Rough idle when warm

[RED98]

My 1998 3.8 has a problem.. when I start it cold, it's fine, no problem. Sometimes, when I have been driving for a while and it's fully warmed up, I'll park it and let it sit for about 20 minutes, like when I stop at the store. I come back, start it up, and it idles and runs very rough. The rpm's drop down to about 600. It hasn't stalled yet, but it feels like it wants to. Then, after I drive it for a few minutes more, it suddenly quits the rough idle on its own and runs normally. Any ideas about the cause of this, and how to fix it? It's an automatic, and has 173,000 miles. I just had a tune-up done, so the spark plugs and wires are new.

 

[wmburns]

I am thinking this may be evaporative purge related. However, it would make sense to rule out some other things first.

It is vital that there are no vacuum leaks. On a car with such high mileage, a through inspection of all vacuum and vapor hoses needs to be done. Today's cars simply will not work correctly with even the tiniest vacuum leak.

Replace your PCV valve if it hasn't been done already.

Next, review the following post #3

http://www.stangnet.com/mustang-forums/810729-starts-but-dies-idle-help.html

Now for the tough part. If this is an evaporative purge issue, tracking it down is going to be a challenge. It would help to have an ODB2 scanner that can monitor operational data. There is a PID that will let you know when the PCM is calling for an evaporative purge.

Another option is to disconnect the evaporative purge vapor line at the engine. Plug BOTH ends of the line. Drive the car and see if the symptom occurs again. Note, disconnecting the evaporative purge lines will throw a different DTC but its OK because it for testing.

 

[Htxgen]

I am thinking this may be evaporative purge related. However, it would make sense to rule out some other things first.

It is vital that there are no vacuum leaks. On a car with such high mileage, a through inspection of all vacuum and vapor hoses needs to be done. Today's cars simply will not work correctly with even the tiniest vacuum leak.

Replace your PCV valve if it hasn't been done already.

Next, review the following post #3

http://www.stangnet.com/mustang-forums/810729-starts-but-dies-idle-help.html

Now for the tough part. If this is an evaporative purge issue, tracking it down is going to be a challenge. It would help to have an ODB2 scanner that can monitor operational data. There is a PID that will let you know when the PCM is calling for an evaporative purge.

Another option is to disconnect the evaporative purge vapor line at the engine. Plug BOTH ends of the line. Drive the car and see if the symptom occurs again. Note, disconnecting the evaporative purge lines will throw a different DTC but its OK because it for testing.

OP did you ever find the issue?

 

[GysHuizenga]

Also wondering if youve found a solution. My 2000 base model v6 has started doing the same thing and i wanna try to figure it out before i start dumping money into it.