Struggling with low power on my converted diesel boat engine

I swapped a small diesel into my fishing boat last year. It idles fine but loses all guts under load. I replaced the fuel filter and bled the system twice. The injection pump timing seems right from my marks. No black smoke or weird noises, just no pull. Could it be a worn injector or something in the lift pump? I'm stuck and the fishing season is coming up fast.

4 comments

4 Comments

taraf832mo ago

Hmm, exhaust backpressure can do that, but on a diesel, you'd usually see some smoke if it was fighting to get out. Since OP says no black smoke, I'd look harder at fuel delivery. Worn injectors or a weak lift pump can't keep up when you throttle up. On my old Perkins swap, a cracked lift pump diaphragm caused the exact same no-pull issue.

ivan_harris2mo ago

Check your exhaust outlet's height above the waterline. Backpressure from a submerged exhaust can kill power under load without obvious smoke.

walker.emery2mo ago

On my buddy's Formula 240, we measured the exhaust outlet at 8 inches above the waterline at the dock. But when he's got a full tank and three people in the back, the stern dips and it's practically underwater. That got me thinking about how backpressure builds up quietly, no black smoke like a clogged filter. So when you say to check the height, are you talking about static measurement or should we be looking at it under load? Because I've seen engines run fine at idle but bog down when you hit the throttle, and I wonder if that's the exhaust gas fighting to get out.

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elliot_roberts1mo agoTop Commenter

Whoa, eight inches at the dock and then underwater with people? That's a huge drop. You absolutely have to check it under load, that static number is useless. The whole point is to keep the outlet clear when the stern is squatting in the water. If it's submerged when you're on plane, you're forcing the engine to push exhaust out against a wall of water. That'll choke it out every time, no smoke needed.