very high idle

nesralyrrej · Feb 19, 2015

its a 1990 2.9, I was driving home and I pushed the clutch in to stop at a light and the idle jumped up to about 2k rpms, not a big deal I though, because its done it before and I usually have to tap the gas and it will drop ,so I did and it stayed at 2k. so I took it home, turned it off visually inspected every inch of vacuum line not a crack or hole on one. i know if the iac were sticking i could unplug it and it wouldn't do anything, i unplug mine it just dies. new map, new tps, new intake air temp sensor,im stumped. when i first start it it idles at800, then i press the gas and it stays at 2k, the throttle is closing entirely by the way any help would be superb

RonD · Feb 19, 2015

An engine is an air pump.
If engine is idling with the throttle plate closed and you dump extra fuel in the RPMs will not go up, engine would be running rich, and may even stall from flooding but RPMs would remain the same.

So the only way to raise the RPMs is to let in more air.
Which is why people always suggest a vacuum leak if RPMs are higher than they should be.
May not be that, but you do have extra air coming in.
IAC valve allows air to bypass throttle plate, it is a "controlled" vacuum leak.
If RPMs were at 2,000 and you unplugged the IAC Valves wires, and the engine RPMs dropped to 500 or it stalled then Computer was opening IAC valve for the 2,000 RPMs, OR the IAC valve is failing.
IAC valve uses a Step or stepper motor, this is an electric motor that has "steps" that can be called on by a voltage pulse(voltage morse code).
If computer sends stepper motor a specific pulse the motor will turn to a specific spot(step), if computer sends it a different pulse it will turn to a different spot(step).
I believe these have just over 100 "steps" that can be called/set.

Throttle plate itself may be loose inside, when it is closed and you start the engine it will stay closed, but if you open it slightly it will be sucked open by the higher air flow and won't close again until engine is shut off.
I would look at that.

nesralyrrej · Feb 20, 2015

I replaced the iac to no avail, still idles just as high, the Ecu uses the cts to tell the iac how much air to let in, right?

Haywire6000 · Feb 20, 2015

Have you tried unplugging the tps?

RonD · Feb 20, 2015

nesralyrrej said:
I replaced the iac to no avail, still idles just as high, the Ecu uses the cts to tell the iac how much air to let in, right?

ECU uses the TFI module(distributor sensor) to tell it what the engine RPMs are.
ECU sets the idle based on target RPMs programmed into it at the factory.

ECU uses ECT(engine coolant temp) sensor to set "choke mode" if engine is cold, which does include a higher idle RPM, 1,100, and also richer fuel mix and advanced spark timing.

If your idle is still high after engine is warmed up, unplug the IAC sensor's wires, IAC Valve should then close, idle should drop to 500 or engine may even stall, either is OK.
It means no vacuum leak, but it also means computer is what is setting the high idle

Andy D · Feb 20, 2015

The Rat's high idle was a stiff cable at the throttle body. A spritz of PB Blaster unstuck it

nesralyrrej · Feb 20, 2015

If I unplug the tps and it idles fine, what does that mean? The tps is new

Haywire6000 · Feb 20, 2015

New just means untested. With it running try to manually move the throttle under the hood to the closed position( if it's not already there) if it moves and idles down you have something bound up. If it doesn't move then you more and likely have a bad tps.

nesralyrrej · Feb 21, 2015

But, if I'm not mistaken, doesn't the tps only add fuel, without the throttle plate opening wouldn't it just run super ritch and die?

If it's helpfull, it only does it when the motor is warm and it doesn't do it all the time.

nesralyrrej · Feb 21, 2015

When I unplug the tps, the idle does just drop back to where it should be, so I re replaced it with the old one, and it still does the same thing

very high idle

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