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Old 06-02-2016, 06:30 PM   #1
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Struggling with idle

Good evening,

I am sorry for putting up a thread that has been beaten to death, but the search function didn't really show me anything new. I have been experiencing a rough idle for about four - five months now and have tried and replaced so much that I'm beginning to go insane. Sitting at idle the car feels like it has a vacuum leak and when accessories are turned on it drops significantly, sometimes dies, and then goes right back to the crappy idle. Under throttle its completely fine and pulls all the way up without issue. So far I have replaced all of the following: IAC (motorcraft 3x), TPS , MAF, Vacuum solenoid, EGR, throttle body, PCV, all of the tubes connecting the valve covers to the intake (including adding a JLT oil separator), handheld can tune, dyno tune (fastlane), upper plenum, header gaskets, intake manifold, alternator (PA performance 130amp) and the vacuum line that runs into the passenger side inner fender from the back of the plenum. I have tried spraying carb cleaner on different spots of the engine and traced every single inch of vacuum lines but had no ideas. I am at the point that I am about to rip the hair from my head it is driving me insane. The modifications to the car include: JLT RAI, Kooks LT/catted x, flowmaster cat back, FRPP intake manifold (stock replacement from LMR) Trickflow plenum, Trickflow 75 mm TB, dyno tuned by Fastlane Performance in Benson, NC. There are only 80 thousand miles on the car and it was completely stock when I bought it. Again I apologize for the super common topic and I really appreciate any help .

V/R,
Matt
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Old 06-02-2016, 11:42 PM   #2
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Have the spark plugs been replaced?
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Old 06-03-2016, 12:16 AM   #3
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Man, I thought my idle issue was frustrating and it lasted about 2 weeks or so. Mine turned out to be the IAC, bought a store brand one and it was junk. Took the one off my 96 GT and put it on my 03 and it fixed it right away and its actually a different IAC and part #. It has the little breather valve on the top of it. I actually read a couple instances where people used this type of IAC on their 02 and up cars and it worked when the other wouldnt for some reason?? IDK. I did all the same stuff as you and then did this and it fixed it. No codes I assume? I assume you have tried the throttle relearn process too of course. It may be time to take it in instead of throwing more parts at it? I know that sucks and I hate that too. It could also be COP related, maybe get a spring and boot set and rebuild your COPs, probably not a bad idea if they havent been done yet anyway.

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Old 06-03-2016, 05:31 AM   #4
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I did the plugs and COP's once with motorcraft and again with MSD. I tried a trick that someone had posted using a pre-PI IAC and it made it worse. I've even tried used IAC's from the pick and pull. I'm actually figured I was getting to the point where I'm going to have to take it in, but thought I'd ask the old interweb before I took that shot to my pride. Bummer. Thank you again for all the help.

V/R,

Matt


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Old 06-03-2016, 09:03 AM   #5
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Quote:
Originally Posted by mattranchero View Post
I did the plugs and COP's once with motorcraft and again with MSD. I tried a trick that someone had posted using a pre-PI IAC and it made it worse. I've even tried used IAC's from the pick and pull. I'm actually figured I was getting to the point where I'm going to have to take it in, but thought I'd ask the old interweb before I took that shot to my pride. Bummer. Thank you again for all the help.

V/R,

Matt


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You did the COPS and the plugs? And are the MSD COPS what you're currently running?

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Old 06-03-2016, 11:25 AM   #6
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Do a compression test. There may be a weak cylinder.

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Old 06-03-2016, 01:42 PM   #7
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Yes I'm running the MSD COP's now with Motorcraft plugs. I had a compression test and bore inspection done prior to the dyno tune to make sure it wouldn't blow up and it was all normal.


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Old 06-03-2016, 02:06 PM   #8
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The Motorcraft COPS are superior to the MSD. You should probably switch back to them at some point.

Was the idle problem happening before the tune?
Or did it start happening after the dyno tune was completed?
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Old 06-03-2016, 05:23 PM   #9
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it was happening when I bought the car. At the time I had just chocked it up to the crappy weld job he po had done on the replacement cats he welded in. I'll try the Motorcraft COP's again and see if it improves. Thank you again to everyone for the help.


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Old 06-03-2016, 10:47 PM   #10
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The Motorcraft COPS are superior to the MSD. You should probably switch back to them at some point.

Was the idle problem happening before the tune?
Or did it start happening after the dyno tune was completed?
I agree with the OEM COP statement. My car came with Accel COPs from the previous owner and I swear the car doesnt start or run as crisp as it should. My 96 GT with coil packs that are OEM replacements and Motorcraft plugs is much snapier upon start up like the other should be. It has no issues running at all it just isnt as crisp as I think it could or should be.

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