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Fuel pump stumper - 2001 115 4 stroke EFI

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Qball70

New member
Ok so...
Motor decided not to start in fall 2023. Spring of 2024 I deduced the high pressure fuel pump was kaput. Replaced with Amazon pump; motor fired right up on muffs. Problem solved!

Once we were on vacation, she initially wouldn't rev fast enough to plane; plowing water at maybe 10mph tops. This is the weird part. It started to fix itself. Turning the boat seemed to help. Modulating the throttle lever seemed to help. After 2-3 days, she was running pretty good; maybe a couple RPMs under 6000. I had even borrowed a fuel pressure tester from the local O'Reillys but the motor was running so well I figured there wasn't much to test so I didn't bother. The motor ran near 100% for the rest of my 10 day vacation. Problem solved!

Back home, we took her out for a lake day a couple weeks later. Same BS as before. Wouldn't rev, plowing water. After an hour of hoping it would magically heal itself again, I trailered it home and parked it. Haven't touched it until last weekend.

I decided to replace the fuel pump with OEM. Maybe that Amazon Chinese pump wasn't up to snuff. Called the Mercury dealer. Guess what's not available anymore. He was very helpful and directed me to the wide variety of pumps available on Amazon. :cautious:

So I procured a different brand of fuel pump. Installed Saturday and took her out for a run. Same issue. Not revving high enough to plane. Local dealer is backed up a month and vacation is in 2 weeks. Where do I start, please? Coil packs maybe? Plugs are new. The vapor separator float chamber feeding the injector pump had plenty of fuel in it so I don't suspect the inline diaphragm pump. Would a failed pressure regulator allow the motor to run fine at idle but not develop RPM as the throttle increases? I should probably check the fuel pressure.

Any other ideas would be greatly appreciated. Thanks!
 

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No surprise that these " high tech " motors and high tech parts are not well supported by the motor " assembler.".-----No wonder I see so many 4 strokes for sale.----High repair costs means some folks just buy a new one.------Great if you can afford that.----Start with cleaning of the vapor separator tank and replace all filters.-----Perhaps test run with a portable 6 gallon tank.
 
Vapor separator interior was pristine. The wee screen filter at the HP pump inlet is clear. The inline filter between the diaphragm pump & the vapor separator looks new as well. I may have replaced it during last year's Troubleshooting Fiesta (fiasco?); I don't recall. But it sure looks clear.

I don't know of any other filters?
 
racerone said:
No surprise that these " high tech " motors and high tech parts are not well supported by the motor " assembler.".-----No wonder I see so many 4 strokes for sale.----High repair costs means some folks just buy a new one.------Great if you can afford that.----Start with cleaning of the vapor separator tank and replace all filters.-----Perhaps test run with a portable 6 gallon tank.
Click to expand...
Why is that? Seems that the motor makers would be wanting to sell as many high tech and expensive parts as they can sell.
 
I think before I replaced the high pressure fuel pump that I would install a fuel pressure gauge to verify that fuel pressure being lost is the problem.

If fuel pressure is lost I would be double checking the power and ground inputs to verify they are in good working order before I bought another fuel pump.
 
In case anyone is interested...
Procured a pressure gauge. Fuel pressure was spot on at idle on muffs. Took the fuel injector rail off to remove / inspect / test injectors. Noticed upon reinstallation that the wee bit of fuel hose betwixt the fuel cooler and the rail appeared flattened; not terribly surprising, considering it's maybe 2 1/2" long and makes a 90 degree bend in such a short span. Replaced with a longer piece of hose that now has a loop in it to provide a more gradual curve. Motor runs like a champ now.
 
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