Weird Engine Issue

[SteedaGT9150]

when i come to a stop, occasionally my rpms will drop to about 500rpm, and it will either kick back up or die. whats up with this? just replaced the plugs, coil packs, and fuel filter. it has been doing it since before the tune up as well. thanks guys

 

[wmburns]

IAC quick tests.

Manual or Automatic?

Have you disconnected the battery recently? It can take a while for the car to re-learn the idle speed. There is a procedure to speed up the process. Idle the car for 1 minute AC off. Turn the AC on and idle for 1 minute.

Just to rule out easy fixes, try cleaning the MAF with MAF cleaner (if not already done during the tune up).

My first thought is the IAC is not adjusting the idle speed. A quick test is to disconnect the electrical connector to the IAC, if the motor dies, it is at least working at some level. If you have a ODB2 scanner, monitor the IAC percent. Vary the engine load (AC on/off rock steering wheel) and watch to see if the IAC percentage changes. Also, if the IAC percentage is < 10% or > 90%, this could indicate a problem (possible vacuum leak).

Many ppl recommend against cleaning the IAC. Consider a new one.

If automatic and the lock up torque converter does not release as it should, this will stall the motor.

If Manual, if the clutch does not fully release, this could stall the motor.

 

[SteedaGT9150]

its a manual, and the po put in a new IAC in March of '07

 

[fast97gt]

well it is september of 09 so how long do you think it (the iac) should last. pull your pugs and double check the gap, sounds like theres to much gap. if the new iac dont fix the problem.

 

[trombonedemon]

Wmburns beat me to it, your symptoms sound sound identical to mine, I just replaced the IAC and no more problems, just food for thought! Don't know how many miles they should get, but sounds like my symptoms, good luck.Stangnet

 

[merc123]

Didn't this used to be a common issue on '01-'04 model mustang where it would just up and die when coming to a stop with the clutch depressed? I know mine's done it 3 times since I've had it. The first time it only had 30,000 miles.

 

[SteedaGT9150]

plugs are fine. gapped at .54. ill check the iac tomorrow. ill unhook the battery as well.

 

[dustang50]

When was the last time you changed the fuel filter?

 

[SteedaGT9150]

When was the last time you changed the fuel filter?

saturday

 

[squeak93]

Do you have any vac leaks? Maybe a cracked rubber hose on the pcv system or vac lines to the egr. Sounds like it could be a vac leak as well. Unhook your iac motor and see if your idle changes. If no change or the car doesn't die right away, then your iac is bad. If it does die or try to when you unplug it, then start looking at all your rubber hoses and vac lines.

 

[wmburns]

Regarding the short life of the IAC. Suggestion. Look at the IAC for blow-by (or EGR) fouling. Sometimes modified motors or high mileage motor can have more PCV blow-by than normal. Maybe that is fouling your IAC reducing its life.

+1 on look close for vac leaks

 

[Kilgore Trout]

saturday

Clean IAC, reset PCM.

 

[Gixxed]

did you have your a/c on

 

[Kilgore Trout]

Yeah I was also thinking low freon but usually low freon will cause idle to bounce but not cause the car to actually die...

Also, asking people if their a/c is on can be problematic because people do not understand what positions the compressor runs in. The compressor runs in all positions except the ones immediately to the left and right of off (straight up).

 

[SteedaGT9150]

ac wasnt on. i just unhooked the battery, well see what happens next time i go somewhere.

 

[wmburns]

Procedure after battery removal

Reminder, after removal of the battery all of the "learned" engine settings are forgotten. This may make your IAC idle problem worse in the short run (the IAC position is fined tuned by the PCM).

To speed up the IAC idle speed learning, idle the engine for 1 full minute with the AC off. Then turn the AC on for a full minute.

I had a 96 that was impossible to idle after removing the battery. It would die just as soon as you took your foot off of the gas. It would run fine after following the idle procedure. Otherwise, it was an absolute bear to drive until the the idle settings were "learned".