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Old 08-20-2005, 07:11 AM
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87 Mercury Grand Marquis

Need Help! I have an 87 Mercury Grand Marquis. My problem is that I am having trouble with the engine idle speed. Sometimes the engine while idling will rev up to very high rev's whether in neutral or drive or reverse. Sometime's it's the opposite and the engine will die out because of low rev's.
I have replaced the a/c idle compensator and there was no change. Could a possible vacuum leak cause the idle speed to fluctuate to these extreem's. Also while driving the car the throttle will stick and the car runs @ 45 mph without my foot on the gas peddle. I can solve this problem by simply cutting the engine off for 1 -2 seconds and it returns to normal.

Thanks for any tip's or info,

D. Popwell

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Old 08-20-2005, 07:35 AM
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i had a problem with my 90 f150 similar to yours. turned out to be the MAP sensor on the fire wall. it has a vaccum hose and wires connected to it.unplug the hose and see if problem occurs. but yes,a vaccum leak could be the problem also.
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Old 08-20-2005, 09:02 AM
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my 88 lincol town car with a 302 motor did the same thing..crazt thiing woulkd run5 0mph and ver touch the gas..than bearly idel...i traded it off..but the man i trard it in sais it was the fast idel valve and another thing..maybe the map ..mat sensor..
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Old 08-20-2005, 12:09 PM
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First you need to scan for codes. This will save you a lot of time and effort.

This sounds like a bad IAC motor. The car most likely needs a throttle valve service (cleaning the deposits from the actual throttle body) and also injecting cleaner into the IAC motor (has to be removed and gasket replaced). If the IAC motor cannot be cleaned, it must be replaced.

The IAC motor actually controls throttle speed as it meters air around the throttle valve for base idle, etc.

I have replaced the a/c idle compensator
You may mean IAC. It is a silver colored cylindical piece at the throttle valve.
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Old 08-30-2005, 10:33 PM
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My '88 T-bird has basically the same engine. It had a cheesy plastic "manifold" in the vacuum line that supplied the power brakes & cruise control. It broke, and the car would go ~45mph with no pressure on the gas pedal and no power to the brakes. A crack in this piece might cause some erratic idling. I replaced mine with a brass tee.

Also, spray the throttle with some throttle valve cleaner. Mine needs it every 50,000 miles or so when the idle gets weird. Shoot some into the IAC (idle air control) passage, too. Be careful with the rubber seal that goes between the plastic air hose & the throttle body. You can't buy them any more.

Might also wiggle the connector to the MAP sensor. This can break through a thin layer of corrosion that may be causing weird electical signals to the computer. Wiggle the IAC & TPS connectors, too. If wiggling helps, pull the connectors, clean them, & replace the protective grease that's in them.
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