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I have a puzzling P0304 code (misfire on # 4 cylinder). I swapped the plugs and coils between #3 and #4 and the code stayed on #4.
The circumstances of the check engine light coming on are suspicious, so here is the entire background...
Some of you will recall that I was wrestling with foamy oil. I took off the intake manifold & changed the PCV parts. Then I dropped the oil pan to replace the o-rings, which fixed the foamy oil problem. At that point, the car developed a nasty engine shake because the 20 year old motor mounts which were mostly perished, did not like the twisting involved in loosening them up to raise the engine.
While waiting for the mounts to come in, I drove the car about 45 miles, until the code reader showed that all was well and it was ready for the emissions inspection. Then I replaced the mounts, which is a very fiddly job on a turbo with the electronic throttle body. Started the car, and it was much smoother. I don't think I got out of the driveway before..BOOM... Check Engine light.
Since swapping the coil didn't help, and I could hear the injector clicking with a stethoscope, I figure a wire had gotten damaged in the course of changing the motor mounts. The parts of the wiring that are visible (i.e. before it goes into that massive plastic sheath that runs above the subframe) did not show any damage. (I assume that the sheath, which is not smashed, would protect any wires inside from damage), and there were no wires pinched between the motor mount and its bracket.
So, I verified 12V and ground on their respective wires on the #4 coil and pronounced myself stumped. I don't have a scope and I don't know a safe and effective way to check whether or not the coil trigger signal is reaching the coil. (Since it comes directly from the computer, I don't want to risk damaging the computer [like by exposing a 5V circuit to 12V]. I'm told that the computers are flashed to be VIN specific, are discontinued (and cost as much as the car is worth), and thus irreplaceable.)
Anybody got any thoughts?
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