changing the fuel pump

[jetman737]

i'm changing a fuel pump on a merc 4 cyl 3.0 i was getting gas in my oil with it just sitting in storage. is there any way to check the pump for a bad diaphram to make sure that is the cause before I throw a new one at it? and do i need to put anything on the threads of the fittings for sealing?

 

[tommays]

Re: changing the fuel pump

They would usally have a Glass Bowl if they were real old that you would see fuel collect in

All of the new ones have a plastic tube that is hooked up to the Carb so the fuel dumps into the motor if it leaks

If you were dumping a lot of extra fuel in from the CARB it would run pretty bad



tommays

 

[ron7000]

Re: changing the fuel pump

what you can do when you pull the fuel pump,
a) if the upper and lower body are bolted together then you can unbolt it and check the diaphram;
b) if the upper and lower body is crimped together which most pumps are these days then you can find a plug and plug the output port and rig up an air fitting to the in port and pressurize the pump to 10psi and listen for a leak through either the vent nipple in the upper housing or out through the pump arm.

thread sealant depends on the fittings you use, the in & out fittings that screws into the pump are NPT threads and that you need to use a thread sealant resistant to gas. The other side of these fittings that may go to a metal fuel line, if they're an AN fitting or compression fitting then you don't use sealant on that joint.


I've always wanted to ask regarding the vent in the upper body of a mechanical fuel pump, where the hose from that is routed to the carb.
When the pump diaphram ruptures and fuel leaks above the diaphram, how can the fuel pump then push fuel all the way to the carb? And considering the carb is usually 2 feet higher than the pump, isn't the fuel just going to leak past the pump arm and into the crankcase anyway? Why even have the hose to the carb?

 

[tommays]

Re: changing the fuel pump

A marine pump has TWO diaphrams with the vent in the middle so if the first diaphram fails the fuel pressure(from the second diaphram) would pump out the vent and NOT just dump into the motor like and AUTO fuel pump would



Tommays

 

[watermellonI]

Re: changing the fuel pump

Jetman make sure its not your carb first, leaky carb will drop fuel into the oil also and cause rough running,

on another note there is a horn on my carb where fuel gets dumped into the carb if the fuel pump goes out, but I noticed during a rebuild that not all carbs have it, mine is located on the helm side of carb.

 

[Robj]

Re: changing the fuel pump

If your pump looks old, it is most likely your pump and I would change it anyways. Consider it a wear item, when a pump fails on a car it is no big deal, it does not run. On a boat, it is a different story, so I take a more pro-active approach. When I bought my boat, the pump looked like the original and I changed it. Otherwise you can start it and see if there is fuel in the clear line. If there is, then it is definitely your pump.

Have a great day,

Rob.

 

[ron7000]

Re: changing the fuel pump

A marine pump has TWO diaphrams with the vent in the middle so if the first diaphram fails the fuel pressure(from the second diaphram) would pump out the vent and NOT just dump into the motor like and AUTO fuel pump would

Tommays

really?

wow! where did you hear that?

dude, that's not how my oem mercruiser mechanical fuel pump on my 2002 3.0L works. OMG, is my boat gonna catch fire and blow up?
What's the chance i have an auto fuel pump, did mercuiser give me an engine with an auto fuel pump?

(not directed at you tommays, just messin' those iboat's few... from a previous post where my humor was apparently too offensive and some couldn't take it )

my vent is on the top of the main diaphragm, just like all marine mechanical fuel pumps that i have seen. maybe on some abortion of a motor like the 470 they used a fuel pump like that, with two diaphragms and a vent in the middle? They would have to have some sort of check valve to prevent the fuel from always pumping out the vent with it being in the middle, no?


below is a pic of a standard merc fuel pump, pic provided by go2marine. Note that the vent which has the hose going to the carb is in the top casting. That is separated by the main diaphragm, which you can see if you're familiar with mechanical fuel pumps. And this is really the only diaphragm. I think I know what you're talking about referring to the 2nd diaphragm, which is just above the open [output] port visible. That isn't a diaphragm. It's only a gasket. My fuel pump has a bowl on the bottom for a fuel filter, this pic doesn't, but the inner workings are going to be exactly the same.

 

[ron7000]

Re: changing the fuel pump

discovery science channel, how it's made. automobile fuel pumps!

it was just on at 9pm EST. and will be on again at midnight.

http://science.discovery.com/tv-schedules/series.html?paid=48.14293.121570.16541.18

 

[starsnstripers]

Re: changing the fuel pump

I believe the pump in that pic is upsidedown atleast mine, ziggys and fishermarks mounts to the engine opposite of that pic. That puts the vent on the bottom if that helps.

 

[ron7000]

Re: changing the fuel pump

That how it's made episode was on electric in-tank fuel pumps, kinda lame.

(1)
here's my pump off my 2002 3.0L, had it off the boat so i figured i would pull it apart just for you guys. Had some rust and sediment in the lower bowl so i'm glad i did.


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(2)
the following pic is of 4 pieces. Not shown is the bowl on the very bottom of the pump. That's only the bowl holding the fuel filter. This is all there is to the pump.


the top right is the top half of the pump, it bolts to the engine. It has the diaphragm along with the vent.

On the top left leaning against the wood is the middle of the pump with the input and output check valves. The side leaning against the wood mates to the top half of the pump. The side you see mates to the gasket just below it; gasket is somewhat oriented correctly as the opening in it goes to the side with the input check valve. The check valve closest to the brass nipple on the left is the output check valve. The output check valve only prevents fuel from flowing from the fuel line going to the carb back into the pump. It maintains pressure in the fuel line going to the carb, once it has exited the pump.
The input check valve is the pressure regulating valve. As the diaphram goes upward it sucks fuel upward through this valve. When the diaphram goes down it pushes fuel through the output check valve to the carb but if it exceeds the pressure limit of the input check valve then that fuel flows back through the input check valve also- going back towards the fuel tank.
Mechanical pumps for carb applications like this regulate around 5-10 psi. But don't assume all mechanical pumps have regulated output pressure, it ultimately depends on what application the pump was made for.

The gasket if you were to pick up with your hand, you would place it on the middle piece of the pump on the top left.

The piece on bottom right is the lower casting of the pump. It has the input port which is visible, it is next to the gasket.
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(3)


The above pic is where the magic happens. Left side is the output nipple with output check valve. Check valve on the right is the pressure regulation check valve. Diaphragm goes up- it sucks fuel in from valve on the right; it can't suck fuel from the left valve because that's a one way (output) valve if it's working properly. Diaphragm goes down- it pushes fuel out the valve on the left but if the pressure on the output side + diaphragm pushing down overcomes the input (regulating) valve on the right then the fuel also flows downward through the input valve.
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(4)

above, here's a better pic of how things go together and hopefully you can understand how the fuel flows through the pump. Just take the gasket and slide it upwards onto the pump part with the check valves. Take the bottom piece and flip it over onto the other pieces. The side you see of the bottom piece goes against the gasket. Input side is on the right. Output side is on the left.
Fuel flows in, from the right going left through the brass barb. It flows below this casting into and through the fuel filter not shown. After getting cleaned through the filter, it comes up through the hole in the bottom piece that is just off center to the right. Fuel then flows through the large open hole in the gasket and through the input regulator valve.