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Discussion Starter #1
For a while now, my 5.0 Mustang has been doing this tempremental high idle thing. Most of the time, it runs normally, but every now and then it decides to idle at high RPM. How high varies, usually 1000 or 1300 or so, but it occasionally does 1500, and one time it decided to idle at 2000(!). It doesn't do anything different when cruising along, but the RPM does jump a little when I shift, like if I shift at 2100, the RPM will go up to 2300 or so before I release the clutch. Normally, the RPMs stay the same or do down slowly when shifting, for a smoother shift, I suppose. It runs normally again if I turn the car off and then restart it. I also notice that when it's in "high idle mode" the RPMs gradually increase when I slow to a stop in neutral. It may drop to 1000 or so rolling in neutral, but it goes up again as the car stops. The really strange part is that this happens seemingly at random. It may do it in the morning or at night, engine warm or cold. It might do it as soon as the car starts, or halfway through a trip. I'm guessing this is some kind of electrical problem, but what could it be? The car has about 170k miles and still runs great (except for this idle thing).
 

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Have you pulled codes?
With that many miles I would also take apart and clean the throttle body and idle air valve.
 

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Pulling the codes and cleaning the idle air controll is the first thing you should do. Also The Throttle position sensor voltage should be checked. Read from the green to black wires, if it is more than 1 volt(say 1.1-1.5) it can cause your problem and not set a code. The voltage should be in the .8-.9 range. If the sensor has never been replaced it is likely your problem.
 

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Discussion Starter #4
Whew! It' s been another busy week over here, but I managed to work on the 'stang a little today. I checked the voltage on the TPS and got .854 volts (throttle closed) I wonder if there is a better way to do it, though - I couldn't get the voltmeter probes to contact anything with it assembled, so I disconnected the little connector just before the sensor, and used wires to make all 3 connections, and then I could measure it. I did pull the codes a while ago when I was having an ignition problem, and it didn't seem to yield any useful information, though I was never sure if I did it right (watching for voltage changes can get tricky). In any case, it might be a good idea to do a little clean-up, though I have to get a new throttle body gasket. I wonder if it might have anything to do with an air leak near the throttle body?
 

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If you've got any 'air', (i.e. vacuum leaks), it will be understandably hard to get a 'good idle'.
First things first. Make sure you DON"T have any vacuum leaks. Then clean throttle body and IAC. Then start car up, (this is after you've tested the tp and it's good). Get the car warm. Shut it off. Disconnect battery for 5 mins. or so. Start it back up, and let it relearn idle strategy. If everything's working correctly, (i.e. base idle is set to specs, proper pcv valve installed, no aftermarket camshaft, etc, etc), the vehicle should relearn idle strategy. my .002
 
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