Adding a second inline fuel pump?

[thormx11]

I've got a Merc 5.7l with a 2bbl carb (Exact year/serial # unknown, late 90's or early 00's vortect engine) I've been battling an issue with for the last month or two at the lake. I believe I have narrowed it down to an issue with the fuel system.

The boat will start up fine and jump right on plane but after a few minutes of cruising the engine will start to bog down and it progressively gets worse the longer it runs. The higher I run the rpms the faster the problem starts. The boat will never stall though, it will bog down and if I back the throttle down it will idle perfectly. Then when trying to get over about 2k rpms it will start bogging down again. It will idle in gear all day long. If I rev the engine in neutral it will rev up to about 5k rpms fine with no bogging (although I won't rev it that high for more than a couple seconds)

I have changed the plugs, wires, coil, cap & rotor, fuel filter, and the oil pressure/fuel pump cut off switch with no change. They were all pretty old and due for changing anyway so that is where I started. I used sierra/quicksilver parts only except for the plugs.

I pulled the fuel sending unit from the tank and looked inside and seen no gunk/trash/water etc. in the tank.

Finally this last weekend I brought along a gas can and ran the engine off the gas can and it ran perfect and I didn't notice any issues. I am starting to think the fuel pump may be getting hot and not being able to keep up the pressure needed to keep the anti siphon valve on the tank opened up fully.

A new mercury fuel pump would run $200+ and quite a bit of work to change as it bolts behind the alternator on the engine. I am thinking about installing a low pressure Holly inline fuel pump close to the tank to help the primary pump keep up and get me through the rest of the season before I can fix it correctly over the winter.

I am looking at either a Holly 12-426 (1.5-4 psi, 25gph) and a Holly 12-427 (4-7psi, 32gph) I can get either for about $40 total and install one in a few minutes and hopefully make it through the last couple weeks on the lake. Would either of these pumps work for what I am trying to do? I'm not really able to find many specs on the Merc pump, except maybe 7-10 psi.

Sorry for the long post but I am just trying to paint a clear picture for you guys!

 

[jimmbo]

Is the Holley pump USCG Certified? Have you tried running with the Filler Cap open?

Revving the engine in Neutral does/proves nothing. There is no load on the engine so it takes next to no fuel to rev

 

[achris]

...Finally this last weekend I brought along a gas can and ran the engine off the gas can and it ran perfect and I didn't notice any issues....

You found the problem :thumb:

Something it stopping air getting into the tank (like a blocked vent line!) and that's what's causing the fuel pump to overwork...

Stop trying to 'band-aid' the symptoms (additional fuel pump), and fix the cause. That'll also be the less expensive, by far!

My bet is that if you run it next time and the problem starts, when you open the fuel filler cap, you're going to hear an almighty rush of air being sucked in....

Chris...........

 

[Bt Doctur]

Try this first, remove the anti-syphon valve and try to pass a this flexible wire/cable/weed wacker string down the pickup tube. It if goes in about 4 inches and stops you found a internal screen IN the pickup tube.

 

[Bt Doctur]

click to enlarge

 

[Scott Danforth]

agreed, your issue is not between the fuel tank fuel line and the exhaust in the water.

your issue is with your fuel tank.

you have a restriction either on the vent side or the anti-siphon valve are.

you could add 1 million fuel pumps and you would have the same problem.

fix the problem

another thing. if that $40 holley pump does not have the SAE J1171 stamp on it, it will cause an explosion in a boat

and I guarantee that line of pumps does not.

 

[thormx11]

I forgot to mention that I did try to run the boat with the gas cap off and it also made no difference. I have also removed the anti siphon valve and checked that the ball and spring inside it moved freely and were not sticking somehow. I also took the vent tube off and blew through both ends (the tank and the vent on the boat) and it wasn't clogged.

When I did look inside the tank I could see the pickup tube, but mine is a little different than the one above. It looks like the top half is aluminum and the bottom half is plastic like the one above. I could not figure out how to get it out though as it seems to be welded to the top of the tank.

 

[achris]

Ok, get a vacuum gauge and fit it to the suction side of the pump. Run the engine until you have the problem. Look at the vacuum gauge. More than 2" of vacuum, you have a 'suck' problem. You need bigger lines or find why it's drawing a vacuum. Not pulling more than 2" of mercury, get a pressure gauge onto the carb and ensure you're not dropping pressure. With those 2 readings you should find the problem quickly.

As for fitting an additional pump... That will not work. Those electric pumps, unlike mechanical valved pumps, can't be 'blown through'.... You'll need to remove the old pump and replace it with new... Yes, it's $200+... Welcome to boating.

