Low idle when put into park

[Bigbertha78]

2004 Tahoe Z71, idles perfect, drives great, until stopped and put into park. The Tahoe's RPM's drop down to around 350, but slowly raises back to normal. No issues on starting the Tahoe, Throttle body was recently cleaned and new battery was also installed. Also, no engine lights or any codes to read. Trying to figure this out before we are stranded, currently the only vehicle we have as we have evacuated due to hurricane, so tools are limited. Thanks

 

[Doubeleive]

might try putting in some bottles of injector cleaner or consider a injector cleaning service some places have a tool that connects right to the fuel rail and runs the cleaner thru the injectors. if you just do the bottles it might take a couple 2-3 tanks of gas ran thru before you notice any difference. could still be the TB even if it was cleaned, you might also want to check the fuel pressure the little tool kit should either be cheap or available as a rental from autozone oreilly's etc.

 

[Bigbertha78]

Copy that, it's worth a try, thanks! Would it be an injector/fuel issue going from drive to park? Could it be a transmission part?

 

[Bigbertha78]

Tried the fuel additive with a few tanks of fuel and no fix! We have driven over 1500 miles, Tahoe does great until we have to stop or slow down, RPM's drop almost to 200 and truck almost stalls. Not fun driving through Denver at rush hour. Hit the gas and it's good to go. Stopped by Autozone to check the alternator just in case, battery and alternator are good, no codes, and no sounds of a vacuum leak or misfire. We still have about 800 miles to go, any ideas on what this issue could be?

 

[SnowDrifter]

It's possible it could be a sticky/non-functioning IAC. It's a cylindrical bit that attaches to the top of the throttle body. How thorough was the cleaning?

Pop it out, give everything a thorough cleaning.

If you could find a pinout, you could check:

1. Power supplied from the PCM
2. Resistance of the controller itself - shorted or open circuit
3. I *think* mechanical actuation

 

[Bigbertha78]

Removed the throttle body and gave it a good cleaning, have to see how it does afterwards, here are a few pictures of after it was removed!

 

[Bigbertha78]

Even after the intense cleaning on the throttle body I drive home and as I’m slowing down the RPM’s drop and almost stall! They go back up and as I put Tahoe in Park, the RPM’s drop again! Any thoughts?

 

[Doubeleive]

unless you want to drop $85 on a diagnostic I would maybe just slap another throttle body on and see, even a used one from the junk yard, there not terribly expensive. on the older throttle bodys you could just change the IAC but I don't think you can on these drive by wire.

 

[OneofFew]

Connect a scan tool. Engine load should drop when going into park, raise with a/c on, raise with electrical load on alternator. TPS could be reading wrong, so look at it also. Usually the RPM will raise when engaged in park... IAC may not be working correctly. Also check for vacuum leaks as that is the no1 cause for idle issues.

 

[Bigbertha78]

Already checked for vacuum leaks and the truck is good. When I removed the throttle body for cleaning, I noticed when you move it there was a noise inside the black cover which I'm guessing is normal but maybe the next thing to look into. Is there a way to swap out just that or will I have to replace the whole throttle body?

 

[Bigbertha78]

Cleaned out the MAF sensor finally and it was kinda dirty, replaced the air filter just Incase also. Worked for a few days and then of course back to the same issue again. Talked with the Chevrolet dealer here and he said try disconnecting the MAF sensor while the engine is running and if the RPM's rise then it's bad. Tried that and there was no change. Any thing else I could try, maybe a way to test the Throttle body?

 

[Doubeleive]

if cleaning the maf sensor fixed the issue even temporarily then I would just replace it, even if unplugging it didn't change the rpm that really doesn't mean anything it could still be bad. the maf's can cause all kinds of issue's even transmission shifting problems

 

[Bigbertha78]

Doubeleive, do you think the catalytic converters could be bad or even the O2 sensors? If so how would I test them!

 

[OneofFew]

O2 sensors rarely fail. Cats neither. Both are susceptible mostly to fouling. A cat can be tested with an IR gun. It should be significantly hotter at the discharge end.

 

[Doubeleive]

Doubeleive, do you think the catalytic converters could be bad or even the O2 sensors? If so how would I test them!

you would be getting codes, I say pick up a maf sensor put it in real careful and see what it does, if it doesn't fix the problem take it back and say it turned out to be something else. or call around any local junkyards and pick up a used one cheap