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Misfire when AC is on but getting worse

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My 2002 DOHC has 128k miles and has been a great car. About a year ago, it would stumble at idle when the AC was on. Over the last year it progressed so now it stumbles at idle whether AC is on or not. Under acceleration, it now stumbles and feels like a misfire. It only happens when the car is at operating temp. It's NOT affected by rain. I inspected the EGR valve and it closes correctly, nevertheless, I replaced the DPFE and EGR vacuum solenoid & IAC. No change. Plugs are less than 10k old. I just removed the coil and tested the four connection leads for resistance. Using the red lead as positive, all three leads achieve the same impedance. Using an ohm meter, I ohm-ed coil pole to pole and all three had the same resistance with .01 differential. I found no cracks on the coil. Wires ohm-ed ok and so did the plugs. I am about to replace the coil for no good reason. 

As an after thought, the alt failed on this DOHC and we all know what a lousy project that is. Could I have damaged something getting it in or out? I used the rotation method to remove it.  Actually, the first & second replacement rebuilt alts failed too so the vendor suggested I buy a new alt for number three. That's been in there for 3 months so here's hoping. Nevertheless, the alt has been in and out 4 times so perhaps I broke something else. Ideas welcome.

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Starting point would be to check for codes and read the "long term fuel trims". LTFT at idle high but not high under power show a vacuum leak. Leaks of vacuum or fresh air inlet to the two valve covers will make for poor idle and hesitation. Fuel trims are measured in % +/- zero. 10% high or low is OK if it is there in idle and under modest power are OK. Much higher at idle indicates a vacuum leak.

Poor PCV performance will lead to idle issues and hesitation. New OE part at 100K miles is very important. My '03 DOHC stuck closed at ~110K and made for very low idle speed. That also made water and oil in the intake accordion. These cars are also common fail is the vacuum elbow from the bottom of the intake to the PCV. Redesigned part Motorcraft fixes this part fail.


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