Up and down idle after intake manifold gasket replacement

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Fixture

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I have searched a bunch on the forums and I haven't found the issue that I have. My knock sensors were bad so I replaced them along with the intake manifold gasket (Fel-Pro). I cleaned the IAC, and the throttle body. New sparkplugs, 4 new 02 sensors and PCV valve. When I start the car cold the idle bounces from 600 to 400. About a minute or 2 later the idle is steady. Is there a cold start sensor or something else that I need to check? The service engine light came on after all the repairs but no error codes, so I reset it. I let it warm up and it idles and runs fine, its just the first minute or 2 where it does this. no error codes have come up yet. I torqued the intake per the factory sequence to 44 in lbs then another pass to 88 in lbs. I have let the truck go thru a couple heat cycles and have re-torqued the intake to 88 in lbs.

The sparkplugs I used were Bosch 4509 Platinum Ir Iridium Fusion Spark Plug, which per the manual are ok to use, but I don't think that is the issue. So mayby the IAC, TPS, and of course a vacuum leak somewhere?

I could do a smoke test to see if it is a vacuum / air leak.

Here is a video of it when I just start it.

 

SnowDrifter

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Surely there's a code if you had a CEL?

Verify all PCV hoses and vaccuum lines are connected in the right spots and not kinked. If you have EGR, check that too. Also verify that your intake tube is all connected together, and that it's fully seated on the throttle body - it's easy to kink it during install and create an air leak post MAF.

How/where did you clean the throttle body? Describe it in detail, or pull up a picture of it and color in where you cleaned.

Disconnect your battery, eat a meal, then connect it again. Just reset everything, go from there. It could be possible that the vehicle's long term fuel trims haven't re calibrated yet.




If the issue still persists:
Grab a scanner, pull IAC, TPS, MAF, oxygen sensor, and short term fuel trim data while the issue is occurring.
 

Scottydoggs

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that type of surging is a vac leak most of the time. go over all your plastic lines make sure they are connected tight or broken. they break pretty easy, you might not know you broke one.

you can also spray the top of the engine down with carb cleaner, if you spray a area and the idle jumps up you found the area the leak is in.

if you got a scanner that will show live data, the long term fuel trims will be pegged at +16. thats a clue to a vac leak.
 

SnowDrifter

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you can also spray the top of the engine down with carb cleaner, if you spray a area and the idle jumps up you found the area the leak is in.


I'd like to add to this: Don't forget under the intake manifold when you're checking. I forgot which year, but GM began putting a foam baffle between the manifold and valley pan to help shield the knock sensors from water intrusion. You'll need to wiggle your way past that
 

Scottydoggs

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all that foam did was trap the water in the knock sensors, in later years they did away with the foam. its recommended to remove the foam on the early model years.
 
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Fixture

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SnowDrifter,

I took off the Tb and took off the IAC, clean the rubber from the IAC which was caked with black stuff. Also cleaned the area inside the TB where the IAC seals in the TB with Carb cleaner and a tooth brush till the beveled edge the IAC contacts was clean. I cleaned all around the TB plate with a toothbrush and carb cleaner. I installed a new TB gasket as well when this was done. Is there another area on the TB that I missed? I hear of a pin hole that needs to be cleaned, but I didn't see anything like that.

I have ODB software for my laptop that reads real-time data so I can record it and post my findings.

Once I get home I will double check what you said and see what I find. I just replaced the battery and I just finished all of the factory drive cycles to get the computer set. I guess I will need to do that again, as you said maybe something is set from that cycle which is affecting my new repairs/fixes.

Jonathan
 

Erickk120

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That is exactly the problem I had years ago with a different car after the engine was replaced, the guy did a piss poor job and left a lot of stuff loose, along with a disconnected vacuum line that made it surge just like that.
 

SnowDrifter

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All sounds good as far as the TB cleaning. I primarily wanted to make sure you cleaned the choke surfaces near the circumference of the throttle plate. Those are the bits that when they get dirty, will cause you issues.

I await your live data

IAC, TPS, MAF should be graphed together if your software allows it

And fuel trims + oxygen sensor voltages should be graphed together
 
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Fixture

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Here is the readings before the battery pull last night and the engine was warm, showing the long term banks.

And then this morning after the battery is connected with it jumping up and down in idle.

I checked the vacuum lines and they appear to all be connected and tight. I dont have the EGR on my 5.3 where is usually is by the passenger side belt tensioner. The service engine light came on last night, on the way home and its just said "Idle speed detected lower than specifications". My software dosent show the ISC data.

Its just strange that as soon as it warms up it idles just fine. Want to get this fixed since I want it working good when I go to Smog it (socal).

Before batt pull.jpg 20181214_103811.jpg 20181214_103813.jpg 20181214_103911.jpg 20181214_103914.jpg
 
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SnowDrifter

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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
 
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