GM TBI sucking air until warmed up

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GM TBI sucking air until warmed up

Postby Grand_Wag_85 » Tue Jul 10, 2018 10:06 pm

I am running a TBI on a 258 in my AMC Eagle. Until the car warms up it sounds and acts as if there is a HUGE vacuum leak. You can hear the moto sucking excessive air and the car does not want to idle until. Once the car is warmed up it idles perfectly and the sucking noise goes away. I thought that this was a leak between the TBI and intake manifold so I sprayed carb cleaner EVERYWHERE while the motor was cold but no surge in idle speed, except for one runner where the intake meets the head. I also went and plugged off every nonessential vacuum port but still no luck.

Any other spots on the TBI where they are known to leak? This is a "rebuilt" unit so who knows what it's been thru.
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Re: GM TBI sucking air until warmed up

Postby shimniok » Thu Jul 12, 2018 5:35 am

That is a heck of a vac leak if you can hear it. Could it be a leaking intake manifold? Are you sure it is really a vacuum leak?

What about the air cleaner housing -- snorkle valves working correctly? If the flap that closes to prevent overrun doesn't open when the engine is running it could make a racket and would block airflow preventing proper idle.
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Re: GM TBI sucking air until warmed up

Postby Cecil14 » Thu Jul 12, 2018 9:14 am

Make sure the intake itself isn't coming loose. I had that happen on a 258 many many years ago, didn't end up tracking it down until it had caused some catastrophic damage. A new intake gasket isn't terribly expensive or difficult to replace on the 258, worth taking a look. If you grab the intake on the outside (near the TBI), can you wiggle it up and down on the head?


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Re: GM TBI sucking air until warmed up

Postby babywag » Sat Jul 14, 2018 1:32 pm

IAC will make noise on cold startup
open element filter? first tbi?
bad fuel map?
bad IAC?
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Re: GM TBI sucking air until warmed up

Postby JeepsAndGuns » Sun Jul 15, 2018 5:38 am

If the IAC valve is pegged wide open it will make a sound like that.
Could be at cold start, it is having to peg wide open to try and keep the engine running, then once warm, it closes back off. Cold engines need more air and fuel to idle than a warm engine. Take the air filter off and do a cold start when you hear your noise. See if it is coming from the throttle body/IAC valve.
If so, my guess is the min idle speed is not set correctly. With the IAC all the way closed, the throttle blades should be open enough to let the warm engine idle at about 100 rpm below your desired (what the ecm is programmed to) rpm. If the throttle blades are closed too much, every bit of the air at idle is having to rush through the IAC valve.
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Re: GM TBI sucking air until warmed up

Postby Curly » Mon Jul 16, 2018 10:28 pm

Agree on the IAC being the culprit, normal sound. They can be quite noisy on cold start as the air rushes through that small opening.
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