On a relatively new to me 99 Trans Am. Started the car a couple of mornings ago and it would not idle. It kept trying to die and would drop below 300 rpm. I drove it about 20 miles. It ran poorly the whole trip. But it would maintain an idle without help after it got hot. When I would let off the throttle, it would drop to 500 rpm and then slowly climb to 750-800 rpm. On the drive home later in the day it started normally and drove normally. While it was acting up, it showed no codes. What gives with this? Thanks.

 

"It showed no codes" -- did you hook up a scanner, or are you basing this on no SES light?

If the latter, scan it and check for pending codes

Best guess:a vacuum leak, or a bad sensor somewhede. Keep driving it; it'll set a code soon enough and send you down the right path.

 

No codes was based on the lack of SES light. I have had a somewhat persistant knock sensor code, however.