The HIGH or LOW faults are typically set when the signal voltage ~ the sensor bias voltage, depending on the code set....in other words, when the ECU sees a value outside of what 'normal' is (which is nominally 1 volt inside the bias voltages)….unplugging the sensor causes the ECU to substitute a nominal value determined by the TPS (going on memory.) You can also use a DVM to measure what is going on at the connector....
Usually if the pressure in the line is ok and consistent (no pulsing) the pump & plumbing are serviceable....
I finally had a couple hours to spend at the marina to delve into this a bit deeper. Pretty confident it's not a fuel delivery issue at this point. Under the right condition, boat will run as long as I want it to at 2,000rpms, and fuel pressure remains constant 2,000 - 3,000 rpms. It just won't idle. I had three MAP sensors at the boat, the old sensor from the affected engine, a "new" sensor that i bought last year for the same motor, and a good working sensor from the perfectly running starboard motor.
I first tried running the motor again with no change, no difference. Next I tried running the motor with the vacuum line attached, but the electronic plug to the sensor unplugged. Motor ran better, but wouldn't idle. ECU put out a nominal reading for the map sensor at approximately 4.76 volts. Motor would run great at 3000rpms, 2000rpms, but when I slowly eeked down the throttle, as soon as it got below 1700 or so, the motor would go into a stall situation, whereby a running condition could not generally be salvaged, without a complete stall. After the motor stalled, it would hard start for 5-10 minutes (barely start, only sputtering, from around 200rpm to barely 1,000rpms, before stalling again). After 5-10 attempts, I let it sit for 10-20 minutes, and I could repeat the good start condition, it would again run 2000rpms, until I pulled the throttle back, and same condition.
Next I tried swapping ECUs (the poorly running port motor had a brand new ECU from ASM, I put the factory ECU from the good running motor on it), no change. Same no idle, same good performance at 2000rpms, same stall below 1700, same no start after, until it sat. Seems like it's running rich, and needs time to let unburnt fuel clear out before it will start again. It's not smoking heavily while it's running, and there's no "oil slick" out of the exhaust (so it's not running that rich...).
Next I tried swapping MAP sensors. I put the "new" sensor in the good running starboard motor, and it ran great, no MAP sensor faults. I put the MAP sensor from the good running starboard motor in the poorly running port motor, same condition, same "low voltage" error code fault. Tested the vacuum line for leaks, no leaks.
At this point, I don't know if there's some mechanical issue causing the stall, which is causing the error codes, which is causing the inability to idle, or if there's some other electrical issue that could be causing the faulty read from a good MAP sensor. I'm guessing the former is the issue, but not sure where to look. The intake has no external leaks, and still has the factory paint on it, covering the gasket mating surface. I can't see it leaking there. The TBI has been rebuilt with new gaskets, and no apparent leaks. Any other ideas?