good morning all......need some serious help....apologize for the LONG thread but wanted to be thorough as possible.
2000 502 MAG MPI serial # OL611651. ( 2000 Baja 272 ) SALT WATER BOAT
History very vague when I bought it but apparently it's been rebuilt at some point as it is holding 160+ psi in all 8 holes....had a full tank of pump gas in it and sat for unknown amount of time, cockpit interior was wasted so it sat outside for some time.
Here are the events that I have experienced since purchasing this boat. Several water tests and running premium 93 octane and NUMEROUS bottles of Startron & Seafoam trying to "clean" the pump gas out.
Changed entire ignition system with GOOD parts except the distributor. Engine will run on the hose no problem for hours....put it in the water and after approx. 1 hour I would lose throttle above 3500 and slowly start to surge at 3500 to the point had to idle back, when throttle up it would spit, cough, backfire ( fuel delivery issue )....towed in twice because it actually wouldn't stay running. Found fuel pressure was 95+ psi when finally checked the fuel pressure....found the screen clogged at FPR, replace with OEM FPR and ***after market $100 "Quantum" eBay fuel pump**** assuming ORIGINAL cool fuel canister still in place at this time**** flushed with carb cleaner....good flow through all holes.
Water test resulted in same type symptoms.....runs about 1 hour and start surging above 3500.....fuel pressure drops down to 10 then would finally die....starts right back up, stumbles and dies.
Suspected vapor lock...removed anti-siphon valve, replaced fuel supply line to the filter, replaced filter housing, replaced fuel supply line from filter to pump, checked fuel pick up tube for obstructions, checked vent line, routed FPR return line from filter to front of the tank.
fuel pressure with key several times 42+ and running 40-bouncing.
Water test would start and run great, watching fuel pressure it would pull down to 36-38 on WOT to get planed out.....idle zone started running rough, fuel pressure surging up and down from 10-30 at best.....unscrewed the fuel filter and heard it "suck" as soon as the seal was broken so I had to unscrew it several times to get it back to the trailer.
Had gas pumped out from the fuel sending unit hole, lots of ethanol gas "poop" that came out....filled with NON ethanol gas.....new filter and plenty of spares on board.
Water test and ran out approx. 20 miles and back 3500 no problem....even pushed it to 4500+ and was running 58 mph when the bimini top decided to implode-oops left it up by accident ( oops ) Saturday & sunday no symptoms.
Next weekend took it out and approx. 1 hour in it starts same symptoms surging, backfire on WOT, fuel pressure 20-40 surging ( checking this with manual gauge at the shrader valve )
Suspected after market fuel pump so swapped back to original OEM pump....fuel pressure 43+ ( as per FPR set for 43 )....water test 43+ and would hold, WOT great....after approx. 2+ hours running above 3500 would start surging again....replaced fuel filter on the water....no change, could not get on plane due to surging....shut it off and key prime numerous times and would start and run 1500 "OK" for about 2-3 mins before it start surging again.
NO CODES.....could this be due to OEM pump with 500 hours just wore out AND cheap after market pump OR could this be Throttle position sensor or Idle Air Control or combination of both due to the salt air and 20 year old components?
Someone PLEASE help ( Good friend of mine is a GM master Mechanic old school GM EFI so he is also helping diagnosis this issue but does NOT believe it could be the TPS or IAC )

Who’s to say you don’t have more crap plugging something in the fuel system ? I would also be nervous the injector filter baskets are full of junk too.

Some pickup tubes in tank have a sock/screen?

I ran a clothes hanger down through the pick up tube to make sure there wasn't any obstruction, nothing there

Here is another odd ball symptom.....while doing all the repairs to this "project" Baja I'm trying to resurrect back to life.....so the fuel sending float comes off the original unit...ok no biggie as it's 20+ years old so I put it back in and ordered a new one...while waiting i take the boat out for more water testing....the old unit is leaking because the seal is wasted ( you assume correct as I did not seal it with anything ) and it runs GREAT 2 full days in a row with gas leaking out of the sending unit.....put the new unit in and the next weekend take it out and runs great for about an hour.....go to take off with my buddy mechanic on board and cough, sputter, backfire.....fuel pressure back to surging again....I literally bypassed EVERYTHING and fuel line DIRECT to the fuel pump....fires up and runs out and get about 4500 rpm for about 2-5 mins out of it and goes right back to it......I've filled it up several times and gas has spit out of the overflow/vent but is there a REMOTE possibility that the tank isn't able to INHALE through the vent tube due to some kind of obstruction creating a VACUUM lock inside the tank?
It's fine on the hose and PSI is awesome at 44, throttle up to 1000+ and I have literally ran it for 2 hours with NO issues.....put it in the water under a load and right back surging after a short time....

So after some discussion and reviewing our findings.....when it ran good a few weekends back the only thing that was consistent was an almost full fuel tank and the fuel sending unit was leaking which would cause the fuel tank to vent.....maybe a combination of both eliminated both the vacuum & vapor lock?
Thoughts?

You need to pull the pickups and examine the outer part of the screen for debris. Just because you used a coat hanger on the inside, doesn't mean there isn't trash or ethanol goop on the outside.

Is this an VST or cool fuel engine? IF VST rebuild. Also the Quantum electric pumps are trash. Mine died and I replaced with OE.

YES!! It sounds like your fuel tank is overfull and/or has an obstruction. When filling mine fuel will spray out of the vent hose and then fuel sits in the vent hose so vapors cannot flow out as the tank heats up when ambient temps. go up. I found this out by fuel being pushed by the pick up hat gasket. I no simply don't try to get a full tank, space needs to be left so the vent hose stays clear.

I'm not on often but I'll try to stick with you on this. I'm 3 or so years into trying to diagnose my idle surge problem.

Help Chasing Erratic Idle Please

Also..... you might as well go ahead and test your IAT, MAP, TPS and other FI and ignition sensors and modules. Also test the coolant temp sensor, or replace as they are relatively cheap. The shop manual says how to test all with a volt meter if you don't have Merc diagnostic tools. Check base timing and timing and fuel pressure when the issue happens. If you have twin engines swap parts side to side including the ECM but only do so one at a time so you know which single unit may be the issue.

Fuel vent line has fuel sitting in it and tank starts to draw a vacuum. If the vent line is not routed correctly, fuel can get trapped in it. There should be no dips in the vent line where fuel can settle

When it does its trouble, loosen the gas cap and listen, if air is being sucked in

I'm sure being the same manufacturer constructed my boat and the OP's contributes to this issue. My vent line is below the fill so it gets backed up first during fill up. Fail IMO so now forced not to fully fill.