Chris......

 

[thormx11]

Does it have to be attached to the pump itself? Or would somewhere along the fuel hose between the tank and engine work for the vacuum gauge? The hose runs from the tank to the water separator filter, then to the pump, then to the carb. The water seperator and pump are tucked in behind/below the alternator and are kind of difficult to get to.

The boat is about an hour away so I probably wont be able to work on it again until this weekend so i would like to find a few things to try when I go to try to figure out the problem.

 

[achris]

Just before the pump is best, but since the filter is part of the engine equipment, the just before the filter should be ok. But I would remove the filter and blow the line from the filter to the pump... There would be a remote chance you have a problem there.

When you hooked up the remote tank, where exactly did you hook it in? Remove the line from the tank (at the filter) and connect the remote tank there? If so, you've proved the line between filter and pump good. One thing to look at closely and because you were running remote tank, likely at a higher elevation than the main tank it would have masked the problem, is the fitting at the filter. Sometimes they get over-tightened and produce a hairline crack. That can be enough for the pump to pull air in... That would also give your symptoms...

Chris......

 

[thormx11]

I disconnected the hose from the anti siphon valve on top of the tank and spliced in a few more feet of new fuel line and just shoved it into a 5 gallon gas can I had sitting on the floor of the boat.

And when I mentioned fuel filter, I meant the water separator. There are no regular inline filters that I can find. There is a rubber line from the tank to the water separator housing, then a metal line going out of the housing to the fuel pump, then another metal line from fuel pump up to the carb.

 

[Scott Danforth]

There are no regular inline filters that I can find. There is a rubber line from the tank to the water separator housing, then a metal line going out of the housing to the fuel pump, then another metal line from fuel pump up to the carb.

that is because what you think of an in-line fuel filter is illegal on the boat.

for the fuel filter it must be metal, it must be fire-resistant

the fuel line is highly recommended to be metal, however appropriate A1 line is acceptable.

CFR (Code of Federal Regulation) fuel plumbing is very specific and based on international boating laws.

from fuel tank to inlet of filter housing is A1 hose. you will notice that the stock fuel barb is longer than anything automotive

from the filter housing to the fuel pump is metal line. from the fuel pump to the carb is also metal line.

 

[thormx11]

The hose running from the tank to the engine is regular automotive fuel line I picked up from a parts store. I changed it last winter.

I did notice the barb on anti siphon valve was a little longer than normal, i cant remember the one on the filter housing.

Is it possible the fuel hose on the boat is getting warm and collapsing in the engine compartment? The line still feels good and stiff but I have not felt it after running the boat when i am having troubles.

 

[jimmbo]

If the pickup tube can't be removed from the Fuel Tank for inspection, a new tank will need to be installed. You have already verified that it ran fine with a Portable Tank.
Unless your 'Auto Shop' Fuel Hose meets USCG certs, get rid of it.

 

[thormx11]

I will replace the hose with the correct hose. I dont believe removing the fuel tank would be possible/worth it. You would have to split the upper and lower halves on the hull to remove it. It is a 76 gallon tank, about 6 foot long.

 

[Scott Danforth]

The hose running from the tank to the engine is regular automotive fuel line I picked up from a parts store. I changed it last winter.
.

change it to correct USCG Type A1-15 hose, or you may just blow up.

 

[jimmbo]

I will replace the hose with the correct hose. I dont believe removing the fuel tank would be possible/worth it. You would have to split the upper and lower halves on the hull to remove it. It is a 76 gallon tank, about 6 foot long.

You might have to cut the floor to get at the tank. and a bit of Glass work afterwards or, you could keep a half dozen Portable Tanks/Jerry Cans aboard and run the engine off of them.

 

[thormx11]

I have ordered the A1-15 hose, hopefully it will be here by the weekend. I will probably take 2 or 3 cans of gas with me this weekend and try to run it a lot longer off the gas cans to make sure that cures the problem.

 

[Bt Doctur]

most pickup tunes unscrew from the tank, all depends on the corrosion

 

[NHGuy]

I'd be willing to bet BT Doctor has your solution. And if the pickup just won't come off of the tank I'd be surprised.
If you are worried that the fitting will break off you can still blow through the pickup INTO the tank once you have removed the anti siphon valve. If I were doing that I'd attach a piece of surgical tubing or some clear vacuum hose to the pickup tube and catch whatever you can blow out of the tube.
But you are way better off trying to see if it has that screen that BT Doc refers to. It's a known issue.