High Idle Need Help!!
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New User
Joined: Jan 2004
Posts: 21
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From: Chicago
High Idle Need Help!!
I have an 88 f150 302 with about 240,000 miles on it. I replaced the tps and iac today. It idles at 2600 rpm and remains there until I put it gear. When in drive, it runs at 1400rpm. Before I replaced the tps and iac, it idled at 2600 rpm and would drop to 1500 rpm after a minute or two. Now it doesn't drop at all. Any ideas on how to fix this before my engine blows? Thanks alot.
Lead Driver
Joined: Nov 2002
Posts: 9,557
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From: Concord, NC
Have you tried pulling codes from the computer? It might give you another idea of what is wrong. I really doubt that the O2 sensor is causing a high idle like this. I'd suspect a vacuum leak or a sticking throttle. What happens to the idle speed when you unplug the IAC?
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New User
Joined: Jan 2004
Posts: 21
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From: Chicago
When I unplug the IAC the idle lowers. It is brand new though. I am going to clean the throttle body, replace the gasket and replace the plenum gasket. Do you think that will do the trick? When I spray carb cleaner around the plenum gasket the idle lowers.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
How low does the idle drop to when the IAC is unplugged? If it is still high, it's most likely either the throttle sticking or a vacuum leak. Sounds like it is a vacuum leak since there is a change in idle when you spray the carb cleaner.
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Post Fiend
Joined: Mar 2002
Posts: 19,402
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From: Bonita Springs FL
Definitely a vacuum leak. Don't worry about the injectors. I don't see how they have anything to do with the problem.
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Junior User
Joined: Nov 2004
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From: Akron NY
$4 Plenum Gasket $4 Plenum Gasket $4 Plenum Gasket
$4 PLENUM GASKET - Order it from your dealer, takes about an hour to replace with 6 bolts (maybe need a long tool for in between plenum stacks). Do it yourself or have the dealer do it in 1 hour. My brother in law's a ford mechanic & did it at his house in under an hour, so take no bull, 1 hour or less by the shop manual. Solved my problem - exactly as you describe.
Wanna check it out? Let your engine cool to ambient. Start her up and with garden hose in hand, spray the engine where it meets the plenum. As the water fills the engine top you'll temporarily plug the vaccum leak and ilde should go down to normal. As the remaining water is sucked into the manifold she'll start to make a loud sucking noise (where the gasket is blown out) and your idle will once again go high - there's your confirmation - git her done
Wanna check it out? Let your engine cool to ambient. Start her up and with garden hose in hand, spray the engine where it meets the plenum. As the water fills the engine top you'll temporarily plug the vaccum leak and ilde should go down to normal. As the remaining water is sucked into the manifold she'll start to make a loud sucking noise (where the gasket is blown out) and your idle will once again go high - there's your confirmation - git her done