Mechanical Fuel Pump - Mercruiser

[stratmagic]

I have a 3.0L Mercruiser with mechanical fuel pump. If the engine sits for several days, it takes some cranking to get it started. Once started, it continues to start easily even if left overnight. My thought is that fuel might be slowly leaking back accross the fuel pump

An initial shot of starter fluid will make the engine catch, but it won't run without more cranking. Haven't found any leaks in the fuel line and don't smell gas in the boat.

The fuel pump is circa 1982 and probably needs replacement anyway....but want to make sure this condition is abnormal before spending the $$.

Thanks!

 

[Bt Doctur]

Re: Mechanical Fuel Pump - Mercruiser

are you pumping the throttle a few times first?
Is the choke adjusted properly?

 

[Bifflefan]

Re: Mechanical Fuel Pump - Mercruiser

It sounds like the fuel is leaking out of the carb, not the pump.
Once the fuel is in the carb it cant go back down the fuel line.
Id remove the carb and inspect it for leaking, whether it be on the outside or the inside down the intake.
Try leaving it sit for a couple days (how ever long it takes normally for it do it) then have some one move the controls while you look down the carb (not cranking) and see if the accelarator pump is shooting fuel. do it 4 or 5 times and i bet it wont have anything coming out.

 

[Bondo]

Re: Mechanical Fuel Pump - Mercruiser

Ayuh,... I'd sooner guess the carb is bleeding off internally, than the fuel leaking back through 2 check-valves in the fuel pump, then the 1 at the tank...
But Maybe,...

 

[zbnutcase]

Re: Mechanical Fuel Pump - Mercruiser

And fuel will evaporate out of the carb, especially in warm weather, so its normal to have do some cranking to refill it. 'nutcase

 

[Showtime22]

Re: Mechanical Fuel Pump - Mercruiser

Put a 45 dollar universal electric fuel pump on it. Starts quicker then mechanicals. And is cheaper and more reliable. A oil pressure safety switch should also be installed. Install a delete plate where the old pump is.

 

[EddiePetty]

Re: Mechanical Fuel Pump - Mercruiser

If the engine sits for several days, it takes some cranking to get it started. Once started, it continues to start easily even if left overnight.
A prime indication of an internal leak in the carb. Do as previously suggested and check for accelerator pump discharge after sitting of a while.
An initial shot of starter fluid will make the engine catch, but it won't run without more cranking. Haven't found any leaks in the fuel line and don't smell gas in the boat.
Again, a prime indication of an internal leak in the carb. Starter fluid is bad news and just a crutch of the real problem.
The fuel pump is circa 1982 and probably needs replacement anyway....but want to make sure this condition is abnormal before spending the $$.
The fuel pump is not a suspect though a pressure test would be advised
FWIW, Ed in 'ol Virginny

 

[JustJason]

Re: Mechanical Fuel Pump - Mercruiser

There's a test in the adults only section that I wrote that will tell you just how to test that fuel pump.

 

[CheapboatKev]

Re: Mechanical Fuel Pump - Mercruiser

Heres the link to Just Jasons excellent post.

http://forums.iboats.com/showthread.php?t=283269

 

[intrepidvoyager]

Re: Mechanical Fuel Pump - Mercruiser

you have a classic ball check valve ( anti - syphon ) problem ..... on top of your fuel tank pick up tube you will see one of these ...

http://store.philaboat.com/philboat/details.php?ITEM=2533802

this one way valve prevents fuel from syphoning out of the carb back to the tank ....... over a period of a few days if the ASV is not working you have depleted carb fuel so you have to crank for a while to get fuel back to the carb ....... this assumes you also do not have a fuel pump issue. If the boat otherwise runs fine at WOT its probably the ASV.

 

[Fishermark]

Re: Mechanical Fuel Pump - Mercruiser

this one way valve prevents fuel from syphoning out of the carb back to the tank ......

That is NOT the purpose of the valve as I understand it. In fact, it is just the opposite. It is a safety feature to keep fuel from siphoning out of the fuel tank into the bilge in case there is a leak in the fuel line.

 

[Don S]

Re: Mechanical Fuel Pump - Mercruiser

you have a classic ball check valve ( anti - syphon ) problem .....

this one way valve prevents fuel from syphoning out of the carb back to the tank ....... over a period of a few days

Not "Classic" by a long shot, since it can't possibly happen.

Think about where the fuel comes into the carb at. The needle and seat, in the TOP of the carb.
Now someone explain to me how fuel is going to siphon from the bottom of the carb out of the top of the carb.

The fuel pump has a check valve in it that prevents the fuel from flowing back to the tank. If that check valve didn't work, the pump wouldn't work either.

