Danger To Manifold!! High idle (it's not the IAC)

codename Bil Doe · Jan 23, 2016

Several weeks ago I was cruising around with my pops in the truckasaurus (sy). We made a couple laps around town, and after boosting onto the freeway one last time I exited and got instant high idle at 1800rpm. This was right before I gave the truck to a friend.

Eventually starting the truck would idle at 3200rpm. Read all threads I could find, saw the usual TPS/IAC responses and none of them solved the problem.

What I did see were a lot of unresolved threads with high idle. Since the truck was at my friend's house now, I told him to block off the tb. Still idling 2700rpm. Then, I told him to remove the tb and block off the intake to see if the tb was the issue. He did that and it still idled at 1800 rpm. That was the giveaway. A LOT of air had to be coming from somewhere if the plenum holes were blocked off.

What I saw was a chunk of the gasket missing from the front driver corner. Taking the plenum off this is what I found:

http://m.imgur.com/a/61BIH

A quick test for anyone who has a high idle and IAC isn't the fix is to block the tb and try to start it. If it starts and the idle is very high you have a major air leak. Random but you just might have a blown gasket. Danger to manifold F&F was right!

dgoodhue · Jan 23, 2016

Re: Danger To Manifold!! High idle (it's not the IAC)

Did you reuse the gasket the last time the upper intake was off?

Don W. · Jan 23, 2016

Re: Danger To Manifold!! High idle (it's not the IAC)

Interesting. I've reused that gasket numerous times. Never an issue. 20# boost maybe 22# on a spike but unusual. Check that both surfaces are flat. Upper plenum not cracked.

How do you remove or block the TB (Throttle Body) and have it still run? Never heard of that. Misprint?

codename Bil Doe · Jan 23, 2016

Re: Danger To Manifold!! High idle (it's not the IAC)

Not a misprint. That was the whole purpose of having him do that. I was troubleshooting the issue over the phone. If he blocked the tb or removed it and blocked the hole but the truck still ran then it had to have a major air leak into the manifold from somewhere. By having him do that I was eliminating the tb or iac as the problem (only other solutions mentioned in other threads).

Only had this truck for about a year. Never had the manifold off. BUT, the upper manifold bolts were only finger tight. The gasket couldn't have been compressed. Never seen any of this on any car ever, boosted or not.

New gasket arriving Tuesday. Anyone know the torque specs for those bolts?

Don W. · Jan 24, 2016

Re: Danger To Manifold!! High idle (it's not the IAC)

Your theory is correct, your method caught me off guard.

Poconojoe · Jan 24, 2016