Mercruiser electric fuel pump issues

[MIboatfreak]

Hi everyone,
thanks in advance for any insight you might help me with here. I have a 1995 4.3 mercruiser I/O that I bought with a fuel issue. The engine only ran on a starting fluid charge.First I replaced the anti siphon valve. No help but cheap peace of mind. Then I traced and replaced the oil pressure cutoff switch and primed the system by disconnecting the fuel line from the carb and putting suction on it until fuel flowed. I replaced the line, primed and cranked. The engine started right up and idled fine. I thought the problem was solved but 2 problems are still here. After a week of sitting, the engine started after 1 priming pump of the throttle but only ran until the bowl emptied. I did my suction trick at the carb and it ran fine all day and the day after. My 2 problems: 1) When checking for an air leak, do I use a suction gauge or pressurize it with 4 or 5 psi? I assume that’s the only thing left after installing a new anti siphon valve? And 2) I notice that the electric pump runs when I first turn the ignition switch on, then stops after about 5 seconds unless I crank the engine. It does run as I crank and also when the engine is running. Other threads say that the pump should only run while cranking and while there is oil pressure. (Unless you have fuel injection which I sadly don’t.) I also read that it is normal for the electric pump to not prime. I’m here to report it does not. It Is vertically mounted on the engine just above the fuel/water separator and appears to be mounted as manufactured. Why is it losing prime? Should it suction feed itself back? Is it ok to run for a few seconds before cranking?(I love that it does that but if it ain’t right...) Thanks again

 

[NHGuy]

You probably have a leak in your pickup (suction) line from the tank to the pump and filter. Or your carb fuel dried up and your pump isn't making enough suction to get the fuel ot the carb.
I do not know why the fuel pump runs before there is oil pressure. Mine will run for a few seconds after cranking if there's no start. But once the oil pressure goes away it stops as designed.

 

[MIboatfreak]

I agree. My pump shuts off without oil pressure as designed so I’m thinking I’m good there safety-wise. I’ll take a peek at the pickup tube. One thing I just read on another thread was to change the fuel/water separator. Another cheap thing to try even if it isn’t the issue. I know to fill the new filter with fuel, but after that should the electric pump prime itself? It’s slightly higher than the tank.

 

[Rick Stephens]

I have an electric fuel pump on a 4.3L that I built to '95 spec. I have a Weber 4 Bbl carburetor and it will bleed down if left for a few weeks without starting. So I added a check valve between the fuel separator and the pump. I dislike, greatly, sitting and cranking an engine to get the fuel system primed. So before my first trip of the season I will cross connect my pump at the starter until it primes and fills the carb, at which point is slows down and shuts off.

Your pump should NOT run when you turn the key on. The wiring system is very simple - there are two sources of power to the fuel pump - the starter when cranking and the oil pressure switch when oil pressure is above 5 PSI. Something is not wired correctly and you are asking for an explosion. Boat are bombs when the fuel system is not set up correctly. If you value you and your family's lives, fix it before you go boom. Can give you complete and exact wiring and test procedures should you desire them, but get it done right.

 

[achris]

@mlboatfreak please don't open multiple threads on the same topic (as per the forum rules). I have closed the other thread.

Chris.....
iboats mod team

 

[NHGuy]

Fine, but what concerns us is the statement that seems to say that your pump comes on BEFORE you crank the engine. If it does there is a safety issue.

 

[MIboatfreak]

Thanks to everyone who replied here. I dug into this and I think I see what’s up here. 1st of all, the starter is automotive. I ordered (ouchie) a marine grade one from my local dealer. There is some little contraption that goes to the fuel pump and that’s what is powering it. I agree with the advice of others- it isn’t what belongs. So now I’ll have that part sorted out. Luckily I bought the boat right so there’s room in the budget. I hate the idea of adding momentary switches to run the fuel pump before cranking almost as much as cranking excessively. Sounds like many people do it though. I want to leak check the lines carefully when I get to the shop and see what I can find. The bowl seems to stay full, it’s just the fuel line that is draining back. Not sure how with a new anti siphon valve and no external leakage or smell. The dip tube in the tank sounds plausible if it’s sucking some air. I’ll report what I find when I dig into this again.
Does anyone know if the electric pump should self-prime? I’m on the edge of ordering a new one but would rather not waste $275 if not required. This engine runs like a top out on the water with the current pump.