Just got a used motor put in my truck two weeks ago. Has 115000 miles on it. Runs real good, except when i come to a stop light, after sitting for maybe a minute or so, sometimes it starts to idle rough, however i can step on the gas and goes like nothing is the problem, OR i can shift it into neutral and it idles fine. Any ideas?

 

Check the plenum. Also, how old are your O2 sensors?

 

My O2 sensor is a little more than one year old... Plenum was replaced already and still looks clean... Anything to do with the throttle body? Thats what im leaning towards.

 

Cleaning the throttle body, and verifying the TPS is doing what it is supposed to wouldn't hurt.

Also check for vacuum leaks.

Did you give the new engine a tune up?

Might wanna try giving the engine a seafoam treatment. (feed it in thru a vacuum line)