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Discussion Starter · #1 ·
93 engine, mostly stock, I have a high idle. I first thought my throttle cable or my throttle body was sticking, but here is what is happening. It usually idles at about 1000 rpm, while driving the idle will climb to about 1500 and has been as high as 2500 rpm. If I hit a bump, it will go up or down, very erratic. I dont know what to make of it. I checked the cable and throttle body and there seems to be no binding or rough travel, is there another part I need to be looking at? Maybe a vacuum line somewhere? If the idle is high and I am where I can shut the engine off, when I restart the idle is fine, only when I drive it does it do this. I dont want to take it to the shop, but I will if I have to. Any ideas would be greatly appreciated.
Thanks
 

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Get a vacuum gauge and make sure your throttle position sensor is set properly at 0.98v.

Have you tried to manually pull the throttle cable back to see if it changes things ? I had the same problem and it turned out to be the throttle body center section broke and wouldnt return properly.
 

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when its doing it, take a small hammer and tap on the IAC valve. thats probably your problem. then you can clean it with carb cleaner but it'll clog up again after a while. so eventually you'll need to replace it. its easy to replace (2 bolts)
 

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Discussion Starter · #5 ·
I will give that a shot, it didnt do it at all yesterday. Thanks for the tips.
 

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It seems this fast idle problem happens often. My 1987 5.0 5 speed mysteriously started idling at 2200 rpm and nothing seems to help. I've tried many things mentioned here including the cleaning, checking for binding, loose or broken stuff etc. I replaced the IAC since cleaning the old one did nothing. I tried to re-educate the chip by disconnecting the battery for a few days. I disconnected the IAC, reconnected the battery and manually forced a 900 rpm idle for a few minutes. The TPS seems fine. Each time I reconnect the IAC the car starts, idles at over 2000 and doesn't change as it warms. If I turn it off, disconnect the IAC plug and manually set the idle it will remain at 900-1000 as desired. Help?? Something is lying to the IAC? The old and new do the same so I wasted $60 but don't know what to do next. Thanks.
 

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test the TPS, directions r in this forum
 

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Fast idle issues - test some things

Cleaning the IAC is a good idea. Take it off the engine and clean it right. Use TB cleaner and dose it up good. I even attached mine to a variable power supply to check the movement of the valve. Now, assuming you have done this....

Before replacing parts, determine if the IAC is being commanded to open or not. I used sister crimps to add wires to the two wires going to the IAC then connected to a DVM. When idling the voltage will be around 5-6 v. Mine was 5.15v to 5.5v at about 800rpm. When driving you will see the voltage go up proportional to rpms. At 2000rpm I had about 10v. Now, when you press the clutch or go into neutral on an automatic does the voltage go up to about 10v with a corresponding rpm? If so, then it's likely NOT a vaccuum leak but a sensor telling the computer something wrong that makes the computer think it needs to give the IAC 10v.

One possible sensor mentioned many times is the TPS, throttle position sensor. This is driven by the same shaft as the butterfly in the throttle body. It has three wires. One is ground, one is 5v and the other is voltage divider output of something between 0 and 5. Through trial and error I set up a volt meter to measure between ground and the variable output. (You can check this voltage with key forward, engine off.) When throttle was at rest I had 0.94v which matches the voltages mentioned in other threads. During operation I might get 2.5v or so without putting my foot into it.

Now, to see if this is the sensor causing the 10v above take your foot off the gas and see if the TPS voltage drops back to 0.9x while the other goes up. If so, the TPS is probably not the issue. That's where I am at with my '94 5.0L.

Someone mentioned an O2 sensor providing bad data. I intend to take similar measurements and see if they coorelate with the problem.


Roger
 
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