Losing Power At High RPM

[DennisG01]

I had this exact same problem on my 225VX. One of the lift pumps had a small tear in the diaphragm. This resulted in a very small fuel leak that wasn't easily detectable. My boat would run great for 30 seconds, then the motor would shut off. Sometimes it would sputter back to idle, but most times it would just die. I could easily restart it and keep going. It would run 4000 RPM all day, but anything over that would kill the motor. The lift pump that was leaking was degrading the amount of fuel being pushed into the VST. Above 4000RPM, the motor was draining the VST. When all the fuel was drained, it died. I couldn't find just the diaphragm, but I got 3 brand new lift pumps from Amazon for $100. Replaced all 3 and haven' had that issue again. Those same 3 lift pumps now cost $130 from Amazon. I guess inflation is getting to everything. That said, given my motor's propensity for chewing up these low pressure lift pumps, I'm considering buying 3 more just to have on hand in the future.

I've been contemplating switching to an electric fuel pump and making block-off plates for the block where the LP pumps would have been. Been contemplating that for about 5 years, now

[Aggiedave]

I had this exact same problem on my 225VX. One of the lift pumps had a small tear in the diaphragm. This resulted in a very small fuel leak that wasn't easily detectable. My boat would run great for 30 seconds, then the motor would shut off. Sometimes it would sputter back to idle, but most times it would just die. I could easily restart it and keep going. It would run 4000 RPM all day, but anything over that would kill the motor. The lift pump that was leaking was degrading the amount of fuel being pushed into the VST. Above 4000RPM, the motor was draining the VST. When all the fuel was drained, it died. I couldn't find just the diaphragm, but I got 3 brand new lift pumps from Amazon for $100. Replaced all 3 and haven' had that issue again. Those same 3 lift pumps now cost $130 from Amazon. I guess inflation is getting to everything. That said, given my motor's propensity for chewing up these low pressure lift pumps, I'm considering buying 3 more just to have on hand in the future.

[99yam40]

to tell the truth, I had never heard of the pumps themselves letting air into the fuel stream.
but I have seen and heard of the pumps leaking around the outer edges when not bolted up to the block.

[DennisG01]

Good to here.

A clear piece of tubing temporarily put in place right after the pumps might show air bubbles if you run into this again.

Good idea!

Interesting... the "test" that I found on You-Tube by a professional that said to loosen the LP pump a bit from the block? It turns out that is NOT a definitive test like he said. I'm glad that I found that test, though, as I still think it has merit. It's just not the "be all, end all" of tests.

I think if I had my Mity Vac with me, that may shown the LP pump to be bad. If I remember, I'll put the Mity Vac on the bad LP pump when I get home and see what happens.

[99yam40]

Good to here.

A clear piece of tubing temporarily put in place right after the pumps might show air bubbles if you run into this again.

[DennisG01]

All good thoughts - and appreaciate them all. You're correct - no recicirculation. No, there is no fuel cooler.

BUT... I played around with the top LP pump and it started leaking out of one of the screw holes. This area should NOT leak as it's sealed off from the gas. I took the LP pump apart (again) and there are no rips or tears in any of the gaskets. But they must have lost elasticity. No combination of various tightening torque eliminated the leak.

Now the engine ran out of fuel at idle. I drained the VST immediately and only about an ounce or two came out. Nowhere near where it should be. LP pump must be letting in air when running - and was probably the reason all along - even though there was no visible leak (not even from behind the pump).

I put a new LP pump on it and we're back in business - ran WOT with absolutely no issues. I'll replace the other two at the beginning of next season.

I'm a happy boater!

[99yam40]

If the lift pumps cannot keep up with the flow needed to keep the VST full.
either they need to be replaced or maybe the Bypass pressure regulator is dumping too much fuel.
But I am not sure there is a recirculation set up for the lift pumps.

also is there a fuel cooler on this motor that might be not getting the water flow it needs to keep the fuel from vaporizing under the pressure and flow needed at high RPMs.

Just some random thoughts I had

[DennisG01]

does your manual give Ohm specs?

Yes, it does (just trying to document as much as I can). It's in the boat and I'm in the cabin right now, but I believe it's something like 5 or 6 ohms... but I read that after running for a bit, it often lowers. Either way, it's low resistance so the pump (from this test, anyways) seems good. High resistance would be a problem.

I just find it hard to believe that a pump could (consistently) be "flaky" where it only fails at high RPM's. I just don't think the pump cares about higher RPM? The FLOW through the pump can increase, but the PSI stays the same. Well, at least, that's wher emy head is on this at this point.

[99yam40]

does your manual give Ohm specs?

[DennisG01]

Oh... the HP pump tested at .6 Ohms across the motor leads. This was after a 5 minute idle from the mooring to the dock.

[DennisG01]

Didn't have a whole lot of time for "fun" today...

The only thing I got accomplished was loosening the low pressure pumps from the block and squeezing the primer bulb to see if they were leaking, which they were not.

I didn't bring my MityVac with me to test them for pressure or vacuum, but apparently that little test above is a decent "quick and dirty" test.

If I have time tomorrow, I'll try collecting the gas from the VST after idling and after the engine shuts down from a WOT run.



Anybody with thoughts on my question about the fuel pressure regulator above?

Also, am I correct in assuming that if the HP pump was failing, it would be failing at all RPM's?

[DennisG01]

I would think turning the key to the off position when the problem shows up at high RPM, would stop the lift pumps from filling

It seems weird (not a good idea) to do that at 40MPH? But I suppose there's nothing really "bad" about doing that? Maybe just toss it into neutral right away? Maybe I'm overthinking this one, too...

[99yam40]

I would think turning the key to the off position when the problem shows up at high RPM, would stop the lift pumps from filling

[DennisG01]

is the VST staying full at high RPM.
if it gets low due to not enough fuel in the VST the pump cannot pump what is not in there

That's kind of what I'm wondering (low pressure pumps).

But, how can one check that? I can drain the fuel in the VST after just idling. But the level is going to vary based on whether it just filled or was getting close to being filled (float position). And how would I drain the VST after running WOT? I have to throttle back to stop the boat... which means the VST tank will fill up again.

With all of those variables, how can the amount of fuel in the VST tank be accurately measured? I must be missing something - how would you do it?

[99yam40]

is the VST staying full at high RPM.
if it gets low due to not enough fuel in the VST the pump cannot pump what is not in there