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Discussion Starter #1
My 2004 impala base 3.4 engine has about 101K miles on it and a new IAC motor replaced since the old one for some odd reason, was idling fast(about 1100 RPM) after driving on freeway with A/C running when at a stop and in park. The throttle body is cleaned, been tuned up (new A/C Delco (iridium) plugs and wires), fuel pressure regulator replaced due to seepage, and fuel filter replaced. I replaced the IAC valve with a BWD brand. After I shut off the car and then restart it after a few minutes, the idle speed would go back to normal. It happened yesterday after driving on the freeway at close to 70 mph for about 5 miles distance. What can cause this intermittent problem?
 

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sounds like the tps(throttle position sensor) is getting stuck. Sometimes it throws a code, but sometimes it doesn't. fairly cheap part and easy to replace. but id start there.
 

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Discussion Starter #3
yeah it might be the TPS, but the fact is, There is no Check engine light on.
 

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That's one choice, lets see the other choices of what other people might have to say about my car's issue. Because whenever money is the factor, we all need to make careful decisions.
 

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It only goes on if the voltage signal is incorrect. If it's not returning to 0% that what makes it idle high. If you have a live data scan you can check it. I know auto zone uses an actron scanner with live data. Go in and check it out. I had mine go out twice before. Once with mil on and one without.
 

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Discussion Starter #6
my MIL never came on. I might replace the TPS sensor later on and soon.
 

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fast idle

I have a 2002 Chevy impala and is doing same thing when u get on highway do about 65 or 70 mpg it stays at a idle that fast they replace 02 sensor and temperature control sensor and throttle still doing it and now is believed to be the computer.
 

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My 2004 impala base 3.4 engine has about 101K miles on it and a new IAC motor replaced since the old one for some odd reason, was idling fast(about 1100 RPM) after driving on freeway with A/C running when at a stop and in park. The throttle body is cleaned, been tuned up (new A/C Delco (iridium) plugs and wires), fuel pressure regulator replaced due to seepage, and fuel filter replaced. I replaced the IAC valve with a BWD brand. After I shut off the car and then restart it after a few minutes, the idle speed would go back to normal. It happened yesterday after driving on the freeway at close to 70 mph for about 5 miles distance. What can cause this intermittent problem?
you should always get the genuine GM Sensor. get the ac delco one. 1100 seems to high. mine idles at like 840-850 with the ac on. if you have a scanner it will tell you if the tps is stuck open
 

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May I ask what is a normal idle RPM for the LS? Mine seems to fluctuate between 600-1000. Unless I'm mistaken, it seems that when the motor is warm, its at the high end of the range; and on a cold start, it's at the low end. Idling isn't rough, so I don't really suspect there's any issues, but I'm just curious as to what a normal idle is.
 

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Mr. Handy
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At cold start the rpm's should be higher (around 900-1000) after warm up (a couple minutes) it should drop to around 7-800 or so. If not, there is likely a MAF or IAC valve problem.

Note: Whenever you replace an electrical component (sensor, valve, etc) you should unhook the battery and turn your key to the "on" position. This resets the computer right away. Otherwise you must do a few driving sessions (usually 100mi or so) for the computer to relearn the new sensor or changes made.
 

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I have just been through it all and finally have it fixed. The TPS are potentiometers except on at least recent Ford pickups (probably others now) where a magnetic rotary position sensor is used. Everybody should use this method. The pots wear out at the bottom end where the idle is set simply because the wiper in the pot spends so much time there and vibration wears it out. This is why the magnetic rotary position sensor was developed. Anyway, the pot read zero even up to significant openings, so the engine controller only closes the throttle using the IAC down to where the zero reading begins. Replacing the TPS fixes this. My experience with my car, 2005, is that the IAC get clogged with oil from the PCV system causing low idle which results in a slow shift error and the engine dying. Once this is replaced the TPS becomes the high idle issue. And of course if it leaks the fuel pressure controller needs replaced. The fuel pressure controller goes out because of corrosion, since it was designed before ethanol became common. All of these are trivial problems, and all common and easily solved. The cost of fixing them in newer cars is not significant, though, I will bet nearly all cars still have pots for TPS's and maybe still pickups. It just should not be. I am sure the corrosion problem is solved. The PCV covering the IAC with oil would be trivial to solve, just by routing the PCV downstream of the IAC. Hopefully car makers are listening. In any case, when you buy a car of any sort these issues may be there and you can check ahead of time.
 

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This is RHJ again. the problem came back. The TPS reads zero with a high idle. I am at a loss. Any help.
You are reading the TPS using a scanner?

My instinct is that you have a broken wire in the harness to the TPS, and that this wire is making intermittent contact. Disconnect it and pull the wires out of the loom looking for a break, or a kink or pinch in the wires. If the copper is broken but the insulation is still holding it together, there will usually be a flat spot or kink that gives it away.

Doug

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