Erratic Idle

[Phone Man]

I am having idle issues when in drive but not in park. The idle surges and then drops and gets more frantic until it stalls. I have to power brake it at stops. I do have a ported Throttle body. Ive already changed the IAC motor. The tranny is shot and is being replaced soon. Are there any sensors or controls in the tranny that might cause this?

thanks

 

[SunlitComet]

If you provide your vehicle info we can help you alot better. It is not a bad idea to put those details in your signature field.

 

[Phone Man]

Good point.

 

[99Yuk]

I had a couple of initial thoughts.

Back before I installed my ported TB, I had shaved my butterfly in the stock TB. When removing the spacer it leaves two holes in the blade. If those holes are not filled back in, the idle is all weird. IE the extra air caused issues, so what if there is extra air getting in. Not likely the trottle blade in your ported TB, but what about the an air leak at the bottom of the TB, or a gasket missing/deformed?

My second thought was a bad Throttle Position Sensor? But then you should be seeing some codes.

 

[Phone Man]

99 ht383e

 

[99Yuk]

Oh no, you have that Vmax intake spacer that I was thinking about getting, don't you? What if it's leaking?

Can you use some smoke while the engine is running to see if/where it's getting sucked in?

 

[Phone Man]

Yeah I do have the spacer. I think using carb cleaner will increase the idle if there is a vacuum leak. I put that spacer on very carefully when the motor was on the engine stand but you can never tell. I'll check that out.

 

[97yukon]

my old truck was doing the exact same thing.....spent endless money on parts trying to narrow it down....come to find out...my ecm was malfunctioning. had to replace the whole damn thing....fixed it though lol.

 

[Phone Man]

Just checked for vacuum leaks. All ok. I unplugged each sensor one at a time and all was well UNTIL I unplugged the IAC. Then the idle smoothed out. This is a new part that I "threw" at the problem. The only other thing I can think of is that the bored out throttle body that I got from CFM-Tech (http://marine-performance-parts.com/cfmtechportedthrottlebodyfor96gmtruckvortec5057.aspx) have is defective. I ordered a new one from another guy on ebay item # 190363292544. Lets see what happens.

 

[Phone Man]

Ok I have done some research and the story is that I may have a vacumme leak at the intake gaskets. I read that if you plug up the PCV and you have vacumme at the oil dip stick then there is a leak at the intake gaskets. I did this test and Have vacumme there.

What do you guys think? A valid test?

P.S. replaced the throttle body and new transmission... same issue. The Idle is wild in drive and then stalls but ALSO stays high when coming to a stop after driving and then when I do finally get to a stop, the idle goes down and then starts its fluctuating until a stall.