95 k1500 350tbi High Idle issues

I have been pulling what hair I have left out over this issue.

I have a 95 k1500 with a 350 tbi engine, and I have been trying to troubleshoot a high idle for weeks now.
So far I have
-replaced the IAC 4 different times (although none of the motors were actually frozen and they all moved)
-performed countless IAC relearns
-checked map vacuum line for leaks
-checked egr vacuum line for leaks
-checked pcv vacuum line for leaks
-checked evap vacuum line for lines
-sprayed down the egr valve base along the intake manifold for leaks with carb cleaner, no leaks detected
-removed the throttle body and replaced the gaskets

The truck is able to move the IAC valve, it just doesnt appear to be seating all the way..Even with my scan tool reporting back 0 counts at the IAC... I can still feel air going through the IAC passage. I have cleaned the IAC passage and the seating area is clean of debris and carbon.

Theres probably some other things I have tried but this sums it up.
Other things worth mention perhaps:
-Truck has had an intake manifold gasket replaced within 75k or so.
-Over the last few months truck has been experiencing a loss of power. Probably couldnt break the wheels loose if you floored it. However it sounds like it runs smooth at idle
-Replaced Cap, Rotor, Plugs (withing 2 weeks), and wires within the last year.
-Truck sounds like its running smooth

Any Help is appreciated

I might be way off base here but it's another thing to check. My 90 Sierra 5.7l TBI was doing the same high idle thing. No codes were set unless I drove on the highway for at least 5 mins. Then my CEL would come on. I replaced the EGR and the 02 sensor and no more problems. I know you said you checked the EGR valve. Is it possible the 02 sensor is bad? As I said mine would only show a code after 5 mins on the highway.

Eh did the o2 sensor less than a year ago, I can check it's waveform looks good.
I'm getting ready to replace the ECM... out of ideas

Check the TPS - Throttle Position Sensor if you haven't already. Sounds like that might be it. I had that happen to one of my trucks. High idle caused by bad TPS

sntrym said:

Check the TPS - Throttle Position Sensor if you haven't already. Sounds like that might be it. I had that happen to one of my trucks. High idle caused by bad TPS

Click to expand...

This fixed my high idle problem. Should be .5V at idle on your scanner

I'll check it, previously I had just been checking TPS % on the scanner... It was reading 0% or 1% at idle. I'll check the voltage

Check brake booster? When they break they hiss.

Just my experience on my 1995 350 TBI:

I recently replaced my IAC. The plastic lock tab on my wiring harness was broken off so I couldn't use that as an index to plug the wires into IAC. I plugged it in with what I thought was correct.

After IAC relearn process, my in gear idle was 1200RPM, out of gear idle 1800RPM.

Turns out my wiring harness plug in was wrong/backwards. I found internet pictures of how to correctly plug in the wiring harness to IAC. My in gear idle is 700RPM, out of gear 900-1000RPM now.



EGR at idle should be closed and not letting any exhaust into intake at idle. If the EGR is stuck open at idle, exhaust gas will be entering intake and causing idle stumble, stalling, low RPM issues. Not high RPM issues.



At least in my truck, TPS is ALWAYS suspect. I've only experienced stumbles and hard shifts from TPS. BUT, if your TPS is a economy brand, get the AC Delco PRO version -- $28. Before the AC Delco TPS, I was always having weird shift and surge issues. I blame cheap TPS. After AC Delco TPS, no issue and truck shifts so nice.

My vote is for TPS also. Don't get a cheap one because it probably won't work right and will just confuse you more. Been there

Currently I have the original tps sensor.
At idle it reads .58 volts

As a test i took 1 of my 5 iac sensors and i pulled the pintle all the way out. I then installed the IAC such so that i could feel the pintle press against its seat inside of the throttle body bore. I then left the harness unplugged and tried to start the truck. I was able to start it but it (as expected) died out, this verified no vacuum leaks.

I did also break the harness connector plug, I believe I do have in connected correctly as per this thread. http://www.gmt400.com/threads/iac-wiring.34025/

And the fact that in order to fully seat the connector, the internal 4 barrels are keyed as to only allow it to seat completely one way.