OK, DD gurus, here’s my latest. When we last saw our hero he was struggling to find his missing 200 rpm on the stbd engine.

To recap, I had been running the boat for a 3 hour trip from New Haven Ct to Westerly RI at about 2150 rpm (DD 6v92 making 435 hp) when my stbd engine dropped to 1950 and would not come back up. After docking I changed out the filters again and that had no effect.

I went thru the throttle linkage and the shutdown solenoid linkage and all were adjusted properly. I bypassed the floscan to eliminate that from the system. Suspecting the fuel pump, I read the manual and based on their recommendation, measured the return volume at 1200rpm for 1 minute. Flow was acceptable: recommended minimum is 0.8gpm at 1200; I was reading 1.1gpm.

In doing so, however, I did note that there was a substantial amount of foam (air) in the returning fuel. Suspecting a leaking line upstream from the pump, I first tested the good engine and determined that there was no air in its return flow. I then replaced all the inlet lines to the offending engine and ran the good engine’s supply line directly to the fuel pump of the bad engine, but that had no effect on the foam in the return line. All fuel lines upstream of the fuel pump are now new and solid. I am definitely getting good clean fuel (no air leaking in) to the pump.

Suspecting the fuel pump (cavitation?) I replaced it this morning, along with its fittings. I still have the same problem.

One interesting hint is that when I cranked the engine to prime the new pump, with the output line disconnected from the secondary fuel filter that it feeds, fuel spurted up out of that filter (i.e. back out the inlet of the secondary filer). That suggests to me that the fuel already in the engine is somehow being pressurized back into the filter. I’m not sure what to look for from here. Suggestions would be appreciated. Thanks.
Bob