 

[intrepidvoyager]

Re: Mechanical Fuel Pump - Mercruiser

That is NOT the purpose of the valve as I understand it. In fact, it is just the opposite. It is a safety feature to keep fuel from siphoning out of the fuel tank into the bilge in case there is a leak in the fuel line.

Ya ...kind of felt the same way until I had this issue with our Rinker...anyway I called Rinker and the tech guy there verified that Merc fuel pumps can work fine but gas can still syphon out of the carb after a few days if the antisyphon is stuck open. We changed the antisyphon ...a ***** to get at as you gotta pull the rear bench seat and work upside down with a mirror under the floor ...after that no more starting problems after the boat had sit for a while .

But you raise an interesting question re fuel pump pressure relief .....I'll have to look into that one.

 

[Bondo]

Re: Mechanical Fuel Pump - Mercruiser

can work fine but gas can still syphon out of the carb after a few days

Ayuh,... ta 'ell with the fuelpump question,....
Tell me how the fuel Siphons out of a Carb bowl,...
There's dead Air from the top of the fuel puddle, UP to the needle/ seat valve...

Like I said, Waaay back at the top,...
Carbs can ONLY leak down,.... No way in 'ell it's Siphoning out....

 

[Don S]

Re: Mechanical Fuel Pump - Mercruiser

You've said it and Ive said it bondo, but it just doesn't seem to sink in that the fuel can't jump up to the fuel line, pass the fuel pump and get buy an anti siphon valve.

If an anti siphon valve is the cause of your fuel leaking back to the tank, then you have some other problems with the fuel system.
You may have stopped the problem from occuring, but you didn't FIX the problem. Only covered it up.

anyway I called Rinker and the tech guy there verified that Merc fuel pumps can work fine but gas can still syphon out of the carb after a few days if the antisyphon is stuck open.

I've heard techs rattle on for hours about things they know nothing about, but they sound convincing. That's like saying "I read it on the net" and that makes if fact. When in fact, many times the info is wrong.
That is one of the bad things about the internet, good or bad can be posted.

 

[intrepidvoyager]

Re: Mechanical Fuel Pump - Mercruiser

GOOD POINT .......syphoning IS the wrong word .....so i suspect whats happened on our Rinker is as you said ...we have masked the problem ....the tiny check valve in the fuel pump is not seating prefectly .....good enough for the boat to run fine but left for a week or so fuel has leaked down from the carb thru the pump and because of a stuck open anti syphon right back into the tank ...so by changing the antisyphon we have kept fuel in the fuel line but still have a fuel pump issue........well this is COOL !!!!! .....not only can I get answers to problems here , but I can also find out about problems I didn't know I had !!! ...... now that is scary !!! .....

 

[Fishermark]

Re: Mechanical Fuel Pump - Mercruiser

so fuel has leaked down from the carb thru the pump and because of a stuck open anti syphon right back into the tank ...

Do you at least understand what they are saying? The fuel cannot "leak" out of the carb down to the fuel pump and then to the tank. It has nothing to do with check valves, etc - it has everything to do with the design of the carb and the laws of physics. There is an air gap, so that rules out siphoning. The inlet is above the fuel bowl... so the only way it can leak out is down through the bottom of the bowl. It can in fact evaporate, but not leak out of the fuel line. Here is a drawing which should help illustrate what is being said: (I'm not saying it is the same carb as yours or the original poster - just an illustration).

 

[intrepidvoyager]

Re: Mechanical Fuel Pump - Mercruiser

Do you at least understand what they are saying? The fuel cannot "leak" out of the carb down to the fuel pump and then to the tank. It has nothing to do with check valves, etc

Hey Fishermark ...... pls note what I said ........ I said leak down FROM the carb ........ not out of the carb ...... I agree syphon IS the wrong word.

As far as the valves in the fuel pump and the antisyphon ...as it seems we have figured out here ......even if the anti syphon was stuck open ..... if the fuel pump check valves are not compromized how can fuel be leaking down from the carb ...thats the fuel in the line from the pump to the carb ... thru the fuel pump and back to the tank.

only thing i can figure, as i said, is that the fuel pump, although it works fine, is on its way out

 

[Don S]

Re: Mechanical Fuel Pump - Mercruiser

Hey Fishermark ...... pls note what I said ........ I said leak down FROM the carb ........ not out of the carb ......

Now you have REALLY got me confused. How the heck is fuel going to leak from/out of the carb, past the fuel pump and THRU the anti siphon valve if it doesn't go back out through the needle and seat.

 

[Bondo]

Re: Mechanical Fuel Pump - Mercruiser

Ayuh,... Forgetabout the Carb,... He gets that,... Now it's the fuel IN the line We're talkin' about...