

Not that you will find this very helpful, but my '01 truck has the same knock sensor code the comes and goes. It has new Delphi sensors, torqued to spec, and a new Dorman harness. It sets this code with 3 different ECMs. It doesn't cause any idle problems.

 

[dyeager535]

Lol its def not -38 in the South. The Fuel Injection Idle Air Control Valve gets replaced sometime tomorrow or this weekend.

Idle air control controls idle speed. If the PCM is getting data saying the engine is cold, I fail to see why the IAC would be suspected or replaced. Especially given the quality of components nowadays. The IAC is probably doing exactly what it's programmed to do with a -38* engine temp.

The way too cold temperature is a known problem. Fix what you know is wrong first, then worry about the what-ifs.

 

[ZooMad75]

Idle air control controls idle speed. If the PCM is getting data saying the engine is cold, I fail to see why the IAC would be suspected or replaced. Especially given the quality of components nowadays. The IAC is probably doing exactly what it's programmed to do with a -38* engine temp.

The way too cold temperature is a known problem. Fix what you know is wrong first, then worry about the what-ifs.

The thing is the IAC does not normally have the ability to force idle rpm to 1500, 2000, 2500 or higher.

Watching my IAC data when mine was acting up with 2000 or better when I came to a stop with the clutch in. Even though the rpm was way up there the IAC was commanded to 0. So the pcm was trying to slow it down. Upon further inspection the broken o-ring was allowing a vacuum leak.

I’ve started my truck in sub-freezing weather and with a IAC that is not leaking and working correctly and it has never had an idle higher than 1000 rpm. Having one bounce to higher rpm’s is not normal and not caused by the IAT sensor not reading correctly. The pcm is looking at coolant temp as a primary reading if the engine is up to temp not the IAT.

 

[dyeager535]

The thing is the IAC does not normally have the ability to force idle rpm to 1500, 2000, 2500 or higher.

Watching my IAC data when mine was acting up with 2000 or better when I came to a stop with the clutch in. Even though the rpm was way up there the IAC was commanded to 0. So the pcm was trying to slow it down. Upon further inspection the broken o-ring was allowing a vacuum leak.

I’ve started my truck in sub-freezing weather and with a IAC that is not leaking and working correctly and it has never had an idle higher than 1000 rpm. Having one bounce to higher rpm’s is not normal and not caused by the IAT sensor not reading correctly. The pcm is looking at coolant temp as a primary reading if the engine is up to temp not the IAT.

I understand, but -38* is incorrect. That circuit has a problem. Regardless if it's THE problem, there is something wrong causing that reading. It may or may not be tied together, but how would you know until you resolve that issue? Even if the IAC can or cannot force that high of an idle, what's to show the IAC is needing replacement? Just assume it COULD force the idle that high. Is there anything yet proving it needs replaced, vs a circuit showing -38* when it's definitely not that temp?

People start down the road of throwing parts at problems they don't understand, and spending thousands of dollars on (questionable quality) parts, which ultimately don't fix the problem. If there is logic to replacing the part, even if wrong, at least that is something. But to replace the IAC here?

 

[fear_nothing]

Iac replaced to no effect. I did however turn this screw down and that seems to have solved it. Runs great now. Idles at normal rpm.

 

[Blue85]

So the cruise control was holding the throttle open?

 

[ZooMad75]

I understand, but -38* is incorrect. That circuit has a problem. Regardless if it's THE problem, there is something wrong causing that reading. It may or may not be tied together, but how would you know until you resolve that issue? Even if the IAC can or cannot force that high of an idle, what's to show the IAC is needing replacement? Just assume it COULD force the idle that high. Is there anything yet proving it needs replaced, vs a circuit showing -38* when it's definitely not that temp?

People start down the road of throwing parts at problems they don't understand, and spending thousands of dollars on (questionable quality) parts, which ultimately don't fix the problem. If there is logic to replacing the part, even if wrong, at least that is something. But to replace the IAC here?

I don’t disagree that the -38 IAT reading is not right. But I’ll tell you this with the crew I had working for me it was pretty common to leave an IAT unplugged after checking the condition of the air filter. It would kick a code and a check engine light but never did it cause the idle to fluctuate ever. Didn’t matter if it was a cable throttle or dbw setup.

If you look back at my previous post on the iat reading being off I did explain why it would do that. An open on the signal circuit by a cut wire or unplugged sensor would simulate the highest resistance possible to the ecm and cause the value go low temp. It was noted to check the connector and look at the wiring leading to it. But knowing it’s not the cause to the high idle it was lower priority to check out.

It’s interesting the cruise cable was hanging up.