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idle speed racing too high 200 1984

posted by JAZ on Fri Jun 16 16:06 CST 2006
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last visit: Wed Aug 11 16:39 CST 2010

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I had been having some troubles with the idle speed going high after it warmed, up, around 1200-1300 rpm, but not always.

It would start normally, run at regular idle and then sometimes runup in idle. If I shut the car down and restarted it, it cleared sometimes. It usually took many cranks to get it going.

Last week after driving for approx 10 miles I stopped in somewhere, came back out in a few minutes, it started right up then stopped and I couldnt start it for approx 30 minutes.

I looked at some of the posts and thought it'd be good to clean the throttle body first.

The throttle body was loaded with all kinds of black residue, problem the result of 240K+ miles; a mechanic friend of my did not see this as unusual.

After cleaning, the car runs in idle position at around 1500 rpm cold then migrate to around 1800-2000 rpm as it warmed up. Restarts have not seemed to be problem as before. If I pull the IAC hose and plug the port at either end there is really no change, but the IAC is full open. I checked the TPS and it clicks and changes to another mode when this occurs. The IAC hoses look fine. Several months ago I pulled out the IAC and cleaned it to free it up. I remember putting a stethocope on it then and could here it buzzing, which if I remember right was the proper operation.

The idle air speed adjustment is all the way in, any adjustment the other way raises the idle.

IF I remove the connector from the AMM, rpm drops a bit and it goes into the default pedulum mode of higher then lower rpm.

It acts like some connection has gone awry.

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idle speed racing too high 200 1984

posted by B.C. on Sat Jun 17 10:06 CST 2006
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last visit: Wed Mar 28 22:28 CST 2012

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The IAC is what controls the idle speed when the throttle plate is closed.

If the TPS clicks just as the plate gets fully closed, and the IAC is not gummed up, there are just a few things left to cause high idle.

You can learn if it is the IAC system by slowly closing off the most accessable hose connected to the IAC. {Use a large pair of pliers.) That will slowly close off the only air supply to the engine, and the idle speed will drop. Fully closing the hose will shut down the engine. UNLESS there is a vacuum leak somewhere.

Another test is to unplug the IAC and use an ohmeter to check continuity between the terminals. Haynes will give you the ohm readings, but usually the coil is either OK or open. Is it is open, the IAC is toast.

Good Luck,

Bob

:>)

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idle speed racing too high 200 1984

posted by JAZ on Sat Jun 17 17:10 CST 2006
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last visit: Wed Aug 11 16:39 CST 2010

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tried the hose pinch, no change, so this does not seem to be the problem.
-even took out IAC checked its movment, moves free, reads continuty, if I fed it voltage it clicks back and forth.
- checked timing, thinking this could speed up idle, it is right on
- checked response of coolant sensor to control unit, started cold around a few ohms, went to 300+ after engine started to warm up, indicating this appears okay.

Any more ideas? I am stumped.

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idle speed racing too high 200 1984

posted by Bruce S. on Sun Jun 18 08:34 CST 2006
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last visit: Sat Oct 4 19:05 CST 2014

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JAZ,
Please post your findings, because I'm chasing the same problem on my 84 245. I've switched out the TB, and the TB switch, no change.

I found that if I pinch off the small vacuum line from the flametrap to the manifold, that kills the rpm's. My first thought was execcive blowby, but the engine is well cared for and runs to strong to be that worn out.

My other thought is that the oil seperator or it's seal between the block has failed.
--
Bruce S. near D.C.

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idle speed racing too high 200 1984

posted by B.C. on Sun Jun 18 21:36 CST 2006
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last visit: Wed Mar 28 22:28 CST 2012

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I found that if I pinch off the small vacuum line from the flametrap to the manifold, that kills the rpm's.

That's odd, since the PCV system is designed to have manifold vacuum draw the oily fumes from the crankcase into the intake so they burn off. Somehow that small hose is allowing s significant amount of air into the I-manifold. From where?

Regards,

Bob

:>)

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idle speed racing too high 200 1984

posted by B.C. on Sat Jun 17 19:57 CST 2006
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last visit: Wed Mar 28 22:28 CST 2012

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The ECU computer "decides" the amount of fuel to feed depending on the amount of air. If it's too rich, or too lean, the O2 sensor feeds that info to the ECU to help with that "decision".

There must be air getting into the engine somewhere after the throttle plate. Find in the FAQs the test for leaks in the intake manifold gasket. Check all large hoses into the I-manifold, such as the one to the IAC.

Good LucL

Bob

:>)

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