Strange idle issues with 4.8 (SOLVED)

[DarkNinjaSG]

Hello!

I have a 2000 Silverado 1500 with the 4.8 that has 140,000 miles on the engine. It’s been acting strange recently. If I drive it for over an hour at highway speeds or for over 30 minutes in stop and go traffic, the engine will start to idle at 1,000 RPM. If I shut the engine off and then immediately start it again it will still have a high idle, but if I shut off the engine and wait about 30 seconds and start it again it will drop back down to its normal idle (500-600 RPM). Also, during the high idle if I blip the throttle it will take over 10 seconds to drop back to idle. I have checked for vacuum leaks and have found none (checked intake manifold gaskets, PCV valve hose, all around the intake manifold, around the throttle body and IAC, and the intake tube and MAF sensor). I also cleaned the throttle body and IAC seat, but that doesn’t seem to have helped. I also checked the IAC and both coils read 52 ohms. I am stumped with this .

Any help would be appreciated. Thanks!

Edit: also, after waiting the 30 seconds and starting it again it almost stalled before catching itself and returning to a steady 500-600 RPM.

 

[99Sierra2500]

Hello!

I have a 2000 Silverado 1500 with the 4.8 that has 140,000 miles on the engine. It’s been acting strange recently. If I drive it for over an hour at highway speeds or for over 30 minutes in stop and go traffic, the engine will start to idle at 1,000 RPM. If I shut the engine off and then immediately start it again it will still have a high idle, but if I shut off the engine and wait about 30 seconds and start it again it will drop back down to its normal idle (500-600 RPM). Also, during the high idle if I blip the throttle it will take over 10 seconds to drop back to idle. I have checked for vacuum leaks and have found none (checked intake manifold gaskets, PCV valve hose, all around the intake manifold, around the throttle body and IAC, and the intake tube and MAF sensor). I also cleaned the throttle body and IAC seat, but that doesn’t seem to have helped. I also checked the IAC and both coils read 52 ohms. I am stumped with this .

Any help would be appreciated. Thanks!

Edit: also, after waiting the 30 seconds and starting it again it almost stalled before catching itself and returning to a steady 500-600 RPM.

Welcome! The action on the throttle cable could be sticking.

 

[DarkNinjaSG]

Welcome! The action on the throttle cable could be sticking.

Good idea, but when it happened the first time i made sure to rotate the throttle to make sure it hit the stop and it still had a high idle.

 

[99Sierra2500]

Then that makes me think IAC or vacuum leak.

 

[DarkNinjaSG]

Yeah I was thinking IAC just didn't want to drop $120 on something that might not fix it. I have no idea where the vacuum leak could cause I've checked everywhere the common ones are on, and on top of that any area I could think of.

 

[ORVietVet]

Before you throw any more money at this, clean the MAF, if you have not done so already. You said you checked at the MAF boot for leaks but did not say you cleaned it. You also did not say, unless I am blind, whether there is any CEL's. Sure sounds like an IAC problem but need to clean the MAF first. Do you have a scanner that will do live data?

Welcome to the forum from Oregon.

 

[99Sierra2500]

Yeah I was thinking IAC just didn't want to drop $120 on something that might not fix it. I have no idea where the vacuum leak could cause I've checked everywhere the common ones are on, and on top of that any area I could think of.

A search says on the IAC the terminals should test out at 12-20 ohms resistance. Yes, I agree, throwing parts at the problem is not good. Another thing to check is the fuel pressure.

 

[DarkNinjaSG]

Before you throw any more money at this, clean the MAF, if you have not done so already. You said you checked at the MAF boot for leaks but did not say you cleaned it. You also did not say, unless I am blind, whether there is any CEL's. Sure sounds like an IAC problem but need to clean the MAF first. Do you have a scanner that will do live data?

Welcome to the forum from Oregon.

I did clean the MAF with CRC MAF cleaner and it also throws the code P0507 as a pending code. The TPS shows 0% at idle and the upstream O2 sensors bounce between .1v and .9v like they're supposed to. The long term fuel trim for both banks usually stays around +3%.

Edit: yes i do have a scanner that can read live data

 

[DarkNinjaSG]

A search says on the IAC the terminals should test out at 12-20 ohms resistance. Yes, I agree, throwing parts at the problem is not good. Another thing to check is the fuel pressure.

I read that was something that could be a problem too, haven't got the tools to do it but will probably rent a tester from AutoZone or Oreilly's

 

[ORVietVet]

Well P0507 is all about the IAC and possible relation with the throttle body. Watch some you tube about that code and get some diagnostic steps for code P0507 and go from there.

 

[DarkNinjaSG]

Well P0507 is all about the IAC and possible relation with the throttle body. Watch some you tube about that code and get some diagnostic steps for code P0507 and go from there.

Sounds good. When I looked up code P0507 I did see it said it's a IAC code but that it could also be caused by other things. Thanks for the help!

 

[99Sierra2500]

Sounds good. When I looked up code P0507 I did see it said it's a IAC code but that it could also be caused by other things. Thanks for the help!

If you figure it out, tell us what it was.

 

[ORVietVet]

If you figure it out, tell us what it was.

Yes, and if you do find the fix, edit the thread title with...SOLVED.

 

[DarkNinjaSG]

Will do!

 

[ORVietVet]

In the future, if you have codes, post them in the beginning please. That alone eliminates some guessing.

 

[DateNight]

Buy a cheap IAC valve from Amazon. If it works buy a better quality unit. Throw that one in a box for troubleshooting. I have a big box full of different sensors for troubleshooting, comes in handy

 

[DarkNinjaSG]

In the future, if you have codes, post them in the beginning please. That alone eliminates some guessing.

Will do. I thought I did but I guess I forgot.

 

[Cadillacmak]

Welcome from Alaska. I too want to see what the fix is for this problem.

 

[DarkNinjaSG]

So 2 days ago I changed the IAC with a cheap one off Amazon and it seems to have fixed the high idle problem as I don’t get code P0507, but it still has an RPM hang in park/neutral if I blip the throttle. Also seems like it’s accelerating a lot more in lower RPMs since replacing it. I’m thinking that the problems are the still the IAC so I’m going to put a Delphi unit it. Also tried replacing the PCV valve the day before and that didn’t seem to do anything since the old was still rattling like it should.

 

[Cadillacmak]

So 2 days ago I changed the IAC with a cheap one off Amazon and it seems to have fixed the high idle problem as I don’t get code P0507, but it still has an RPM hang in park/neutral if I blip the throttle. Also seems like it’s accelerating a lot more in lower RPMs since replacing it. I’m thinking that the problems are the still the IAC so I’m going to put a Delphi unit it. Also tried replacing the PCV valve the day before and that didn’t seem to do anything since the old was still rattling like it should.

Thanks for posting the fix!