ok, i have a quick question for you guys. i have a 1990 firebird with the 5.0 TBI in it. for some reason it has a pretty rough idle, has had it since I bought it. but heres the thing, it idles smoother in drive when the rpms drop down a bit, than it does in park. anyone know what might cause this? oh, by the way, it just got a full tuneup. any ideas?

 

From those symptoms, it could be engine mount, transmission mount, bad sensor, sometimes even if they are brand new a spark plug can break or crack, could be somthing connected to the tb, could possably be a torque converter.

 

is it throwing any codes. when you start the car does the service engine light come on??? if it doesn't the bulb is blown and diagnosing afterwards would steer you towards an answer. otherwise it's going to be throwing money at guessing.

 

The SES light does come on when I first start it. It comes on, flashes one time, then stays on until I start the car. After that the light goes out..
I checked for codes and it only has the code 12, which according to my book is normal.

I've also changed the IAC valve and TPS sensor. I adjusted the IAC, but I can't adjust the TPS, it's the non-adjustable style. Oh, the EGR valve is new also. I had this same problem even before the full tune-up, so I'm thinking it's not related to the plugs or plug wires.

Any other ideas on what it could possibly be? Thanks.

 

check for vacume leaks?

 

when you had the EGR out...how did the intake look? I had the same problems...EGR was trying to work, but when you looked inside the intake, you couldn't even see down there, it was perfectly gummed up in all corners.

 

Intake looked pretty good, didn't seem gummed up or anything. I did check for vacuum leaks, but I couldn't find any. I was using starting fluid looking for a change in the idle, but noticed none. I squirted around all my vacuum lines and around the intake manifold. Could the leak be small enough to cause me this trouble, but not really raise the idle noticably when I squirt it with starting fluid?