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Old 03-24-2010, 06:46 PM   #1 (permalink)
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92 f150 5.8 very fast idle 2500 rpm

hi and thanks in advance
have a 92 ford f150 idles very high. 2500 rpm new iac valve. have backed out idle screw and covered throtle body compleatly then it idled slower 700 rpm but still ran i would have thought this would have killed the engine but it still ran have been told it could be a a vacum leak have checked the brake booster and all large hoses i could see have not found vacum line to trans. does this truck use vacum to trans? could it be a intake gasket leak tryed the spraying carb cleaner trick but did not find the leak if there is one

any ideas?

also i checked the egr valve and it is not stuck open
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Old 03-24-2010, 06:51 PM   #2 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

2500 is very high. Did this start all of a sudden? Check and see if the bottom of PCV valve fell out. Sometimes this happens with plastic valves. If it has overdrive transmission then it will not have vacuum hose to transmission.
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Old 03-24-2010, 07:06 PM   #3 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

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Originally Posted by gbic1 View Post
2500 is very high. Did this start all of a sudden? Check and see if the bottom of PCV valve fell out. Sometimes this happens with plastic valves. If it has overdrive transmission then it will not have vacuum hose to transmission.

Vacuum leak seems probable, check gaskets.
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Old 03-24-2010, 07:17 PM   #4 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

I would guess it is a defective Idle Control Valve. They are known to get stuck part part way open. It is mounted to the throttle body.
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Old 03-24-2010, 07:38 PM   #5 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

if you can't find a vacuum leak, see you replaced the IAC , check wires to the iac. can't recall cold idle rpm, test ect unit & wire , not the sender to gage
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Old 03-24-2010, 07:52 PM   #6 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

hi gbic1 and all who replied
checked the pcv valve held my thumb over it so i dont think that is it also closed the throtle body compleatly and held my finger over the hole for the iac where it bypases the throtle plate and no effect is there any wher else the iac could get air from i would geuss that it all come frome the filter
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Old 03-25-2010, 06:48 PM   #7 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

Have you tried to run a diagnostic check on the ecu? If so, what kind of codes is it spitting out? If there are any codes present, that would help in diagnosing your problem.

This may not be your problem, but I had a '87 Ranger that did pretty much the same thing. I would come to a stop and the motor would stay at about 2000 rpm for a minute or so and then come back down to normal. Took a long time to figure out that problem, but it ended up being a faulty MAP sensor. The problem was that it wasn't bad enough to throw any codes. It was telling my ecu that I was at about 12,000 feet in elevation when I was in fact at about 100 feet above sea level.
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Old 03-25-2010, 07:08 PM   #8 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

getting air from some place . check the plugs , real lean one's could give a clue , such as intake gasket
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Old 03-26-2010, 08:03 PM   #9 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

Air leak somewhere. Tracking it down might be challenging....

The IAC. Try unplugging it while the engine is running. Just for giggles. Also, you can pull the hoses off of the throttle bodies (dual TB setup) and just before the butterflies you'll see a hole in each TB. That is the hole that the IAC uses to increase the amount of air going into the engine. Use your fingers to block those holes off and see if the idle drops to around 500 RPM give or take. Used that trick to find my 5.0's problem ('95 F150). Mine, it was the booster hose. It wasn't broken-just cracked in a gazillion places. I ended up replacing it...couldn't find a piece of hose that would fit so I used a piece of an air hose. DONT do that. The air hose does a good job of holding pressure in...but it collapses under vacuum. Found out that hard way (no brakes!).

Also on the upper intake there is a vacuum tree that has a bunch of hoses attatched to it. Lots of those hoses are plastic and they crack easily. On mine, the tee itself was cracked & leaking. I just brazed it up...but a new one would probably have been a better option (but remember...I'm super cheap...)

Good luck. Let us know what ya find.
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Old 03-26-2010, 08:19 PM   #10 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

Definately vacuum leak. Upper intake manifold leaks are common. They leak where the upper and lower intake manifolds bolt together.

Try running with IAC unplugged to make sure the pcm isn't commanding high idle for some reason. Unplugging the IAC causes it to close so if the idle is still high you are getting air from somewhere else.

Eliminate all other possible leaks by closing off all vacuum hoses including pcv.

Replacing the upper intake (plenum) gasket is fairly easy. Pre-soak egr tube with penetrant before removal.
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Old 03-26-2010, 09:26 PM   #11 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

I had the same problem on a 91f150 5.8 about 3 weeks ago. It ended up being the TPS sensor, it was reading part throttle when at closed throttle position. Definetly check for vacuum leaks first. It would really help if you could read the live datasream from a scanner, if you have access to one. I confirmed no vacuum leaks first, second I unplugged vacuum line to MAP sensor, even though motor was idling hi it began to load up. Third I unplugged TPS sensor,idle came back to normal. When IAC valve is unplugged it would do the same thing, but it was new. TPS was sending wrong signal to computer which in turn was causing hi idle. I am a fulltime mechanic of 21 years, I definetly love what I do, but I'am not very good at translating what I know into words,but I hope this might help you.
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Old 03-30-2010, 03:43 PM   #12 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

hi all i am the one workinking on the f 150 high idle and wanted to give you a update.

talked to a man at the parts house he thinks the injector driver in the (ECM) is out and i do not get a check eng light when i turn the key on it should come on and then go out so i am buying a scaner By the way i am getting a code reader and going to try that before buying parts also he explained the high idle by saying the ignition timing is also advanced.

Ps. if i had hair i would pull it out. my dream boat will not have a computer hooked to engine
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Old 03-30-2010, 05:44 PM   #13 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

think your parts guy is out. no light-- did you just buy this truck ? check for missing or burnt bulb
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Old 04-02-2010, 04:53 PM   #14 (permalink)
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Re: 92 f150 5.8 very fast idle 2500 rpm

My wood mule has the same thing going on. Start up the idle is all over the place and might even die. Once warmed up idles 2500, sometimes it idles correctly for 5 or 10 min and then back up it goes to 2500. Hard to drive slow thru hayfields and such with a load of wood on when it wants to run fast like that!! Hope you all can help the OP and I find out whats up, I'll go out and run all the suggestions tomorrow.
Thanks
Rick

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