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1979-2004 V6 Mustang Tech Discussions

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Unread 09-04-2009   #1 (permalink)
cover3 is offline Apprentice

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Default 1999 Mustang RPM idle problem

This is hard to explain, but sometimes idling my 99 v6 will start like misfiring/backfiring and idle will flucate a bit. On the highway at like 40-50+ if i coast in neutral/clucth in the rpm will flucate from 900 to 500 and sometimes the car will even shut off but if i roll with the clutch out in gear and the rpm up it everythings fine. Also i find that at lower rpm like 1000-3500 or so i find the car isnt responsive like it should be almost like it takes a second or so for the engine to get its fuel but at higher rpm it seems fine. I am just wondering what could be a cause for this? Sorry, i am no good at explaining things.

thanks!
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Unread 09-04-2009   #2 (permalink)
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Quote:
Originally Posted by cover3 View Post
This is hard to explain, but sometimes idling my 99 v6 will start like misfiring/backfiring and idle will flucate a bit. On the highway at like 40-50+ if i coast in neutral/clucth in the rpm will flucate from 900 to 500 and sometimes the car will even shut off but if i roll with the clutch out in gear and the rpm up it everythings fine. Also i find that at lower rpm like 1000-3500 or so i find the car isnt responsive like it should be almost like it takes a second or so for the engine to get its fuel but at higher rpm it seems fine. I am just wondering what could be a cause for this? Sorry, i am no good at explaining things.

thanks!
i was having an issue with that for a little while. tried cleaning the MAF, it didnt help. finally a CEL popped up, p0171 & p0174. the codes would come and go intermittently. pulled off my IAC and bench tested it, found out that it would sometimes stick wide open because it was full of carbon deposits. so i cleaned it out real good with some brake clean and let it sit for a while to dry, then put it back together. the result was a perfect idle.
these should to be cleaned regurlarly every 30k miles or so once you start getting higher miles on your car (around 100k + miles).

symptoms of a dirty/sticking IAC include; hard starting, high idle, bouncy idle, no idle at all, and stalling when slowing down/coming to a stop. it can also cause the car to bog down on low-end acceleration. if it starts sticking open a lot, it'll set a CEL indicating a lean condition on both banks.
well anyways, it cleared my idle up almost perfectly.

here's your IAC,
1999 Mustang RPM idle problem-iac-3.jpg


unplug this connector,
1999 Mustang RPM idle problem-iac-1.jpg

and loosen these (2) 10mm bolts.
1999 Mustang RPM idle problem-iac-2.jpg

and be careful with the gasket, dont break it or lose it.
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Unread 09-04-2009   #3 (permalink)
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heres what the IAC valve looks like once you unbolt it, take some intake cleaner/brake cleaner and spray everything down to clean all the carbon deposits out of it.

1999 Mustang RPM idle problem-iac-close.jpg

this is what it looked like AFTER cleaning, so you can imagine how dirty it was before. :hihi:

1999 Mustang RPM idle problem-iac-closer.jpg


if it gets better, but still isnt quite perfect, my suggestion would be to replace it.
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-2000 3.8L TKO600, 1.8 60ft, 8.71 @ 81mph
-1988 Jeep XJ, bone stock 2wd
-1994 Jeep XJ 4wd, 3" lift, 31" tires.. rust in peace..
-1997 Jeep XJ 4wd, 7" lift, 35" tires, front/rear lockers

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Unread 09-04-2009   #4 (permalink)
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the car has 150 000 miles(only 12 000 miles on engine), i will clean it out tomorrow and hopefully i will see a difference, ill also get new plugs/wires and fuel filter. Will post the results.
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Unread 09-04-2009   #5 (permalink)
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Also, in the morning/day when the car is first started, starting the car is fine but first starting off moving it almost shuts off boging need to WOT to keep it running just for a second but then its perfect for the rest of the day in that case. I also dont know why its only at high speeds why the rpms flucate coasting and not around 30-40km coasting?
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Unread 09-28-2009   #6 (permalink)
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Cleaned the IAC(wasnt that dirty) and seafomed the car and is still the same? What else could be causing this? When the car is cold it has alot better throttle response and feels like what the car should feel like, until it warms up a bit then it gets sluggish again.
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Unread 10-08-2009   #7 (permalink)
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Any ideas? Id like to resolve this problem.
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Unread 10-11-2009   #8 (permalink)
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Check the MAF Sensor on the Air Intake, there will be two elements on the inside and make sure they are clean, not dirty. If they are dirty go to the parts store and get a MAF Sensor cleaner spray and clean it out.

Or it could also be a plugged EGR tube. Might want to check that as well.
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