Engine racing and strong vacuum sound

[toomanyhobbies]

After a warm start this am, my truck sort of stumbled at idle this AM and then started racing to about 2100-2200. There is a loud vacuum hiss coming from what appears to be the front right side of the engine. I looked around and can’t find any loose hoses or anything like that. No CEL from a few minutes running.

Any ideas? I am far from home and truck is not stock.

Thanks


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[toomanyhobbies]

I should have mentioned it’s an L31 with 3 years/7K miles with a black bear tune, egr delete and a zz4 cam.

Thanks


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[SAATR]

IAC could be stuck in the open position allowing a high idle, or it could be hung open by a dirty IAC passage. A can of carb and choke cleaner sprayed around the base of the throttle body, the injector harness connector on top of the intake, the joint where the upper intake meets the lower, and any other connection or joint in the manifolds could help find a potential leak. The brake booster could have failed internally causing a huge vacuum leak, which can be confirmed by disconnecting and plugging the hose. Any post MAF air leaks can cause what you describe.

Oh, and try to have a fire extinguisher handy when using flammable chemicals on a running engine.

 

[toomanyhobbies]

There is no booster it’s hydro which gets me to thinking about where the hose is blocked off...

I know the method you described but was trying to avoid for the reason you mentioned.


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[toomanyhobbies]

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This is the culprit but I can’t figure out what goes on here. Back of manifold toward distributor.

Thanks


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[454cid]

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This is the culprit but I can’t figure out what goes on here. Back of manifold toward distributor.

Thanks


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Is that where your PCV valve connects? I'm not all that familair with the details of the small block or V6.

 

[SAATR]

That's the power brake booster's vacuum supply port. Buy some rubber caps made for the purpose, or a piece of hose with a bolt and a hose clamp.

In days gone by, I once set fire to the engine on a '98 Z71 looking for a vacuum leaking. Turned out it had a bad cylinder and a cracked distributor cap. When the fumes from the carb cleaner found the crack, it lit off with a loud WHOOSH and I almost pissed myself. The engine looked like the aftermath of a bakery explosion by the time I put it out.