I am trying to get a boat sold for a buddy. Has been sitting for 2 years has a 500 EFI the boat is a 01 Sonic Prowler. We did plugs, distributor cap, fuel filter and pulled the injectors and them cleaned and flow tested. When we first fired the boat it was only running on 5 or 6 cylinders so we pulled the injectors and did the service. Ran the boat on the hose for about an hour everything seemed fine. I put 50 gallons of ethanol free premium in the boat, the was about 15 gallons of old fuel still in the tank. Splashed the boat today for a test drive. Idled out then took it easy out of the hole tabs down trim down and couldn't get it to plane out before it started missing. Tried 5-6 times but no go. It will go along for 20-30 seconds at 3500-4000 or so then start to die. Was backfiring and RPM's would die back to idle. I am thinking to replace the fuel pump any other suggestions would be greatly appreciated.

Greg

I,D check the fuel pressure before I spent the money on a new pump. Also you said you replaced the plugs, cap & rotor did you replace the plug wires as well if not test the wires as well. Check the coil and timing as well.

hook up a fuel psi gauge under load,probably dropping

Agree with the fuel pump. Did you replace the inline canister fuel filter on the back of the engine?

We did not replace the plug wires. I will put a pressure gauge on it this weekend what kind of pressures should we see?

Double check IAC valve and MAP sensor.

I believe it's in the neighborhood of 42 psi