I'm working on an '89 Chevy 2.8L with an intermittent misfire. A *very* rough drawing is attached. All cylinders have approximately the same ignition pattern, except #6 shows an intermittent spike (red line on drawing) during the firing section when the engine misfires. It's not the plug or the wire, and after closing the gap, it may be less frequent at idle but the engine still misses when car is driven. Seems to be worse after car warms up. Any ideas of what this could be? I'm guessing ignition module or coil pack (hopefully not the ECM).
This is the first time I've used a scope to troubleshoot a misfire, and was surprised to learn it can show ignition as well as fuel problems. I'd suspect an injector, but from what I've read, it seems lean/rich (clogged/leaking injector) affects the slope of the firing section instead of causing a spike. I hooked the number #6 plug wire to a grounded (uninstalled) plug and the firing section showed a solid straight horizontal (instead of a somewhat changing slope depending on fuel injection), and I thought I saw the spike (which would eliminate the injector) but couldn't repeat it after driving the car again. It was GOOD to get a consistant misfire after pulling the plug wire (I don't do this with HEI when the engine's running...) as it confirmed the problem was with cyl #6.
This is the first time I've used a scope to troubleshoot a misfire, and was surprised to learn it can show ignition as well as fuel problems. I'd suspect an injector, but from what I've read, it seems lean/rich (clogged/leaking injector) affects the slope of the firing section instead of causing a spike. I hooked the number #6 plug wire to a grounded (uninstalled) plug and the firing section showed a solid straight horizontal (instead of a somewhat changing slope depending on fuel injection), and I thought I saw the spike (which would eliminate the injector) but couldn't repeat it after driving the car again. It was GOOD to get a consistant misfire after pulling the plug wire (I don't do this with HEI when the engine's running...) as it confirmed the problem was with cyl #6.