There's an intermittent problem with an electrical or mechanical part. "Mechanic-in-a-can" isn't going to solve this type of problem. A fuel quality-related problem will be constant; it won't come and go like you're describing.
The symptoms sound typical of an intermittent MAP or throttle position sensor. A glitch from a speck of dirt in the throttle position sensor will usually cause a stumble when the gas pedal is moving, not when holding it steady. The MAP sensor measures engine intake manifold vacuum and is the main sensor used to determine fuel delivery. Vacuum is directly proportional to engine load. MAP sensors usually fail rather quickly, not over a period of many days. The problem is, there is a specific range of signal voltage that is acceptable to the engine computer. The signal voltage from a failing sensor can be wrong, and cause a problem, but as long as the sensor's voltage is within the limits, the computer will not detect a problem. Eventually the voltage will fall outside acceptable limits. Then the engine computer will detect the problem. This is the type of scenario where you might have to put up with a problem until it gets worse. When the computer detects the problem, a diagnostic fault code will be memorized that will lead you to the circuit or system with the problem.
A tool available to your mechanic is a hand-held computer with a "record / playback" function. It can record information from various sensors a few seconds before and after the record button is pressed when the problem occurs. Later, the information can be played back to find which sensor acted up or responded to the problem. Sometimes paying the mechanic to find the problem is less expensive and more effective than blindly throwing parts at it in hopes one will stick. Also, every part you replace or change you make introduces another variable that could insert another problem making the original one harder to find.
Tuesday, February 9th, 2010 AT 10:17 PM