1999 Merc 50 4 stroke - Fuel filter sucks dry and motor dies

[lugoismad]

I asked over on thehulltruth as well, but I figured I should ask here too since there are some very talented mechanics here.

1999 Mercury 50 ELPT 4 stroke.

I started with replacing all of the hose clamps with the zip tie style clamps I purchased from a local marina.

Started it up, still having the same problem, the motor will start and idle just fine, and I can idle it out through a long no wake zone with no problems. But as soon as I give it gas, it doesn't want to go.

At home, off the water, I can rev it up to the rev limiter with no problems but after about 10-15 seconds it sucks the filter dry and starts sputtering.

Next, I replaced the fuel filter, thinking maybe it had a small crack that was letting air in. Nope, still same problem.

Then, I took my old fuel line, and went to a completely different fuel tank, and hooked the fuel line directly to the filter to bypass the connector on the cowling.


Same problem. You can prime it with the bulb, filter fills up, motor will run, and eventually the filter just empties and the motor sputters.

I've replaced the fuel pump with a rebuilt unit I purchased online. The problem occurs with either fuel pump.

The fuel bulb will stay stiff. I can prime the fuel system, disconnect the fuel hose from the cowling, let it sit and a while later press the little valve where the fuel hose connects to the cowling and fuel will spray out, so its holding pressure.

I have tried multiple fuel hoses. Multiple tanks. I replaced all of the fuel hose clamps. No matter what I do the motor does not seem to be able to draw enough fuel to maintain itself at anything over idle.

I'm stumped, and I have a fishing trip coming up in a week that I really want to be able to take the boat on, or I'll be fishing from the dock for 3 days.

Page 134 of the Service Manual -

https://www.scribd.com/doc/297811678/Mercury-service-manual-40-50-55-60-90

I've done all of this. I don't know what else to do.

 

[Weldorthemagnificent]

The tank vent is open?

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[lugoismad]

The tank vent is open?

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I've removed the fitting from the end of the fuel hose and stuck it directly down into the fuel and tried running it that way with the same result.

 

[DaleH]

Sounded to me like the tank wasn’t venting either. And those zip-ties are the ‘rounded’ ones made expressly for round hoses, right? And are not just regular zip-ties?


Uhhhhhhh, how are you running it ip to the rev limiter? On muffs? In gear? Darn good way to blow a top bearing you know ...

 

[lugoismad]

Sounded to me like the tank wasn’t venting either. And those zip-ties are the ‘rounded’ ones made expressly for round hoses, right? And are not just regular zip-ties?


Uhhhhhhh, how are you running it ip to the rev limiter? On muffs? In gear? Darn good way to blow a top bearing you know ...

Yes, they are the special hose zipties, I bought them at a marina.

Yes, its ran on muffs. I didn't realize running it up to higher RPMs on the hose was dangerous, I'll avoid it from now on.

I've tried multiple tanks, and I finally tried just taking the tank fitting of the fuel line and sticking it directly down into the gas, so I don't think its a venting issue.

 

[Weldorthemagnificent]

I'm far from an expert (hopefully one will be along soon) but don't those motors have 2 fuel pumps? A vacuum pump and the EFI pump?

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[lugoismad]

I'm far from an expert (hopefully one will be along soon) but don't those motors have 2 fuel pumps? A vacuum pump and the EFI pump?

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No Fuel Injection, it has carbs.

 

[Weldorthemagnificent]

So the fuel system holds pressure as evidenced by the bulb. Have you vacuum tested it? Probably buy a cheap vacuum tester from an auto parts store, or even borrow one. Some times a hose will hold pressure but leak under vacuum.

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[DaleH]

Appears to be a well known issue, at least as reported over on “iboats”:

Mercury issued a Service Advisory that provides some fuel starvation troubleshooting guidance. It applies best to carbed motors. Link = https://forums.iboats.com/forum/engine-repair-and-maintenance/mercury-mariner-outboards/229975-mariner-fuel-starvation-hesitation?t=224246&highlight=fuel

Also Google or search for terms “50hp mercury 4 stroke starves fuel” for more links & info.

 

[Pappy]

Let's clear up a common misconception.......
A fuel primer bulb can never and will never hold pressure while an engine is running. That is not and never has been a valid test or observation.

 

[lugoismad]

Thanks everyone for your advice.

Today I took the new fuel pump off and was going to install the old one, and noticed that I had not transferred over the seal from the old pump to the new one that seals the pump against the crankcase.

 

[jethro]

Let's clear up a common misconception.......
A fuel primer bulb can never and will never hold pressure while an engine is running. That is not and never has been a valid test or observation.

That's right, all it's for is priming the motor before starting. Once the motor starts and the fuel pump start doing the job, the primer bulb becomes nothing more than un-pressurized fuel line. The bulb does not need to be hard at all, but it should be full of fuel, not air. If it does, you have an air leak somewhere, and I have found the most likely candidate is the fitting from the line to the motor itself. Turning the motor all the time takes it's toll on that fitting and the seal.