88 Pontiac Grand Am 2.3 Quad Four
Here is how I do ignition testing on these, use plug wires between the output terminals to the plugs. Now its simple to put the scope on each cylinder, or do power balance. Testing revealed two dead cylinders, 2 and 3. Both had adequate spark and the plugs were like new.
I suspected injectors not firing, so I went directly to the harness that fires them, one wire is power, the other two pcm control. After hooking up the amp probe below is the image off the power wire.
Looking at both banks firing on the power wire indicates a definite problem.
Cylinders 1 and 4 look ok, pcm control wire.
Cylinders 2 and 3 are the offenders, this pair of injectors are not firing at all. Checking resistance on these is fairly easy, and after checking the specs, 2.0- 2.4 ohms injector number 2 was 1.2 ohms, the rest were at 2.1- 2.2. injector number 2 was causing the pcm to invoke its protection of drivers and shut down this pair of injectors effectively killing output on cylinders 2 and 3. Customer opted for all four injectors replaced, since 147k miles on injectors.
Many of you might have pressure tested cylinders after see the dead cylinders, I would after looking at injectors. Sometimes you just suspect them, and it can be quicker looking at them first.