Kinda need to know the engine size when talking about an engine problem. If yours has separate ignition coils, you can try switching two of them to see if the misfire goes to a different cylinder. You can do the same thing with injectors. GM has more trouble with injectors than any other manufacturer. They don't necessarily fail, but unlike Chrysler, GM does not buy their injectors in matched sets. They just grab a handful out of a big bin and stuff them in. They are not flow-matched. When the engine is relatively new that usually doesn't cause a problem, but over time the difference in flow rates shows up as a lean condition in one or two cylinders. The unburned oxygen is detected by the oxygen sensor, and the Engine Computer responds by commanding more fuel to all the cylinders on that side of the engine. No matter how much extra fuel it commands, there will still be that unburned oxygen. You smell raw fuel at the tail pipe but the computer sees a lean condition. The fix for that is to install a set of rebuilt injectors that have been flow-matched.
That injector flow problem usually results in the computer detecting and setting a misfire code but you typically won't feel it or even know it's running too rich. What you described sounds more severe. As far as compression tests, it's not whether or not a cylinder has compression. It's how much compression does it have and is it the same as all the other cylinders. From the way you described how it acts, it doesn't really sound like a compression / valve / piston ring problem. My first vote is for a coil.
Saturday, January 26th, 2013 AT 11:18 PM