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High Idle
Topic Started: Apr 18 2010, 04:48 PM (4,531 Views)
Skrilla
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Is there a way to adjust the idle, I need to turn it down a little?
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Johnny Mullet
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No. You have either a vacuum leak or the coolant passages/hoses to the throttle body are plugged.
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crankcase


Some common problems I’ve found for high idle after recently dealing with the issue on a 1992 are the IAC (idle air control valve) and clogged coolant lines around the throttle body. There were no engine codes and I had idle around 1500 RPM.

The IAC solenoid is an electrically controlled solenoid valve that is pulsed by the ECM (Engine Control Module). When the ECM decides the engine idle is too low, it opens the IAC by applying 12 volts to the IAC solenoid coil in a series of pulses which makes a "chattering" noise. This noise is loud enough that the car designers incorporated a silencer to muffle the sound. The silencer is a little plastic cylinder about the size of a 35mm film can with two hoses attached, one goes to the air filter area and the other goes to the IAC.

The silencer should not be stoped up. If you blow into the silencer, air should pass through. If plugged, it will defeat the function of the IAC. The IAC allows a regulated amount of extra air into the intake manifold, controled by the ECM, thus causing the engine to idle faster. This happens if you put a heavy load on the engine or from a cold start. The ECM detects this and pulses the IAC accordingly to maintain proper idle speed or to warm up the engine.

Now for what can go wrong: The IAC has a little brass diaphragm that acts as a spring with a rubber molded center. It works like a faucet washer in a faucet. The brass surrounding the rubber fatigues over time and shatters, causing the valve NOT to close with no signal is applied.
You should not be able to suck air through the IAC top hose connection (the one that goes to the intake manifold) when the solenoid is off (not energized). If you can suck air through it, the valve is not functioning. Extra air will enter the intake when the ECM is not calling for it and the idle speed will be too fast and/or not steady.

The car had the IAC problem and a clog in a coolant lines at the throttle body. The small coolant hoses around the throttle body, such as the one that goes to the black metal coolant pipe leading into the water pump, were not getting hot!! These lines should get hot about the same time the big radiator lines do. The clog in the car was actually in the throttle body though. Anyway, there was no coolant flowing across the coolant temperature sensor.

The small hoses are on the passenger’s side. Hot coolant goes through a short hose on throttle body to another section of the throttle body a few inches above it. Coolant then flows past the coolant sensor and exits the throttle body toward the top and directly to the rear of the car. This is the hose that goes to the black metal pipe leading to the water pump. Try checking for scale or rust inside the holes.

I hope this helps.


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Johnny Mullet
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:gp
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Skrilla
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I have checked the coolant hoses and they get warm when running.And I tried blowing through those hoses and all checked clear.My car still idles I dont have a tach so I can say where but I guess around 1000-1200
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mcmancuso
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My 91 idles 1500-2000RPM, hoses heat up, so I'll check the IAC in the morning and see if that fixes mine. Closing off the vacuum hose through the IAC should drop the idle if that's the problem correct?
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crankcase


Yes, if all else is the same on the 91 Vert. I checked my daughter-in-law's 1992 by temporarily pluging the IAC line that goes to the air filter, it's the easiest to get to. Her husband has a buddy that liked to get loaded and "work" on cars. He made some "adjustments" that I decided to undo (throttle stop screw) . I think the buddy situation is fixed now, but we all know about those kind of things potentially happening to our cars...... :rasp


If it does lowers the idle and you decide to leave it that way, you will loose the idle speed control however. The IAC was not designed to be serviced, but for the most part, it can be done.

Edited by crankcase, Apr 21 2010, 01:55 AM.
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slander
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Johnny Mullet
Apr 18 2010, 05:22 PM
No. You have either a vacuum leak or the coolant passages/hoses to the throttle body are plugged.
Actually, there is an idle adjustment screw on the TBI. Sorry Johnny, not trying to step on toes here.
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Skrilla
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Whats the TBI and where is it?
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Johnny Mullet
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The TBI is the carb looking thing under air cleaner housing.

Slander, the preset idle screw should never be adjusted.
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Skrilla
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So I should leave it alone
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mcmancuso
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TBI idle is set at the factory, and adjusting it is not recommended. :( Problem should be elsewhere unless someone before you changed it there.
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slander
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It shouldn't be adjusted under normal circumstances, no. You always look for contributing factors first, and adjustment last. But, there has been a time or two I have had to adjust the idle.
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DesmondGhostRider
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Is that a challenge?

This is a very delicate problem. The last guy had adjusted mine lower and left a note in the glove box saying so. He said it saved gas...but the car almost died all the time. I raised it up until it would run smoothly at idle then left it alone. Be very careful and write down how far you turned it. I took a video of me doing it so I would know exactly how much I had moved it
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Skrilla
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Ok when I plugged the house the idle went down,so besides plugging the hose how cani fix this?
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