2002 gt, 76k miles, mods listed in sig, my normal idle speed was around 780, but one day it changed to around 1000rpm. its been pretty constant between 1000-1100rpm for about a week now, but today it started to hang close to 1500rom until i slightly let the clutch out while in gear and it will drop to 1100rpm. i tried cleaning the IAC, no change. i tried unplugging the IAC and the motor sits at 1100rpm. i looked through all the live data section on the DiabloSport and couldnt find anything about the IAC Duty Cylcle, and before i replace the IAC i want to make sure thats the problem. Ive checked for vacuum leaks but cant find any. i have the restriction plate installed, but without it there is no change except that the rpm's hang much longer. any ideas? thanks guys.
high idle, please help
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wmburns
SN Certified Technician
First allways check all of the fuses. Esp fuse F2.8 in the CJB for the 1999-2004 MY.
Disconnection the IAC and no change means bad IAC, Throttle body set screw too high allowiing too much air to pass, or vacuum leak. The idle must drop when the IAC is disconnected.
Have you altered the TB set screw in an attempt to resolve this problem? If so, likely too high. Remember that the IAC can only ADD air (can not remove any).
Disconnection the IAC and no change means bad IAC, Throttle body set screw too high allowiing too much air to pass, or vacuum leak. The idle must drop when the IAC is disconnected.
Have you altered the TB set screw in an attempt to resolve this problem? If so, likely too high. Remember that the IAC can only ADD air (can not remove any).
no i have not altered anything except for adding the restriction plate, but when i removed it there was no change. where would be a likely vacuum leak? i have looked at every visible line and they all seem to be intact, but there is one branching off the rear of the plenum, looks to be about 3/8 inch diameter and runs across the passenger side fire wall and drops down behind the pass. shock, that looks in poor condition. it still seems to have a good connection, but maybe thats the leak? my first thought was bad IAC because in the past i was able to adjust the idle speed via my DiabloSport but no change made on the tuner works anymore. i just want to be sure before i spend $100 on something. I am a starving college student so every penny counts. thanks for the help so far.
wmburns
SN Certified Technician
The vaccum leak has to be AFTER the throttle body. PCV line or the like. Today's cars will NOT run right with even the smallest vacuum leak. Do yourself a favor and replace any/all lines that are not in tip top shape.
There are two ways to do this. Take your best guess and change some parts. Or do the tests and POST the RESULTS.
Have you checked all of the fuses?
If the car has been running fine and then all of a sudden, idles high, as stated before, the odds favor either a blown fuse, bad IAC, bad TPS, or Vaccum leak.
Do you have an ODB2 scanner? If not, narrowing this down is not going to be easy. It is vital to confirm that the TP-MODE PID is "CT" when the throttle is actually closed. If not, the TPS is bad.
But consider that you have already stated that disconnecting the electrical connector to the IAC does NOT reduce the idle at all. IMO, this makes a blown fuse or a bad IAC near the top of the list. But as stated before, a vacuum leak should be ruled out.
There are two ways to do this. Take your best guess and change some parts. Or do the tests and POST the RESULTS.
Have you checked all of the fuses?
If the car has been running fine and then all of a sudden, idles high, as stated before, the odds favor either a blown fuse, bad IAC, bad TPS, or Vaccum leak.
Do you have an ODB2 scanner? If not, narrowing this down is not going to be easy. It is vital to confirm that the TP-MODE PID is "CT" when the throttle is actually closed. If not, the TPS is bad.
But consider that you have already stated that disconnecting the electrical connector to the IAC does NOT reduce the idle at all. IMO, this makes a blown fuse or a bad IAC near the top of the list. But as stated before, a vacuum leak should be ruled out.
ok, everything seems logical, and im going to try to find a guy to swap IAC's with to see if thats the problem too (like i said, starving college student)
is there a more accurate way to test for vacuum leaks than looking for old tubing? everything is connected as it should be, but like i stated above, there is one line behind the plenum that looks like it could be an issue. i also noticed that the hose from the passenger side PCV has after market clamps (zip ties
) holding the hose on. would a leak from there cause an issue?
thanks
is there a more accurate way to test for vacuum leaks than looking for old tubing? everything is connected as it should be, but like i stated above, there is one line behind the plenum that looks like it could be an issue. i also noticed that the hose from the passenger side PCV has after market clamps (zip ties
) holding the hose on. would a leak from there cause an issue? thanks
trinity_gt
10 Year Member
A quick way to find a leak is to get a can of "Quickstart" (ether) and spritz some on vacuum lines, fittings, TB gaskets, inlet tubes and the like while the engine is idling. If the engine ingests any of this ether you'll hear a definite change in idle speed for a second and will have found your leak.
As wmburns notes, the IAC can't add air, it simple throttles a bypass opening. If the idle is 1100 with the IAC unplugged I'm going to wager the IAC is not the problem. More likely is that air is finding an alternate path (e.g. a vacuum leak.)
Have you ever cleaned the throttle body? Gone in there with a can of carb-cleaner and a toothbrush to make the throttle plate and bore all spiffy and clean?
As wmburns notes, the IAC can't add air, it simple throttles a bypass opening. If the idle is 1100 with the IAC unplugged I'm going to wager the IAC is not the problem. More likely is that air is finding an alternate path (e.g. a vacuum leak.)
Have you ever cleaned the throttle body? Gone in there with a can of carb-cleaner and a toothbrush to make the throttle plate and bore all spiffy and clean?
trinity_gt
10 Year Member
No, don't do that: It was a question, not a suggestion.
If you clean the throttle body you stand a chance of removing a special sealant that Ford put in there to seal the throttle plate to the bore when closed, thus allowing the IAC valve the job of metering idle air. If that seal is broken or removed, air will leak past the plate and cause the idle to increase out of the control of the IAC valve.
Do NOT clean the throttle body. Per the warning in the Ford shop manual: "CAUTION: The throttle body bore and plate area have a special coating and cannot be cleaned."
well crap, i wish i had read that before i cleaned the throttle body.... i used throttle body cleaner that protects the teflon type coating so i hope i didnt screw something up. however, as i was cleaning the TB, i notice that the soft line connecting the passenger PCV to the intake had two significant cuts in it. i replaced it with 5/8 inch heater tubing and a couple hose clamps and its good as new. im going to test the functionality of the IAC later today with the adjustment on my DiabloSport to see if everything is back to normal. thanks for the advice guys.
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