Mercruiser 7.4 LX MPI Fuel Pump Question

[tpenfield]

I have the combo fuel pump and sea water pump on my Mercruiser 7.4's. (yes, wonderful) . . . I was changing out the impellers and was having trouble getting the impeller off of the shaft. So, I decided to take out the whole pump assembly. In the process I disassembled the fuel pump from the sea water pump exposing the oil reservoir of the sea water pump bearings and could immediately smell gasoline.

So, that seemed to indicate that the fuel pump has been leaking into the sea water pump reservoir.

My thinking is the the diaphragm in the pump separates the fuel in/out fittings from the overflow fitting as shown below.



FWIW - I replaced the fuel pump on the starboard engine in 2012. Not sure when/if the port engine fuel pump has been replaced.

The starboard pump (newer one) had a quite noticeable gasoline smell in the reservoir, the port pump did not.

If I replace the pumps, I'll do both of them now.

 

[Bondo]

So, my question is if that sort of a test would indicate that the diaphragms in the pumps have a pinhole or some other defect?

Ayuh,.... I would think there should be No leakage, but I have never tried such a test, either,.....

 

[tpenfield]

Just to be safe, I order 2 new pumps. Wondering if I can rebuild the old ones :noidea:

 

[alldodge]

Doubt you can rebuild them, no one has parts.
There is also only one diaphragm in these pumps, don't no where the second is unless its twice as thick

 

[alldodge]

You know I'm running without a antisiphone valve (I know not according to USCG) and the original cool fuel. Reason for no valve is builder didn't want the restriction for the pump. If your up to it you could switch to cool fuel with some plumbing changes

 

[Bt Doctur]

I would eliminate the pump and cap the opening with GM fuel pump block off plates and run a electric pump

 

[tpenfield]

I received 1 fuel pump, but the place I ordered from neglected to ship 2 pumps, so the second one is on its way. I should be in business to replace the fuel pumps next weekend (May 19-20th) . . . Then I can start the engines and get everything all set for launching.

Since I'll have new pumps, I'll run a quick test to see if the overflow tube gets any pressure from the inlet side and update this thread accordingly.

 

[alldodge]

A note for future thought, there is a discussion on OSO about a guy running out of fuel with brand new Airtex fuel pumps. He is running over 600hp but noticed the pump won't even keep up about 4000 rpm. Next time I need a pump I'm going to think twice about Airtex

 

[tpenfield]

I got the fuel pumps. So, I'll probably give them a quick test, the same way I did the old pumps just to see if there is air passage from the input fitting to the overflow fitting.

I should have the engines running this weekend. :thumb:



I think these will keep up with 300 hp of engine (each).

 

[tpenfield]

I ran a quick test of the new vs. old fuel pump to demonstrate the theory of not leaking from the 'pump' side to the 'diaphragm' side of the pump.

Here is the video of the test.

FWIW - the pump should not pass air/liquid from the inlet/outlet side (pump side) over to the diaphragm side where it would go out the overflow (safety return) side of the pump.

Marine fuel pumps have the overflow fitting as a safety feature - the overflow tube is routed back to the air intake so that any fuel that leaks to the diaphragm side would be pulled back into the air intake of the engine. It is best not to have any leaks, regardless of the safety feature.

 

[dubs283]

FWIW every fuel pump/sea water pump assy Ive ever dealt with always smell like fuel in the actuator rod/gear lube housing, have no idea why unless every mechanical fuel pump leaks a little fuel past the actuating rod?

I guess on a "normal" setup the fuel would just enter the crankcase and mix with the engine oil, perhaps its not enough to severely dilute tho oil or even cause it to smell like fuel.

Also note that mercruiser recommends high perf gear lube in the actuator housing on your setup, change it every time you replace your sea water pump impeller/housing