Expert: Roland Finston - 1/11/2012

Question
My very good mech. Owner. Of76 station over 30 yrs. Cannot figure out y my car doesn't. Want to go at steady speed! It doesn't get gas goes 10 mph then all of a sudden it goes fast. When I stop it starts all over. Some bac kfiring., but the diognostic machine shows nothing.they put new catalyc converter.runs better but not fixed.. PLEASE HELP ME. IVE NEVER KNOWN MY MECH. NOT TO FIX A CAR. THANK- YOU

Answer
Hi Susan,
I would be reluctant to recommend replacing anything without a fault code in hand. You can waste a lot of money that way.
One possibility is that there is a intermittent vacuum leak in one of the hoses attached to the intake manifold and the engine controller is automatically compensating for that by changing the opening of the idle speed motor/injector duration. So look over all of the vacuum hoses for a crack/leak/loose fitting.
About the only other part that controls speed (assuming this has nothing to do with a faulty cruise control system which could be taken off the table by removing fuse #11 behind the end cap at the left end of the dash) would be the automatic idle control motor either being faulty or its wiring harness being damaged and intermittently shorting out. So that is where I would recommend that he look after first removing fuse 11`as a verification that it is not the cruise system.
It is an easily accessible harness and part and so that might be the only item to consider replacing if the vacuum hoses and idle speed motor harness doesn't look damaged and the wires check out. The wiring connections are pin D to pin 57, pin C to pin 49, pin A to pin 58, and pin B to pin 48. Verify continuity and shake the harness to be sure of that and that none of the wires is intermittently shorting to ground or to voltage of an improper value.
Is he getting verification that there are indeed no fault codes, i.e., that it is truly reading out? It may be a disconnect to the socket for the code reader.
Roland