Engine idles up and down, old owner say its converter?

[Suburban94]

Just bought 94 suburban with 175k and runs good but idles up and down when in park, never quits or comes close just sounds weird. Any ideas?

Also does a weird like scraping sound when drive forward or reverse for first 5 minutes of driving. Any ideas?

[Oldtruckfanatic]

In most cases an engine searching at idle has to do with the tps. make sure the ac compressor isn't kicking on and off in sequence with the idle going up and down too. I would definitely get it to a transmission shop and have the trans checked out...when I busted the sun shell in my 01 chevy it made a noise like that for a few weeks and eventually because I didn't get it checked out I had to buy a new trans I don't know exactly what finally happened but it wrecked the case so it wasn't a good core. I don't know that this is a problem for your truck and it may very well be a torque converter problem but definitely have a transmission shop or someone that knows trannys have a listen. Hope you get it worked out.

[jsbgump]

Welcome to the board

[Suburban94]

Thank you for the advice and I will do that. Really glad this site is here as I am a novice when it comes to fixing anything beyond having to use a hand tool as no garage or heavy lifting equipment for engines and such.

[Oldtruckfanatic]

Welcome to the board. There are tons of great people here with knowledge in just about anything that has to do with chevy trucks.

[oldyeller]

hunting or low idle could also be a stuck open egr valve. something to check, I just ran into this about 2 weeks ago.

[ChevyTech]

Welcome to the board.

Here are a couple more things to consider:
Check for vacuum leaks, especially the gasket under the TBI (throttle Body Injection) unit.
It is possible the IAC (Idle Air Control) is starting to fail.

Here is where the IAC is located on the rear corner of the TBI.

[gizmo1]

your IAC could be clogged with carbon , pull it off and clean it with carb cleaner also spray some in the opening where it mounts. wait a while for the cleaner to dispurse before starting your truck

[Suburban94]

Originally Posted by Oldtruckfanatic

In most cases an engine searching at idle has to do with the tps. make sure the ac compressor isn't kicking on and off in sequence with the idle going up and down too. I would definitely get it to a transmission shop and have the trans checked out...when I busted the sun shell in my 01 chevy it made a noise like that for a few weeks and eventually because I didn't get it checked out I had to buy a new trans I don't know exactly what finally happened but it wrecked the case so it wasn't a good core. I don't know that this is a problem for your truck and it may very well be a torque converter problem but definitely have a transmission shop or someone that knows trannys have a listen. Hope you get it worked out.

Took into tran shop and lools like I need a new catalytic converter as one I have has broken pieces flying around in it and that is the scraping sound as the circle piece inside has broke and that is also causeing engine to go up and down, not really an idle problem apparently, is that something I can replace easily and can I do the trans oil flush as well. I am not too mechanical but can do a lot o little things but never down either one of these. He quoted me $175 for cat replace and $130 for flush. What do you think?

[HotRodChevy]

Hello all. I have been having a similar problem with my 89 chevy. It runs great in gear, but idles up and down in park. Has a new engine. Have replaced computer, tbi, iac, fuel pressure regulator, all sensors, distributor, cap, wires. Have replaced everything I can think of. Have removed catalytic converter just to see if that was the problem. Still no change. I'm at my wit's end. Any ideas, suggestions, etc. would be greatly appreciated.

[ChevyTech]

Hi HotRod Chevy.
I see it's your first post, so welcome to the board.

If I were working on your truck the first thing I would do is get it up to operating temp and look at the IAC count with a scan tool. I would like to see it low but not zero. Off the top of my head I believe the desired range is a count of 1 - 50

Next I would look at the integrator and block learn counts to see where the fuel mixture is.

I would also look at the o2 sensor reading.

Do you have headers and a one wire o2 sensor that is running to cold at idle?

[HotRodChevy]

Originally Posted by ChevyTech

Hi HotRod Chevy.
I see it's your first post, so welcome to the board.

If I were working on your truck the first thing I would do is get it up to operating temp and look at the IAC count with a scan tool. I would like to see it low but not zero. Off the top of my head I believe the desired range is a count of 1 - 50

Next I would look at the integrator and block learn counts to see where the fuel mixture is.

I would also look at the o2 sensor reading.

Do you have headers and a one wire o2 sensor that is running to cold at idle?

Thanks. I don't have headers and no codes are present. O2 sensor is a one wire and is new. I don't have a scan tool. IAC is new also. What is an integrator and where is it located?

[ChevyTech]

What is an integrator and where is it located?

Block learn is long term or slow fuel trim – computer fuel adjustment currant setting – determines fuel injected
Integrator is fast or short term fuel trim – computer fuel adjustment currant setting - determines fuel injected.

HotRodChevy you have replaced so many parts, at this point it would be a good idea to find someone with a scan tool and experience using it to determine why the idle is not being controlled.

Here are some possible causes:
IAC sticking / binding or failed
IAC passages in the TBI plugging up
Lean mixture
Vacuum leak – check your hoses and TBI base gasket
Minimum air rate out of adjustment.
Bad information to the computer from a failing sensor.

With all HotRodChevy's new parts he has eliminated most of the possible causes.