I have a 1998 Lesabre with just shy of 160k miles. When I was given the car it was giving a P0171 code, which I believe I have now solved. I have replaced the PCV valve, plugs & wires, battery, mass air flow sensor, upstream o2 sensor, alternator, purge valve solenoid and the idle air control (IAC) valve. Now when I drive the car for a while, let it sit for 15-20 minutes, I start it back up and the car wants to stall when put into gear. I am not getting a check engine light. Any ideas as to what my problem might be?

Get us those codes. We can play guessing games all day long as to what'* wrong.

The car is not throwing any codes at this time.

Does it jerk?

The car does jerk sometimes when its about to stall out. The car idles fine on cold start up, shifts fine when first started. The stalling & jerking happens after I have driven it a while and let it sit for 15-20 minutes and restart it.

IAC came to mind, but you replaced it...

Pull the vac line off your fuel pressure regulator, and check for fuel in the line.....if present replace regulator.....

Are you actually checking or just going by the CEL?

I used a scan tool. Checked for codes again today, no codes were found, but still had the stalling problem after driving the car for 15 minutes, letting it sit for 20 minutes, starting it and putting it in gear.

When you replaced the IAC valve, did you clean inside where the pintle seals?

I did not clean inside where the pintle seals. What do you recommend using to clean the area?