As the title states, I fixed P0506 (low idle) with a new iac valve and cleaning the tb, ever since I have had P0507 (high idle) haunting me. All I notice is the engine will rev up to about 900rpm sometimes when I’m stopped at a red light, however this is only while I have the AC going so it’s normal for the engine to have a higher idle because it has more load on it? Other than that it idles at 600-700 rpm. I’ve taken out the new iac and reinstalled to make sure it was sealed nicely aswell as checked around for vacuum leaks. When I hook up my scan tool fuel trims are showing good (0% or small negative). If anyone has any ideas to what could be going on that would be much appreciated!

Edit 1: Added more information.

 

Have you looked at TPS values in live data to make sure they are stable and as expected both at idle and throughout the travel range?


George

 

Yes I had a look at the TPS data yesterday and it was showing normal values.

 

Is the high idle constant while the AC is on or only when the compressor disengages or engages?

Is there any freeze frame data with the high idle code?


George

 

Only when the compressor engages and disengages. My scanner didn’t show freeze frame data for the P0507, it only showed me freeze frame data from 0420 code. I’m going to take it for another drive this morning and try to get the code to come up again hopefully it will show the freeze frame too.

 

Sounds like poor IAC idle control with changing loads. Have you had any AC trouble? This could be caused by a compressor that requires high torque to run such as from overcharging or a failing compressor. If the throttle body is cleaned and the IAC is new then it could also be the control voltages and grounds to the IAC valve itself. One more easy thing to try is a new PCV valve.


George

 

I will checkout the PCV valve more later today. Just took it for a drive and couldn’t get any codes to come back. However I noticed that after driving it, just sitting it will idle around 800rpm. Then I shut off the truck and start it back up and it idles in the high 600’s or low 700’s.

 

Okay guys, done some thinking here. Checked around more. Cant find anything to be wrong, I’m really thinking of going back to where it all started, getting a genuine acdelco iac valve and see if that fixes it. I usually always get acdelco but when I replaced the iac my truck would not idle at all and I needed to get it back up and running so I settled with an aftermarket lordco one. Which was like $150 may I add so not cheap either. I’m now reading online some people having problems just like me with aftermarket iac valves and high idle.

 

Since your still in the weeds on this, here is the bigger picture:

In a professional repair environment, the tech would test the IAC system with scanner commands and measurements but most of us cannot do that. The control circuit is a bi-directional DC stepper motor control so measuring the commanded values wont mean much to the shade tree mechanic. You can however disconnect the wiring at both ends and measure continuity and check for shorts to ground and power and inspect all connectors.

After that other suspects are:

Throttle body and plate
Vacuum leaks
PVC valve
Throttle linkage
TPS wiring, pwr/gnd, and sensor values throughout range
Any other unmetered air
Throttle set screw
MAF sensor, O2 sensors
Loads straining the IAC control (AC compressor, alternator, PS pump, etc)
PS pump switch if so equipped
The IAC itself as you have mentioned. If the new IAC did not change anything at all then its probably not the IAC (new or old).


Good luck,

George

 

Thanks a lot George, my scanner does not show me the iac counts which would make it easier to see what’s going on. I’m going to look into a few of those things you have mentioned that I haven’t already to make sure everything is in good order. Replacing the iac valve fixed the P0506 code and made it idle “normal” again. Then ever since replacing the iac valve I have had P0507.