Jeep missed up bad

peanut_girlNovember 22, 2009

My BF replaced head on my 1995 jeep cherokee. It has the 6 cylinder engine in it.

The problem we have now,is when you start it, it revs to over 3000 rpms.

Everything was cleaned to the bare metal with a surface disk he calls them.

So he says the seal to block and manifold to head is not the problem.

He wants to know if the air flow control valve could of got damaged in the process as one person told him is the problem.

Thanks for any help or info. Peanut_Girl

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The idle speed is controlled by the computer with the idle speed control motor (ISC). Its unlikely the ISC got damaged with the cylinder head being taken off and put back on. There are two likely causes for such a high idle speed, one would be a large vacuum leak, something like the power brake booster hose not being reconnected yet. The other likely cause is either the computer is commanding the high idle speed, or else the computer cannot control the ISC. Inputs like power steering pressure, AC clutch command, and the transmission shift position cause the engine to need to idle faster to prevent stalling. A false input from one of these causes an excessive fast idle problem. The computer looks at inputs such as engine coolant temperature to control fuel delivery and to produce the cold engine idle speed which would be about 3000rpm right when the car is first started, and then it should drop quickly. A break in the wiring between the computer and ISC could be the cause for this problem. The ISC allows air to flow around the throttle plate to control the idle speed. This passage is right at the top of the throttle body, and it should be easy to confirm if the air entering the engine is in fact flowing through the ISC passage. (Loud sucking sound, and idle speed can be affected by restricting the ISC intake port with a finger.

Its always possible the ISC has simply failed in the stuck open position, but its not anyone's fault. Cars break whenever they want to.

    Bookmark   November 23, 2009 at 8:44AM
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My jeep has sat since I posted on this forum.
The jeep revs and idles at 5000 RPM and not 3000.
That was a typo on my fault.
Could what you mention above john g be the problem still or is it something more serious because of the 5000 RPM idle?
It stays at 5000 RPM and does not drop down.

    Bookmark   December 18, 2009 at 10:10PM
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5000 rpm? Now the only thing in play is the throttle plate must be stuck partly open. No other single source of air entering the engine would allow enough to flow to let the engine run that fast.

    Bookmark   December 19, 2009 at 3:20AM
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That was checked couple times john_g.
It was in the closed positiion.

He is going out to check it now again.

    Bookmark   December 19, 2009 at 9:29AM
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It is closed and operating properly as opening and closing.


    Bookmark   December 19, 2009 at 10:52AM
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The high idle can only be caused by air entering the engine.
Have him block off potential vacuum leaks such as the power brake booster.

Listen to see if the air is entering the engine through the idle air control valve in the throttle body, but that can only increase the engine speed to about 3000 rpm by itself.

If he has a big vacuum leak, common to a number of the cylinders such as the wrong intake manifold gasket, that possibly would allow enough air to enter the engine around the throttle plate, but that is a stretch.

    Bookmark   December 19, 2009 at 11:32AM
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Could it be the wrong head?
The head was used and we never checked the gasket placement on it.
Just cleaned the head with surface disks.
That was thought that the problem had to be at the TB for the fuel to mix right with the air to get a steady 5000 RPM with out any back fire or stumble.
It sounds good other than the 5000 RPM idle.
But will do as you say first about vacum lines.

    Bookmark   December 19, 2009 at 2:02PM
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If you have the 6 cylinder engine, you are damaging the engine at 5000 rpm. That speed is enough to loosen bearings - never mind that the red line is a bit higher.

Air is getting into the intake manifold somewhere. Are you sure that the vacuum hose for the power brake is connected?

Maybe the IACS (idle air control) is wide open. On that engine, it typically runs at 1000 rpm on start-up and tapers back to 700 rpm within a minute. If there are no air leaks, the IACS is not closing as it should. Maybe the electrical connection to the IACS was damaged.

    Bookmark   December 21, 2009 at 3:02AM
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Did not check anything other than to see if the throttle plate was opened.
It was not.
Could the idle air control allow the engine to idle at 5000 though without the throttle plate being open at all.
It's been cold here in PA and maybe get a chance to check the IAC tomorro.

    Bookmark   December 21, 2009 at 4:27PM
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