I have a 1996 mustang gt 4.6 (it has a cai, pullys, trick flow plenum, x pipe cat backs, and a few other small things) a couple months ago I put a bbk cai on to replace the old ratty cai that was on before and for awhile it was fine. A week later it started idling at about 2k-2500 and sometimes past 3k, I replaced the tps two times, the iac, egr, maf, checked for air leaks, cleaned the tb, put the old cai back on and none of that worked. The only time I ever fixed it was when I reset the computer by disconnecting the battery and pulling the fuse but slowly the high idle would come back. I also bought a bama tuner and the made everything normal for a bit the the idle came back again so I returned it back tO the old settings before the tuner and it went back to normal again, but again for only a couple days. I've researched this like crazy and tried everything and still have not fixed it. So help would be much appreciated.
High Idle! Need Help!
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wmburns
SN Certified Technician
Here's some information on how to trouble shoot IAC/idle issues.
Troubleshoot IAC idle problems 1996-2004 | Mustang Forums at StangNet
Note the part about the role that the TPS plays in idle determination. Also note HOW the TPS "ratch" function works and the importance of having a STABLE closed throttle position. The value of the TPS does not matter for the 96+ as long as the TPS sends the SAME value when the throttle is closed.
Since you have an aftermarket throttle body, the correct setting of the idle stop screw is vital. Were the manufacturer's recommended procedures followed?
How about the throttle linkage? Does it move freely with no binding? Are the throttle return springs in place? Does the throttle rest squarely on the idle stop screw?
Does the speedometer work and is it accurate? Note that the PCM uses the car's speed to decide when to switch from fast idle to slow idle. The car needs to be stopped before the slow idle will be selected.
Again, are you positive there aren't any vacuum leaks? Even a tiny vacuum leak can cause problems.
Troubleshoot IAC idle problems 1996-2004 | Mustang Forums at StangNet
Note the part about the role that the TPS plays in idle determination. Also note HOW the TPS "ratch" function works and the importance of having a STABLE closed throttle position. The value of the TPS does not matter for the 96+ as long as the TPS sends the SAME value when the throttle is closed.
Since you have an aftermarket throttle body, the correct setting of the idle stop screw is vital. Were the manufacturer's recommended procedures followed?
How about the throttle linkage? Does it move freely with no binding? Are the throttle return springs in place? Does the throttle rest squarely on the idle stop screw?
Does the speedometer work and is it accurate? Note that the PCM uses the car's speed to decide when to switch from fast idle to slow idle. The car needs to be stopped before the slow idle will be selected.
Again, are you positive there aren't any vacuum leaks? Even a tiny vacuum leak can cause problems.
Last edited:
usaf_branham
15 Year Member
well, if its not the iac, leaks in the cai, mal-adjustment of the tb, etc... have you considered that the plastic intake manifold may have cracked somewhere? i find it hard to believe that you have a high idle problem without a vacuum leak...
Not a after market tb, it's stock. Doubt it's a linkage or binding because it will idle up and down almost like the computer is thinking it's idling fine. When I pull the battery cables and pull the EEC fuse everything goes back to normal for a couple days. Positive there are no air leaks. Speedo is working fine. Been talking to some people who have had the problem and they agreed with me and said it's probably the o2 sensors. So working on fixing those
wmburns
SN Certified Technician
Just wondering. If there were any current DTC codes, you would have posted them right?
How's this car's gas mileage?
Is the fuel pressure regulator intake vacuum reference line connected and leak free?
Did not state the result of the IAC basic function test where the IAC electrical connector is disconnected. Did the RPM's drop or the motor die? If not expect to have idle problems.
Would love to hear if new O2 sensors fix this problem.
How's this car's gas mileage?
Is the fuel pressure regulator intake vacuum reference line connected and leak free?
Did not state the result of the IAC basic function test where the IAC electrical connector is disconnected. Did the RPM's drop or the motor die? If not expect to have idle problems.
Would love to hear if new O2 sensors fix this problem.
Yeah I did the iac test and the car died right away. O2 sensor eliminators are working fine, only got two tho haha the car will idle high for awhile but now as it warms up it acts normal idling at about 800
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