Engine Mechanical problem
1994 GMC Safari 6 cyl Two Wheel Drive Automatic
After a recent tune up ie: plugs, cap & rotor, air cleaner, and PCV Valve the engine idles nicely but under load it pings and occasionally back-fires. My 1st thought was a timming problem, even though I never altered the timming. Advancing or retarding the distributor, even the slightest makes the problem worse. I confirmed the firing order which I believe to be 1-6-5-4-3-2. I inspected the dist. Cap and all the plugs for any cracks, pulled each plug wire while the engine was running to confirm a change in operation and it seems to be fireing on all 6 cylinders. I even changed the fuel filter. Aside from a faulty sensor somewhere I'm stumped.