Question about Ford F-150

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I own a 1998 Ford F150 V8, 4.6L engine with approx. 273,000 miles. My truck has a high idle problem. Whenever I am driving for awhile and come to a stop, the engine idles well pass 1500 RPMs. In fact, the idle is so high, it requires me to keep my foot on the brakes to prevent running into the car in front of me. Once it starts idling high, it continues until I turn the truck off. Can you advise on what is causing the truck to idle at very high RPMs after being driven?

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  • reginald_777 Apr 30, 2010

    Yes, the solution was very helpful. This gives me a starting place to begin fixing the problem.

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  • Ford Master
  • 17,970 Answers

Vacuum leak or the IAC. Check the PCV hose behind the throttle body at the elbow and see if it is sucking air! Common problem on all F series with the 4.6.

The first thing to look for are vacuum leaks, pcv, lines going under battery tray, possible leaks between mass air and throtle body, plastic intake, start engine and listen closely you will have to visualy inspect tube from mass air to throtle body, while ideling disconect conector from IAC see if this changes idle also can be leak to evap valve or bad evap valve located by booster on fire wall this should also hold vacuum.

You can eletricaly check the IAC should be 6to 13ohms between the two terminals and 10000 ohms or mor from ground to each of the terminals.

As a last chance to elimanate the IAC you can remove it and block off the pasages if it is IAC related it will idle down also beshure the throtle plate is closing all the way and that the TPS is not bad or sticking.

Hope some of this helps.

Posted on Apr 30, 2010

  • ZJ Limited
    ZJ Limited Apr 30, 2010

    Your welocme, TY for using Fixya.

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