RPM's Up and Down at IDLE.

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Race_Red_5Oh

Race me if you DARE
#1
#1
Sup guys, having a problem that seems to be getting more and more noticable. At idle my 99 GT with 130k mi, 5 speed, CAI, SCT Tuner, 4.10 gears, ACCEL yellow coilpacks, is suppose to be idling at 670 rpms. it take a while to get down from a Rev, and when it finally gets down to right rpm it then goes up to about 750 and down to about 600. very noticable. Lets say i rev up to 4k. let off the gas completely, my car goes all the way downto about 1100 rpm then creeps very slowly to where it suppose idle at (670). While Driving it seems to be fine or not noticable at least. car pulls hard feels like the power is there,only acts up at idle. i took throttle body off cleaned it. cleaned the air filter. Cleaned most things attached to the throttle body. didnt feel or hear any vacuum or air leaks, my fuel rails got clogged at about 112k (original ones) i had them cleaned and put back on.Any ideas????!! it is bad enough to the point where a guy at a light thought i had Cammed my car with some bigass cams :thumbsdown: HELP ME! Tune up on the car was done by me @ about 110k mi. spark plugs, coilpacks, fuel filter. i run 91 Ocane fuel all the time and my sct tuner is programmed for 91 octane. anything else you need to know just ask. Gonna clean MAF and Sea foam it now (sunday 8pm)
 
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Grkstang

Member Plus
#4
#4
yea you should def check IAC valve...maybe it needs cleaning. It happened to me something similar couple of months ago. I clean IAC , seafoamed and cleaned CAI and throttle body and since then it works perfectly.
 
R

Race_Red_5Oh

Race me if you DARE
#5
#5
OK cleaned everything best as possible. Now the only thing I notice is when I clutch at a light after driving it comes very slowly down to idle from about 1100. So to paint the picture, lets say I'm driving up to a red light @ 40 MPH clutch in (3rd gear) Down Shift to 2nd, release, then clutch in again put tranny into first and roll up to light (clutch still in) rpms sitting at 1100 as I roll to the light. I come to a full stop and rpms come Down very very slowly from 1100 to about 700-800. Is something getting stuck a little open?
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R

Race_Red_5Oh

Race me if you DARE
#7
#7
O OK frostbyte. I actually.never noticed that so maybe me cleaning the iac helped to work properly. Well then I think that's it folks. Thanks for the tips
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Chief Gilly

Chief Gilly

Member Plus
#9
#9
My friend, I had similar problem. I changed IAC and still the same. A few days after not driving the mustang the battery was dead. It was the original battery from 2000. Replaced with new Motorcraft battery and idle problem is gone. I believe it was from the alternator running balls out all the time to try to charge an old battery. I don't know if the IAC affects idle because the alternator is working extra because of beat battery but the problem is gone like I said. So my old IAC is probably good but didn't swap to check.
How old is your battery? If it's a Deca or Optima I would be concerned at the 3 year mark. If Motorcraft or interstate 5-7 year should still be good. Optima batteries are different type then MC or Interstate and don't last as long.
 
onlycodered

onlycodered

Well-Known Member
#10
#10
Chief Gilly said:
My friend, I had similar problem. I changed IAC and still the same. A few days after not driving the mustang the battery was dead. It was the original battery from 2000. Replaced with new Motorcraft battery and idle problem is gone. I believe it was from the alternator running balls out all the time to try to charge an old battery. I don't know if the IAC affects idle because the alternator is working extra because of beat battery but the problem is gone like I said. So my old IAC is probably good but didn't swap to check.
How old is your battery? If it's a Deca or Optima I would be concerned at the 3 year mark. If Motorcraft or interstate 5-7 year should still be good. Optima batteries are different type then MC or Interstate and don't last as long.
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Interesting that you mention that. Member redlawyers had the same issue and replaced his IAC valve and I believe a few months after that had to replace his battery. Could very well be related.
 
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