idle creeps up
 

Our 88GT is pretty much stock - we do have: long-tube headers, underdrive pullies, electric fan, 130A alt. Nothing major to date - still is speed density.

Here's our problem - the car sometimes idles fine and than sometimes the idle slowly creeps up to around 2500 rpm. Do you guys think it's the EGR, TPS sensor, or something else? How can I test each of these to determine whether or not they're functioning?

I would check the 10 pin connector. If your car hasn't been back to ford for the 10 pin connector recall I would bring it in. Its the black and white connector behind the intake. Ford made one side of the connector round, and the other side oval shaped. It results in a loose connection making for an idle problem similar to what you described. The jumper wire assembly takes car of that.

We just replaced the valve cover gaskets over the weekend. Are you talking about the connectors that clip on to the upper intake?

Would you rule out the EGR and TPS for a creeping idle?

Yes the connector that clips on the back of the intake. Its a free fix from Ford if it hasen't already been done. Check for codes, see what you come up with. Did you mess with the TPS when you changed the valve cover gaskets? Whats it set at?

Didn't check for codes, not quite sure how to.. The TPS was disconnected when we took off upper intake. Problem was happening way before we changed valve cover gaskets.