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If it's not really about tuning or wiring. Then it belongs in here.
I have an odd issue on a 2002 WRX with an engine that will not start. The vehicle does not have any recent modifications and was running fine until it shut off while driving. The engine cranks, has compression, fuel injectors are pulsing consistently during cranking (verified with oscilloscope) but the spark plugs do not fire as they should. The oscilloscope shows all coil packs are receiving the 12v constant and a good ground path. The signal from the ECU to fire the spark plugs is inconsistent though. Often no signal is received from the ECU at all but sometime (roughly once out of every 4 starting attempts) the coil packs will receive a single signal from the ECU to fire once just at the start of cranking but will never receive a second signal to fire the plug a second time and keep the engine running. This condition was verified at all 4 coil packs and at each of the ignition trigger pins on the ECU.
The oscilloscope shows consistent signals from the crankshaft and camshaft position sensors during cranking both directly off of the sensor and at the pins on the ECU. Regardless the cam and crank sensors were both swapped with known good sensors from another car. The ECU was swapped with a known good OEM unit and a known good standalone unit. The security system was bypassed, even thought it's not an immobilizing type system. The ground paths were cleaned and checked for high resistance.
I have spend about 12 hours diagnosing this issue and have run out of ideas. Can anyone think of something I may be overlooking?
Wear in the engine harness? Wire break or wire to earth fault on the ignition outputs?
I back probed the connector at the ECU and there was no signal for the ignition outputs. I think that rolls out a wiring issue with the ignition output circuit... Or at least getting a signal out of the ECU is a higher priority to start with.
When scoping the cam and crank signals, I tested both directly at the sensors and also back probed the wires at the ECU connector. Both location show a good signal.
As I'm writing this I realize a broken cam or crankshaft sensor wire would give intermittent issues and might be leading me to believe the wiring harness is actually good. I did only enough testing with the oscilloscope to see a good signal and I never had both channels of the scope running at the same time to view ignition outputs and cam sensor output simultaneously. Anytime I didn't see a signal when cranking I assumed the probe was not making good contact with the conductors so I would adjust the probe. Maybe there was good contact and there just wasn't a consistent signal.
Thanks for your input! I'll do some more testing.