Seemingly random, intermittent high idle?

[JustinJ94FSB]

Firstly, i viewed the threads that came up as suggestions when naming this thread. They did not provide any answers for my issue.
Secondly, To avoid a massive wall of TLDR text, my previous post that i edited the title to say *Solved* provides backstory on things ive done prior when solving other problems.
basically i replaced intake gasket, and some "while im in here" stuff. nearly everything GM genuine or delco original equipment. The low idle, stalling, stinky exhaust solved with new gm genuine EGR. i thought she was fixed!

--New Problem:

After getting new egr in, truck started, idled, and runs/drives absolutely perfect. Finally! I took it out for a drive. Maybe 150 miles round trip. It performed perfect the entire time (idle, driving, etc) After a hour or 2 i took a pit stop at a store to pee/get a drink. I come back out and start the truck and it fires up basically immediately. kind of just thinking, huh that was fast. As im sitting there looking at google maps determining which way i want to drive to go back towards home, the idle decides to go higher and higher up to about 1200-1500 on the factory tach. Im like what the heck? so i shut it off, and restart it and it sounds a little odd when it starts, and goes back to high idle. So i shut it back off. About 10 seconds later i start it back up and it idles perfectly normal. I tap the gas, it revs normal and drops back like it should and idles perfectly around 650ish. I decide to start driving towards home incase im going to have a problem. It drives perfectly fine all the way home, at least 50 miles with coming to intersections and stop lights , on and off of highways etc. I get home and park outside my house and it is idling perfect. I shut it off. Just for the hell of it, after about 20 seconds, i start it back up and it idles normal but within about 5 seconds it decides to just shoot up to 1200-1500 idle and i wait to see if it will change but it doesnt. So i turn the engine off, i wait 10 seconds and start it back up and it is idling perfect again around 650. It does this for like almost 10 minutes and i give up, shut it down, and call it a night. I Started it up today and drove it into town to get gas and it started, idled, and ran/drove flawless all the way to town. When the tank is full, and gas cap back on, i start the truck and it immediately goes up to 1500ish and idles there. I shut it off, wait 5 seconds start it up again and same thing. So i put it in drive and it drops to probably 750-900, put it in neutral and it goes back to 1500. I put back in drive, roll out of the parking lot under its current idle power, and baby it to the next intersection, i put it in neutral while sitting at the light and its idling normal again.

i cannot seem to pinpoint any rhyme or reason to it. It sometimes wants to shoot up to high idle and just sit there forever, or sometimes it idles fine forever. its sporadic behavior, does not currently seem to be related to open or closed loop but i think the highest idle speeds it reached were when it was full temp. i will try to start it again one of these mornings and let it idle up to operating temp to see if it does it during that process.

I've poked/wiggled various wires and ground straps (the one from ECT to thermostat housing, the braided ground straps from engine to frame, even the connector to the ECM, the column mounted ignition switch, and others. Ive attached a vinyl tube to a propane bottle with a valve, and ive opened it and ran the tube all over the place trying to see if i can find vacuum leaks. ive actually checked for vacuum leaks probably 10 times now since ive been fixing things on this truck since doing the intake gaskets. Ive never once found a vacuum leak on this truck and with every intake related gasket replaced it makes sense. All of the hoses (pcv, map, evap, egr, etc) I have a feeling it is electrical either a bad ground somewhere thats changing with heat and becoming weak, or a sensor that is diagnosing fine but isnt actually working, or maybe the IAC passage is dirty and my new IAC sometimes the pintle cant reach closed position? i have no idea.

send help! lol

 

[Schurkey]

WHAT VEHICLE? WHAT ENGINE?

Connect a scan tool, verify EVERY sensor, and the computer outputs--IAC especially, spark advance, etc.

 

[JustinJ94FSB]

ah my bad, 1994 k1500 blazer 5.7 tbi auto trans, do you recommend a scan tool for the obd1, I only have a obd2 scan tool.

replaced IAC, ECT, coil, cap, rotor, plugs, fuel filter, air filter, catalytic converter, egr, tb to intake gasket, intake gasket, oil pressure switch, cleaned ECT/TPS ground, cleaned inside/outside of throttle body, recorded injector pattern in slow motion (looks great visually), checked for vacuum leaks using propane through a vinyl tube many times now.

