Question about 1989 Ford Mustang

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So a couple months ago, on my 1989 Mustang LX 5.0, it started acting weird. When I come to a stop my RPMs will continue to go up to about 3000 RPMs. My Mustang is a 5 speed. So when I put in the clutch the RPMs will continue to go up to right around 3000 RPMs. There is a faint hissing noise which I think is coming from my brake pedal. But I am unsure. There weird thing about it is it doesn't happen all the time. It only happens some of the time, probalby 75% of the time I am driving. And very rarly when I am at a stop and the RPMs are still reveving really high, it will suddenly stop, and drop back down to normal of less than 1000. Could it be a vacuum leak? Maybe a bad sensor? Thank you.

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Go under that hood and have someone run the engine in neutral with the emergency brake on and listen for a hiss while they are pushing the brake pedal down, if you hear it look in the direction of the power brake booster...mounted on the rear of the master cylinder....check the approx. 3/8" black rubber vac. hose that plugs into it for a leak and also test the plastic piece the hose slips onto that goes into the booster is a one way valve and can go both ways causing havoc.
Visually inspect all the vacumn lines for any breaks, cracks or disconnects.

Posted on Aug 08, 2010

  • schrantzjj Aug 09, 2010

    Ok should the plastic piece be loose at all? The hose is ok, I've check that. But I can somewhat wiggle the plastic piece that goes into the booster unit. But it doesn't seem too loose.

    Can it be the Idle Air bypass valve? How can you tell if that bad?

  • JOHN DAVIDSON Aug 09, 2010

    If the brake pedal gets real hard to push that indicates a vac. leak at the power brake booster the plastic piece sometimes feels loose but if no air hissing is heard around it, with the car idling, it is presumed good.

    Have the computer scanned for codes please, everything here is a "guestamation" if you know what i mean. Auto Zone does this as a free service.

    9 times out of 10 that particular problem is a vacuum leak. Check all vacuum hoses for a good connection. Make sure they aren't cracked or rotted. If everything is OK, get a can of carb cleaner and give a quick spray at avery vacuum hose and vacuum connection. then spray every gasket on the intake, if you find the leaking area the idle will raise up.
    During this erratic idle condition, what happens if you disconnect the IAC valve? If the idle stays up, the problem may be a vacuum or engine sealing condition. If the idle stays down, then you would look at an electrical problem in the IAC circuit or a sensor input issue. I dont believe it would be a input issue because you dont have any input codes. (TP, ECT, ACT... ect). Worst case scenario would be a harness problem. idle problem as you described turned out to be induction of the output wire to the IAC holding up the idle erratically. I overlayed the wire (w/lt blue) from the pcm (pin 21) to the iac with a new one and the problem went away.
    Best guess would be to look for vac leak.



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