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for the past year i have been dealing with this problem. when at a stop lite the engine idle will go up and down. I have had it in the dealership they can't figure it out. I've done a full tune up. I've checked for vacuum leaks Their are none. I also have replaced the IAC and MAF sensor as well as the throttle body and still the same problems does anybody have any ideas
 

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Have you cleaned the TB with TB cleaner?

Possible vacuum leak. Use a scan tool to look at long term fuel trims on both banks. Higher positive trims at idle that drop toward zero with a load on the engine = vacuum leak.
 

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Have you cleaned the TB with TB cleaner?

Possible vacuum leak. Use a scan tool to look at long term fuel trims on both banks. Higher positive trims at idle that drop toward zero with a load on the engine = vacuum leak.
once again no one is seeing my original post. their ARE " NO" vacuum leaks! and the replacement parts were OEM = Motorcraft=FORD
 

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for the past year i have been dealing with this problem. when at a stop lite the engine idle will go up and down. I have had it in the dealership they can't figure it out. I've done a full tune up. I've checked for vacuum leaks Their are none. I also have replaced the IAC and MAF sensor as well as the throttle body and still the same problems does anybody have any ideas
Something's loading your engine at idle. My ex wife's 1990 Taurus, when the AC compressor went bad, would barely run at idle, because the compressor was dragging on the engine. So... That would be my suggestion. Check for something creating excess drag. What ever it is, though, it's not doing it constantly.
 

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Something's loading your engine at idle. My ex wife's 1990 Taurus, when the AC compressor went bad, would barely run at idle, because the compressor was dragging on the engine. So... That would be my suggestion. Check for something creating excess drag. What ever it is, though, it's not doing it constantly.
No the A/C is never in use it's winter. This is every time i sit at a stop sign or light.
 

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How did you check for vac leaks? The "spray carb cleaner/ propane / brake cleaner around and watch for idle speed change" is not the best way to locate vac leaks on a modern engine. Even a very small leak will cause issues. LTFTs and / or a smoke test are the accepted methods for confirming a vac leak and finding the leaks location on a modern engine.
 

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How did you check for vac leaks? The "spray carb cleaner/ propane / brake cleaner around and watch for idle speed change" is not the best way to locate vac leaks on a modern engine. Even a very small leak will cause issues. LTFTs and / or a smoke test are the accepted methods for confirming a vac leak and finding the leaks location on a modern engine.
how about the FORD Dealer
 

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Wrong Wrong! my A/C is not in use the switch inside is off... the clutch is NOT engaged nor turning ..
Since about 1990, vehicles with A/C have run the compressor when the air selector is set to defrost or floor/defrost. My wife's Taurus did that. My 2004 Focus did that. The 1998 Sable that we had for a while did that. Running A/C while in defrost mode removes humidity from the interior of the car, and helps to keep the windows clear. None of the Fords that I've had (or my Honda) turned on the A/C light while this was happening, but the 2003 Pontiac Sunfire did (it also ran the A/C compressor while in defrost or floor/defrost).

So, if you have your heater set on defrost or floor/defrost (I always do in the winter), your A/C is running, whether you know it or not, and whether you believe it or not. The only way to disable that is to pull the A/C fuse, disconnect the wire on the back of the climate control system, or unplug the compressor.
 

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No the A/C is never in use it's winter. This is every time i sit at a stop sign or light.
A/C isn't the only thing that causes drag on the engine. There are other belt driven accessories on the front of your engine, like the power steering pump, alternator, water pump, and even a tensioner pulley. Unfortunately, a bad Torque Converter could also cause drag on the engine (sort of like letting the clutch out on a manual transmission).
 

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A/C fuse often also is the keep alive for the PCM. Better pull the relay. The PCM adds throttle equivalent through the IAC in the expectation the A/C pulls the normal load. If the A/C is low but still runs, it draws low power and the added throttle will cause the rpm to rise but the A/C may still work. If the PCM thinks the A/C is on, it adds speed no matter what actually happens. PMC also adds throttle when you put it in gear, and when the PS pressure rises due to turning the wheel. PCM is blind.
-chart-
 

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A/C fuse often also is the keep alive for the PCM. Better pull the relay. The PCM adds throttle equivalent through the IAC in the expectation the A/C pulls the normal load. If the A/C is low but still runs, it draws low power and the added throttle will cause the rpm to rise but the A/C may still work. If the PCM thinks the A/C is on, it adds speed no matter what actually happens. PMC also adds throttle when you put it in gear, and when the PS pressure rises due to turning the wheel. PCM is blind.
-chart-
I have been working on cars for 50yrs when A/C is off it's off. my problem is at a traffic light.. rpms drop 100 plus rpm and goes up above normal idle by 100 rpm. this isn't an A/C nor PCM issue.
 

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I have been working on cars for 50yrs when A/C is off it's off. my problem is at a traffic light.. rpms drop 100 plus rpm and goes up above normal idle by 100 rpm. this isn't an A/C nor PCM issue.
If that's the case, then explain why I could feel the A/C cut in/out on all of my post 1990 vehicles with working A/C?
 

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A/C fuse often also is the keep alive for the PCM. Better pull the relay. The PCM adds throttle equivalent through the IAC in the expectation the A/C pulls the normal load. If the A/C is low but still runs, it draws low power and the added throttle will cause the rpm to rise but the A/C may still work. If the PCM thinks the A/C is on, it adds speed no matter what actually happens. PMC also adds throttle when you put it in gear, and when the PS pressure rises due to turning the wheel. PCM is blind.
-chart-
Oh. I didn't realize that (about the a/c fuse being the keep alive for the PCM). Thanks for pointing it out.
 

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Discussion Starter #19
If that's the case, then explain why I could feel the A/C cut in/out on all of my post 1990 vehicles with working A/C?
I guess you don't know how your cars A/C really works. there are two sensors one on the low pressure line and one on the high pressure hose these switches are set for certain pressures. these two sensors "switches" turn on and off the A/C compressor it's called "cycling" it keeps the A/C from icing up. read an auto A/C book on how the system works and you can clear the PCM by undoing the battery cable for 30 minutes.
 

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I guess you don't know how your cars A/C really works. there are two sensors one on the low pressure line and one on the high pressure hose these switches are set for certain pressures. these two sensors "switches" turn on and off the A/C compressor it's called "cycling" it keeps the A/C from icing up. read an auto A/C book on how the system works and you can clear the PCM by undoing the battery cable for 30 minutes.
I know how my A/C <censored> works, thanks. That wasn't my <censored> point.

Here, let me spell it out for you: If my A/C is not turned on, then why is my compressor cycling? See? It's cycling because even though I didn't press the "on" button, or turn the dial to "A/C" or "Max A/C" (the more efficient setting), my compressor is cycling because my air selector (or what ever you want to call it) is set on DEFROST or FLOOR/DEFROST (MIX). Cars have been doing this since 1990. It's automatic.

Now, answer me this: If you've been working on cars for 50 years as you claim, then how the <censored> can you not know about something that has been a feature in cars for 30 years?
 
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