Loss of Power - 93 GMC Sonoma – 4.3L (Z) – 186,000 Miles – 7/3/2012
SYMPTOMS: Intermittent problem. Low rough idle, poor acceleration, stalls, check engine
light sometimes. The only code retrieved is 12 when using paper clip at diagnostic port. There is no odor (fuel, burning, etc.), vibration, mechanical noise, or apparent smoke present.
RECENT WORK: Last week: plugs (platinum – first time use), wires, distributor cap & rotor, ignition module, coolant
temperature sensor, and catalytic converter. Three weeks ago: fuel filter
DESCRIPTION: The engine did seem to develop a moderately fast idle for some time prior to the tune-up. Current problems developed shortly (less than 20 miles driven) after tune-up. Runs well 95% of the time but that percentage is decreasing the more it’s driven. The problems are more severe when climbing hills. If the throttle is advanced during these episodes, power loss (speed) is faster. There appears to be a “sweet spot” of the throttle position where speed is maintained well on level and downhill driving. When climbing hills, the “sweet spot” keeps me moving but speed decreases continuously. Of course if I reduce the throttle from the “sweet spot” the speed decreases.
The ignition module was replaced after the engine stopped running and the truck was towed back to the mechanic’s shop. There were two quick backfires before the engine shut down. No backfires since then.
My mechanic is, one might say, bewildered.