Curb Idle

[PatrickStapler]

So this past weekend I completed the valve cover gasket job on my ‘99 3400. Drove the car yesterday and all was good. Drove the car to work today and when I got to work and when I parked then engine idled up to 1500 RPM. I put it back in drive and it idled down to 700 RPM. Put it back in park and back up to 1500. So I shut it off and went into work. Drove it home, same thing. I replaced two vacuum lines and one coolant line. I didn’t touch the throttle body other than to clean it and lubricate the springs and cams. They appear to be returning to normal position when the throttle is off. There is a tiny bit of slack in all three cables when idling.

What am I missing?

 

[octanejunkie]

So this past weekend I completed the valve cover gasket job on my ‘99 3400. Drove the car yesterday and all was good. Drove the car to work today and when I got to work and when I parked then engine idled up to 1500 RPM. I put it back in drive and it idled down to 700 RPM. Put it back in park and back up to 1500. So I shut it off and went into work. Drove it home, same thing. I replaced two vacuum lines and one coolant line. I didn’t touch the throttle body other than to clean it and lubricate the springs and cams. They appear to be returning to normal position when the throttle is off. There is a tiny bit of slack in all three cables when idling.

What am I missing?

Did you refresh/replace the TBU gasket? Did you remove the IAC from the TBU?

 

[PatrickStapler]

I didn’t remove anything from the upper intake assembly. Just cleaned it.

 

[spartacus]

I didn’t remove anything from the upper intake assembly. Just cleaned it.

double check your vacuum lines all connected. check to see that the butterfly valve closes completely and operates freely.

 

[spartacus]

unplug the positive battery terminal for ten minutes.

 

[PatrickStapler]

Double checked all vacuum lines this afternoon. Will disconnect battery when I get back home. Why positive and not negative?

 

[thegipper]

Another vote for disconnecting the battery cable. I normally disconnect the negative terminal, it's what I've always been taught/done. If it still does it, check for vacuum leaks.

If it still does it, it might be the IAC valve.

 

[spartacus]

Double checked all vacuum lines this afternoon. Will disconnect battery when I get back home. Why positive and not negative?

disconnect the negative first then disconnect the positive. the idea is to reset the ecu so it recalibrates based on current readings.

 

[PatrickStapler]

Is the IAC the small pancake valve on the rear passenger side of the throttle body? The reason I ask is, as a test I did disconnect that vacuum line just to see if anything would happen and the idle did increase by about 500 RPM to 2000 RPM. Unfortunately I don’t know if that still says it’s good or bad or what it even is.

 

[spartacus]

the pancake valve I think is fuel pressure regulator. the I ac valve is below the TB and has passage leading into it it looks like this

 

[thegipper]

Is the IAC the small pancake valve on the rear passenger side of the throttle body? The reason I ask is, as a test I did disconnect that vacuum line just to see if anything would happen and the idle did increase by about 500 RPM to 2000 RPM. Unfortunately I don’t know if that still says it’s good or bad or what it even is.

No.

It's the little gizmo underneath the throttle body, near the intake tube that comes from the air box.

https://www.youtube.com/watch?v=2-V1P9dReis

I had almost the exact same thing happen on my truck when I swapped in a new/used engine. Idled at 1500rpm, even after it warmed up. Ended up swapping out throttle bodies from my bad engine and reset the ECM (disconnect the battery for 20 min).

It fixed it which means it almost certainly was a bad IAC on my donor motors throttle body. Some people have success cleaning them, others don't and they have to be replaced.

You should definitely try disconnecting the battery first. Sometimes the ECM just needs to relearn the idle after messing around with stuff...

 

[PatrickStapler]

Cool. Thanks guys. I just parked it and disconnected it. I assume it’s ok to leave it disconnected overnight and expect the same results. Sorry, I’m not much on these EFI setups. Can tune the hell out of a Autolite 4100 or Holley DP though.

 

[PatrickStapler]

Let the truck sit overnight with the battery completely disconnected. Now it idles at 1700 RPM. So, I guess I will remove the TB and IAC and thoroughly clean again. At least I have the gasket.

One thing you guys mentioned was vacuum leak. Because I am uncertain about EFI engines, wouldn't the engine idle and run rough with a vacuum leak? I know NA engines do.

 

[JayRolla]

Let the truck sit overnight with the battery completely disconnected. Now it idles at 1700 RPM. So, I guess I will remove the TB and IAC and thoroughly clean again. At least I have the gasket.

One thing you guys mentioned was vacuum leak. Because I am uncertain about EFI engines, wouldn't the engine idle and run rough with a vacuum leak? I know NA engines do.

When cleaning the tb it will gunk up the iac. Sometimes all it takes are some good taps on the iac while motor is running. Bet your idle drops as you tap on it.

 

[19963.4lsr5]

I've already shot carb cleaner through its air bypass port with engine at idle. It's the lowest part of the throttle body and collects dirt


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[PatrickStapler]

:wtf:...I don’t think I have ever fixed anything on an engine by hitting it with a ball peen hammer...until today. 6-8 light taps on top of the TB and the idle fell right down to 750 RPM. Not sure if that is where it’s supposed to be, but that’s a hell of a lot better than 1700. And still no leaks...:banana:

Thanks for the tip.

 

[octanejunkie]

Good on you

It's somewhat likely that either IAC or TPS service/replacement are in your future

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