99 Mustang Gt Won't Idle

Eric Jones · Mar 13, 2017

A few weeks ago I purchased a 99 Mustang GT with a radiator leak. So, naturally, I bought a new radiator and installed it. After the install, the car won't idle. Car starts up fine but then dies immediately unless I give it gas. If I keep trying it then it'll occasionally idle very rough and low—around 800rpm or so.
I can't imagine putting in a new radiator having anything to do with the idling problem, but I still listed it anyway just by some chance it be relevant in some way.

I already replaced the IAC (I've read that is a lot of peoples problem) but it didn't change anything.

wmburns · Mar 14, 2017

Did you disconnect the battery? IF so it would be expected to have idle issues during the "re-learn" process. I has a 96 automatic that was impossible to drive after a battery disconnect until the PCM relearned new idle trim values.

In my case had to let the motor idle for several minutes with the AC off. Then turn the AC on and let idle for several minutes.

Note. IF the AC is low on Freon or the AC compressor is short cycling, EXPECT problems with idle with the AC on as the short cycle will prevent the proper re-leaning.

Here's some more information on how to trouble shoot idle/IAC issues.

https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/
http://www.allfordmustangs.com/foru...-dieing-idle-idling-rough-14.html#post2260649

Eric Jones · Mar 14, 2017

wmburns said:
Did you disconnect the battery? IF so it would be expected to have idle issues during the "re-learn" process. I has a 96 automatic that was impossible to drive after a battery disconnect until the PCM relearned new idle trim values.

In my case had to let the motor idle for several minutes with the AC off. Then turn the AC on and let idle for several minutes.

Note. IF the AC is low on Freon or the AC compressor is short cycling, EXPECT problems with idle with the AC on as the short cycle will prevent the proper re-leaning.

I've never actually heard of the car having to "re - learn" to idle. It makes sense, though. Yes, I did disconnect the battery for a few hours. But I think it'd be impossible/nearly impossible to do the relearn procedure as the car won't idle at all. Maybe every 2 out of 10 tries it'll idle real unstable for a few seconds then die.

trinity_gt · Mar 14, 2017

If you touched any of the intake ducting, the airbox, MAF sensor etc during the rad change check to make sure you plugged everything back in correctly and tightened all clamps. It almost sounds like you have a sizeable vacuum leak ahead of the throttlebody.

wmburns · Mar 15, 2017

Has the battery been tested? Are the terminals clean and tight?

Has the alternator been tested for correct output and excessive AC ripple (bad diode)? No excuse to skip this step as almost all autoparts stores will test for free. Bottom line. Today's cars simply will not run right without a strong battery and charging system.

Confirm that the car did idle correctly before the work was done.

Confirm that the testing is being done with the AC off.

Here's some more information on how to trouble shoot idle/IAC issues.

https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/
http://www.allfordmustangs.com/foru...-dieing-idle-idling-rough-14.html#post2260649

99 Mustang Gt Won't Idle

Eric Jones

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99 Mustang Gt Won't Idle

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