Questions and Answers

GMC Sonoma Stumbles, Misfires Q. I have a 1997 GMC Sonoma 4.3 liter V-6 Vortec 'X' motor, 5 speed manual transmission, 4WD with throttle body injection, A/C, P/S, cruise control, ABS brakes and 85,000 miles. The motor stumbles or misfires, the tachometer needle drops then jumps back up one to two hundred RPM rapidly and you can really feel it running down the road at 55 mph.

It only does this when it is up to running temperature when going down a slight incline at highway speeds. When you lift completely out of the gas it does not do it; it only stumbles when you partially let off the throttle. It is a little more apparent when the cruise control is on.

I have recently replaced plugs, wires, distributor cap, rotor, fuel filter and ignition coil. I do not get a trouble code to tell me what to look for with the OBD-II.

It seems to have plenty of power and idles smoothly. The truck has no problem starting and the fuel mileage is the same. Please guide me in the right direction as I have no idea where to look next without starting to replace sensors and parts that I am not sure if they are the culprits.

Thank you,
Matt

A. You must have a scan tool if you read the codes. You should be able to scan the data and see if any misfires are accumulating and in which cylinders, if any. If your scanner won't allow it, you'll need to see a dealer with a Tech 2 scanner. Also look at the EGR data and make sure the fuel pressure is up to specs.