could be sediment in the tank getting sucked up to the pick ups, the engines could starve for fuel even with the filters full if theirs blockage at the pickup tube, where are you reading fuel pressure ?? id put a gauge on the rail to compare #'s if you haven't already

I'm reading pressures via Smartcraft display. Pulled relays, to check for corrosion, was spotless clean. This is a fresh water only stored inside boat...

Both engines just started at ramp, seemed a tad lean, but fuel psi was 39-41 (Smartcraft). Spent 45 min to get 43 gals ethanol free to mix with the 67 gals 87 octane.

Got back to ramp, both engines fuel psi readings arouund 12. Stbd started for a bit, but electric fuel pump stayed on. Port would start.

Gonna change gear oil in mechanical fuel pumps now...Ruling things out..

what does this mean, are you calling fuel pickup tubes relays ?

No. A 4 prong prong plug/relay, checked it, it was fine and clean.

I dont know where a single pick up would be for both engines, given each has its own fuel line going into the tank.

you have 2 pickups one feeding each engine correct ( single tank ) ? where the fuel line connects at the tank the is a pickup that extends into the bottom of the tank which usually has a sock and a check ball in them

Great diagram!!!! Thanks!!

Thoughts that both my mechanical fuel pumps.have gone out? Blown diaphragms????

Do you change the oil in them when you do the impellers?

Hi. No, looked for it, didn't see. Where is it? That is/was a bear of a job!!!!

Yes, and even more often than that. Just did it yesterday to see if would help, didn't effect anything.

Going to pull the sight tube off the mechanical pump today, see if fuel in it. Will have key bumped too with it off, see of fuel spits out that nozzle.. if so,, bad diaphragm. If not,, on to next possibility.

Pulled sight tube from each mechanical pump. Neither dripped any fuel, nor did fuel spit out of nipple when I turned key to prime system. Boat been sitting, electric pumps come on with key. Port only has 6psi, stbd has 30. Both are below the required 40psi.
Guess I'll check the fuel line pick ups in tank next.

This is an absolute puzzle...