Mercruiser 5.7 I/O Dumping out at 3500 RPMs

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dwarren

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I know we've all been down this road a few times but I'm a little lost and need a refresher.

A friends 1996 (new block in 2006) RWC Merc 5.7 A1 Gen II ran great. He just bought boat. Fueled up in Babylon and then found that the boat would just dump out as soon as he planed off. Boat will pretty much play at 2500 all day but as soon as planed off, it just drops out.

So..I pulled inline filter at carb and replaced. Replaced Fuel/Water Sep. PUlled vent line and verified clean. Pulled pickup from fuel tank and no clog (and no screen btw). I dumped fuel from water/sep into glass but couldn't really see any major settling there.

My gut says this is bad gas as it started happeneing as soon as he fueled up.

He did put in ethenol treatment and dry gas but may be a day late on that.

I'm thinking if not bad gas, has to be maybe fuel pump but seems odd that at 2500 rpms, plowing through the water, that it would be a bad pump.

Anyone got any other suggestions? Carb/arrestor both clean btw.
 
I had a similar problem a few years ago, took me forever to figure out what it was. When the boat was below planing speed, and bow was pointed high into the sky, ran perfect, and got up on plane just fine, then died in under a minute every time. My problem was a small leak in the fuel pickup tube, inside the tank. When coming on plane, because of the angle of the boat, the leak was submerged in gas, so no problems. When just idling, I guess the leak was small enough that I was getting enough fuel to idle. But on plane, sucking air. I had two fuel tanks, so it was somewhat easy to isolate - because ran fine when I ran both motors from the 'good' tank. I thought it was bad gas, as a last ditch effort before I paid to dispose of the gas, I filled a five gallon can from the bad tank, and ran the motor off it - ran just fine. Which is when a problem with the fuel line became apparent.

Not sure if your problem is the same, but perhaps somewhere to start? Run the motor off a can of good gas, with a piece of fuel line? If it runs ok, you know it's your gas or plumbing - if it still dies, you know it's something else?

Is the boat fuel injected or carb? I have a Jeep sitting in my garage that won't go up hill, everyone tells me it may be a problem with the floats in the carb not working right depending on angle.
 
Suggest it might help to rig in a fuel pressure test gauge, and as alk suggest , run off a temporary clean portable tank with nice fresh fuel attached at the fuel pump inlet. That bypasses the rest of the upstream fuel system and shows whether the fuel pump is overheating or otherwise losing ability to supply sufficient fuel above the critical 2500 rpm point underway.

If that works reliably, move the portable back to between the tank and filter/separator.

If that works consistently, issue is on the tank side ( including possibility of bad gas), but if it still "dumps" , it is likely the other direction downstream before the pump. You can still move the portable to each fuel line connection to further isolate the culprit if you don't find anything obvious by rechecking.

Alk also has a good point about the carb float possibility.
 
quote:
Originally posted by Sandy

Suggest it might help to rig in a fuel pressure test gauge, and as alk suggest , run off a temporary clean portable tank with nice fresh fuel attached at the fuel pump inlet. That bypasses the rest of the upstream fuel system and shows whether the fuel pump is overheating or otherwise losing ability to supply sufficient fuel above the critical 2500 rpm point underway.

If that works reliably, move the portable back to between the tank and filter/separator.

I had a friend bring that up to me as well. I should have thought of that. Thanks.
If that works consistently, issue is on the tank side ( including possibility of bad gas), but if it still "dumps" , it is likely the other direction downstream before the pump. You can still move the portable to each fuel line connection to further isolate the culprit if you don't find anything obvious by rechecking.

Alk also has a good point about the carb float possibility.

Click to expand...
 
Tell him to quit adding "dry gas". That stuff is alcohol that mixes with water, settles to bottom of tank and corrodes it from the inside.
 
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