Up and down idle after intake manifold gasket replacement

[Fixture]

Thanks SnowDrifter. I might just go out and get a new IAC and then take the TB off and really clean it good. Thanks again for the assitance all and I will report back with what I find.

Fixture
aka -Jonathan

 

[Tonyrodz]

Thanks SnowDrifter. I might just go out and get a new IAC and then take the TB off and really clean it good. Thanks again for the assitance all and I will report back with what I find.

Fixture
aka -Jonathan

I needed an iac for my LS equipped Express van and no parts stores even list it. Even the dealer doesn't sell it by itself. I got mine from Amazon, and 1 was bad out of the box. It still doesn't idle right either.

 

[afpj]

If I'm reading that data correctly op was in open loop with engine warmed up (199)?

 

[Fixture]

I took off the TB and gave it another good cleaning. I took the IAC out and it seemed to be out more than it should. I could see the spring. So i pushed it in a little and it stayed. It does appear that it is getting stuck some how internally. I put everything back together and fired it up. The idle is higher that it should, but no more bouncing around. It fired up and its staying at 900 rock solid. So it does appear to be the IAC is getting stuck some how. I ordered another IAC and that should do the trick. Once its installed I will post my results.

Thanks all again for the assistance.

 

[ScottyBoy]

I'm going to have to check the same things on my son's 99 Silverado. When he first starts the engine, it's doing EXACTLY like in your video. He just replaced all four oxygen sensors, and he still keeps getting a P0171 code, which basically means it's running lean. I'm going to have to put my scanner on it and check the fuel trims like Scottydogs suggested.

 

[bottomline2000]

Info there looks OK. Fuel trims don't indicate to me a vaccuum leak. And the read between the lines bit with this would suggest that your oxygen sensors are OK.

MAP looks OK.

MAF at the freeze frame looks ok, though you'd really want to graph it to verify. But given that your issue is only at idle vs. a moving drivability concern, I'd think you're OK there.


SO.. with the info provided, here's what I'm thinking: Something's wonked with your IAC. Mechanically, it's not operating at it should. But electrically, its fine - else. IACs aren't mechanically bound like a bolt, the encoder in there moves a pin back and forth, and the plunger follows that via spring tension. If the encoder were messed up, you'd see a code for that. But since you don't, that means the encoder is working fine but there's something obstructing either the mechanical movement of the plunger, or the air bypass on the TB for the intake.


As far as next steps, I'd remove your IAC and clean the hell out of it again. Check the passages as well. Lets see where that gets you

maybe clean it with something more sensor friendly instead of brake cleaner.

 

[Fixture]

Installed the new IAC and its still doing the bouncy idle when cold. Idle steady at 450 when warm. What else should I look at now?

Here is the new IAC and the old one. You can see how far out the plunger is from the new one, but its still doing the up / down idle.

Is their a cold air or cold coolant temp sensor that is giving bad readings when cold>

Thoughts?

Jonathan

 

[SnowDrifter]

What's your fuel pressure at time of issue? Does it trend with any issues?

 

[Tonyrodz]

Installed the new IAC and its still doing the bouncy idle when cold. Idle steady at 450 when warm. What else should I look at now?

Here is the new IAC and the old one. You can see how far out the plunger is from the new one, but its still doing the up / down idle.

Is their a cold air or cold coolant temp sensor that is giving bad readings when cold>

Thoughts?

Jonathan

View attachment 212532

Where'd you buy it from?

 

[Fixture]

Tonyrodz, I got this one from rockauto.com. that is where I get all my parts from.

Snowdrifter, I will have to check the pressure. I might also do a smoke test to verify that their is no more air/vacuum leaks.

This issue didn't happen before all the maintenance.