Fast idle at start / rough idle warm - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars
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post #1 of 5 (permalink) Old 04-12-05, 08:01 AM
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Fast idle at start / rough idle warm

I recently replaced the intake manifold gasket on my 91 Ford Explorer
Sport. I had to remove all of the vaccuum hoses and many of the sensor
connectors during this process.
Now that it is back together, it does the following:

1. Engine revs up to about 1500-2000 rpm immediately after start.
2. After about 30 sec to a minute it settles down to about 800rpm, but
idles rough.

Everything was fine before the repair work.

Could this be a missed vaccuum line? Or a missing connector?
I'm pretty sure I conencted everything the way it was, and I donyt hear
a vaccuum leak.

Power seems normal during regular driving.

Any thoughts?


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post #2 of 5 (permalink) Old 04-12-05, 01:02 PM
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Re: Fast idle at start / rough idle warm

Two additional pieces of info. When I am driving, if I accelerate
hard, I hear a slight leak-like sound for about 1 second then it goes
It does not do this if I rev the engine in neutral, only when there is
a load on it.
Also, I read another post where someone mentioned that when driving, if
he stepped on the clutch the engine would stay idling at about
1500-2000 rpm as long as the car was moving. When he came to a stop,
the engine would drop back to a rough 800RPM.
I checked, and my explorer shares this trait as well. His problem was
with the Idle Air Bypass Valve. Does this sound like a good place to

But why did this only start after replacing the manifold gasket?

post #3 of 5 (permalink) Old 04-12-05, 05:01 PM
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Re: Fast idle at start / rough idle warm

Although not terribly sure when I read your first post the IAC was my first
thought. I believe there is a way to reset it. Someone here should be able
to give you more info.
Good Luck

post #4 of 5 (permalink) Old 04-13-05, 02:02 PM
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Re: Fast idle at start / rough idle warm

OK, some more info...
I found a loose vacuum hose (carbon canister to the throttle housing).
This fixed the high idle while moving problem.
It still revs up at first start, and idles rough. It idles OK for a
few seconds, then dips down almost stalling, then back up to about
800-1000 rpm.
The Check engine light came on and reported "Mass Air Flow Sensor Out
of Range (low)".
I have a new Mass Air flow sensor, so I'll try that tonight. Curious
as to why that would fail right after the gasket replacement.
Coincidence? Or maybe just the rough idle is causing the sensor to
pass bad info.

I'll post my results in case anyone is interested.

post #5 of 5 (permalink) Old 04-14-05, 11:02 AM
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Re: Fast idle at start / rough idle warm

In case anyne cares...

The new Mass airFlow Sensor had no effect. But I noticed that there
was some slight leakage around the vacuum hose going to the PCV valve.
I put a new hose clamp on it and my problem has all but gone away. So
I am guessing that there is a very slight vacuum leak somewhere that
was causing the problem. The leaks all added together made the idle
rough, and when the engine sputtered, the sensors all went out of


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