Still no luck. I bought four new spark plugs and a new engine coil, none of which made the least difference in how the engine runs. I did, however, finally manage to prove to myself pretty definitively that it’s only running on three cylinders, and that the fourth (back) cylinder is not firing. I fiddled for a while, and finally managed to get the engine to start again. I let it warm up a bit, then removed the back coil entirely. I got it started again, and it ran precisely the same. Assuming the engine won’t run on two cylinders (probably a safe bet), it seems safe to assume the back cylinder is dead. I guess it doesn’t rule out the fuel injector entirely, but given how the engine backfires when it runs, and given the wet spark plug in the back cylinder, I think it’s safe to assume the fuel injector is working. I’ve asked again for Viking to give me a pinout of the ECU cable so I can go through and check the wiring harness with an ohmmeter. If that fails, I’m not sure what to try next. Fingers crossed…
Update: I just got home and checked all four fuel injectors one at a time. All four click when energized, ruling out injector wiring and complete injector failure (though not ruling out a clogged injector). I’m going to do a compression check next, just to rule out possible failure with rings and/or valves. I want a baseline for annual compression checks, anyway, since that’s good practice for the annual inspection. Assuming the compression checks are good, I’ll try a continuity check for each coil wire back to the ECU connector. I’m assuming I can puzzle out which wire is which (unless one is broken, in which case I’ll only be able to find three of the four and will know I have a wiring harness problem). I’m hoping that’s what it is, even though it’s unlikely, because that’s something I can easily fix.
Failing that, I guess I’ll pull the injector(s) and make a visit to the junkyard for some parts to swap in.
Jan’s got good troubleshooting advice. I’m sure it’s something simple and that we’ll nail it down shortly. I still think it’s likely it’s something trivial like a broken wire or a clogged injector. Just doesn’t seem likely that it’s anything substantial. Which of course doesn’t make the process any less difficult.
Update: Did a compression test on the questionable cylinder, and it was 120+ PSI without bothering to warm the engine first. That was so good that I didn’t bother to check the other three yet (since I’m not worried about them). I pulled the injector rail, cleaned it, then pulled all four injectors and cleaned them. Reassembled everything and started the engine. Ran like crap, wanted to vibrate off the front of the plane. Shut it down quickly. No change. Sent an e-mail to Viking, I suspect the next step is to replace injector(s), but will await word from on high.