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91 Chevy 350 has high idle speed
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rickcaspari
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Sep 26, 2008, 12:17 PM
Post #1 of 12
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91 Chevy 350 has high idle speed
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I'm having nothing but problems.Now my pick up has developed a high idle condition,1500 RPM. What should I check first and how do I check it?I need a cheap fix for I am out of money due to these Hurricanes down here in Louisiana.Thanks RICK.
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DanD
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/ Moderator
Sep 27, 2008, 2:57 AM
Post #2 of 12
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Re: 91 Chevy 350 has high idle speed
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Check the gasket in between the throttle body unit and the intake manifold; they were well known for failing.
When they fail; extra air is allowed into the engine which will increase idle.
Dan
Canadian "EH"
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ducks13
New User
Sep 27, 2008, 4:59 PM
Post #3 of 12
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Re: 91 Chevy 350 has high idle speed
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Any vacuum leak will cause it.
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rickcaspari
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Sep 29, 2008, 8:04 AM
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Re: 91 Chevy 350 has high idle speed
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What is the best way to check for a vacuum leak?I lightly sprayed some fuel injector cleaner around the TBI with the engine running.I did not notice any stumbles or higher revs.Thanks RICK.
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ducks13
New User
Sep 29, 2008, 6:14 PM
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Re: 91 Chevy 350 has high idle speed
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carb cleaner and the like will not work on a fuel injected engine like it does on a carbrurated engine. Visual inspection will find most vacuum leaks, after that the only thing I know of that works on fuel injected engines is a electronic vacuum leak detector, kind of like a freon leak detector but vacuum leak detectors listen for the high frequency sounds that vacuum leaks make.
The first thing to look at on your engine is the idle air control (IAC) valve, it should be all the way closed if you have a vacuum leak. I suspect it is stuck open or has failed open. If it is closed then the engine has to be getting air past the throttle valve some other way, most likely a vacuum leak. If you have one bad enough that the best the IAC can do is keep the RPM at 1500 it will have to be pretty good size leak, like a 5/16" line or bigger.
Good luck
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rickcaspari
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Sep 30, 2008, 7:30 AM
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Re: 91 Chevy 350 has high idle speed
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How can I tell if the IAC valve has failed ? RICK
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DanD
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Oct 2, 2008, 3:02 AM
Post #8 of 12
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Re: 91 Chevy 350 has high idle speed
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With you saying that the running didn’t change, when you sprayed the base with a cleaner, it may not be the problem?
Or the rpm is so high that the engine just swallowed it and you won’t hear a change?
The gasket is only worth a couple of bucks and it only takes a half hour or so to change it; so yea, pull the TBI unit and have a look.
That way you’ll know that it is or isn’t the problem, for sure.
Dan.
Canadian "EH"
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rickcaspari
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Oct 2, 2008, 8:11 AM
Post #9 of 12
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Re: 91 Chevy 350 has high idle speed
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I was looking at the TBI and it looks pretty simple to pull off.I'll try to do that this weekend and go ahead and change the gasket.Is there anything else I should do while the TBI is removed?I should probably go ahead and clean it real good.Also how do I clean the IAC valve?Thanks RICK.
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way2old
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Oct 2, 2008, 9:17 AM
Post #10 of 12
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Re: 91 Chevy 350 has high idle speed
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Look at the intermediate gasket also. It will also suck into the throttle body. Open the throttle plate and look down inside the throttle body. You can usually see the gasket pieces hanging inside the bore.
Being way2old is why I need help from younger minds
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DanD
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Oct 2, 2008, 9:59 AM
Post #11 of 12
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Re: 91 Chevy 350 has high idle speed
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The IAC motor just threads out of the TBI unit; just spray it down with an air intake cleaner & wipe it down with a cloth.
Dan.
Canadian "EH"
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rickcaspari
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Oct 2, 2008, 12:25 PM
Post #12 of 12
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Re: 91 Chevy 350 has high idle speed
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I'll try all this over the weekend.Thanks for your advice.RICK.
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