 

[Schurkey]

do you recommend a scan tool for the obd1

Several existing threads detailing scan tool recommendations.

replaced IAC, ECT, coil, cap, rotor, plugs, fuel filter, air filter, catalytic converter, egr, tb to intake gasket, intake gasket, oil pressure switch, cleaned ECT/TPS ground, cleaned inside/outside of throttle body, recorded injector pattern in slow motion (looks great visually), checked for vacuum leaks using propane through a vinyl tube many times now.

Inspected plug wires?

Initial timing? Timing advance?

Checked EGR system for operation?

Is the IAC being properly commanded by the computer?

 

[movietvet]

@Schurkey beat me to it. I was wondering what vehicle/engine we are dealing with. Best to put all that info in your signature for the future.

Two things come to mind: The IAC valve passage carbon build up and the coolant temp sensor, for the computer, not the dash gauge.

I actually had the same problem on a customer's vehicle and turned out the IAC passage was filthy. Ended up replacing the entire throttle body.

 

[Keeper]

coolant system needs topped off or burped a time or two?

 

[DerekTheGreat]

Get that scan tool. Being that it can idle low, I suspect the TPS sensor. The throttle shaft bushings can wear too, causing it to bind up and not return all the way as well. Starting with a fully warmed up engine and said engine not running, connect your scan tool to it. Take note of the initial TPS voltage reading. Start it and give it thirty seconds or so to let it come off high idle, paying attention to the IAC counts. If they hit 0 and it's still idling high, that's an issue to investigate. If the TPS voltage reading fluctuates at all, that's an issue to investigate. Blip the throttle, if the TPS voltage reading is higher or lower than initial, that's an issue to investigate.

 

[Schurkey]

@Schurkey beat me to it. I was wondering what vehicle/engine we are dealing with. Best to put all that info in your signature for the future.

If it's in the signature, we get into trouble when they're actually asking about a second or third vehicle. And plenty of folks ignore the signature anyway.

I'd prefer it to be in the first post, main text so that there's no ambiguity.

 

[JustinJ94FSB]

Two things come to mind: The IAC valve passage carbon build up and the coolant temp sensor, for the computer, not the dash gauge.

I actually had the same problem on a customer's vehicle and turned out the IAC passage was filthy. Ended up replacing the entire throttle body.

this is something ive seen a few times posted when people are talking about issues similar to mine, so i will check this out again for sure. When i cleaned the TB, i sprayed it with TB cleaner with it removed from the intake. I sprayed it down good, probably 2 cans of cleaner. Basically sprayed it down and used a toothbrush and such and got that baby shined up nice. I remember when i had it upside down there was a specific passage that was exceptionally filthy, but i dont think it was the IAC, and i did not remove the IAC when i was doing this cleaning. So I have added this to my list for after christmas when things slow down. will report back

 

[JustinJ94FSB]

Get that scan tool. Being that it can idle low, I suspect the TPS sensor. The throttle shaft bushings can wear too, causing it to bind up and not return all the way as well. Starting with a fully warmed up engine and said engine not running, connect your scan tool to it. Take note of the initial TPS voltage reading. Start it and give it thirty seconds or so to let it come off high idle, paying attention to the IAC counts. If they hit 0 and it's still idling high, that's an issue to investigate. If the TPS voltage reading fluctuates at all, that's an issue to investigate. Blip the throttle, if the TPS voltage reading is higher or lower than initial, that's an issue to investigate.

i have done some multimeter testing of the TPS while mounted to the TB and moving the throttle plates manually but this was with a digital HF multi meter, so i might first try with an analog one. I also did test for voltage at the connector with ignition on, and also pulled the TPS and tested for resistance while rotating it manually with a screw driver basically. I will look into the actual scan tool, i'd love to have one. or even a connector for the laptop with some software, i'd be happy with just about anything for live data, so i will check out the threads on that. I have already read through some of them and have been looking online here and there, just have been busy with the holidays and work etc. i have a week off after christmas so i can finally take some of these suggestions and hopefully get this last issue worked out.