F115 Electrical Troubleshooting Help

[Cascade Cruiser]

Ok, if you replaced the regulator, then I think you're good there. The spec from the manual I have says it should be 35 psi at idle and 44 at key-on. And I really don't think it should fluctuate unless you change the throttle plate position quickly. So I would do the pressure test again, making sure you bleed the gauge this time.

It does seem like an electrical issue, because it's a very intermittent misfire/backfire. The loud noise indicates that fuel is igniting when it shouldn't be, obviously. That's typically due to ignition/stator/ECM/timing. But the fact is, there was a slight improvement when you made a change to the fuel mixture, and it doesn't seem that your fuel pressure is exactly within spec. So I would say you can't completely rule out the fuel system yet. The HP pump is really the only component you haven't fully ruled out (assuming you really went through the VST, checked the needle valve and everything). In addition to performing the fuel pressure test again, I would take a good look at the wiring for the HP pump, and make sure the insulation is all good.

If you find that the fuel pressure doesn't drop when the issue occurs next time, I think you have to move on to testing the ECM output peak voltage. The manual I have gives this as a possibility for incorrect fuel pressure.

BTW the 12.5 volts spec is the lower limit. So it it's 14.something, that's ok. It just can't be lower than 12.5.

Agreed I haven’t completely ruled out the high pressure pump, I will get the pressure gauge on it again once I have this charging issue sorted out. I’ve chased many of the wires and haven’t found any bad spots yet.

[Cascade Cruiser]

is that peak volts measured with the proper voltmeter or with a DVA and regular meter.
I also would expect to see voltage at the battery above 13 even at idle

The battery voltage I’m seeing is being measured directly on YDS.

[Cascade Cruiser]

I believe I have solved my problem. Last month I discovered my batteries were no longer charging. Voltage rectifier/regulator was fried along with a few pins at its connector. I replaced the rectifier and connector...it fixed my charging problem but not the overall misfire. However I kept at the electrical troubleshooting and decided to swap in a borrowed ECU. The motor ran great on the borrowed ECU for 15 to 20 minutes with no sign of misfire at any rpm. Today I ran it again just to be sure. Ran great on the borrowed ECU, then swapped in my old ECU and the misfire came back after 2.5 minutes...I assume because it heats up to a temperature where it starts to fail. As to the overall cause of the bad ECU, I suppose it may have been related to the fried rectifier/regulator and perhaps some battery wiring that I cleaned up over a year ago. As an aside, I've read about stuck ISC valves causing ECU's to go bad, but I think my ISC is fine as I can hear it clatter every time the motor shuts down and rpm's at startup and idle behave appropriately. Anyhow, nice to have closure on this one.