50hp EFI diaphragm fuel pump pressure

[Jeff J]

A friend asked me to look at his 2006 Mercury 50hp EFI. Motor runs okay as long as someone pumps the primer bulb every few minutes. I think I have the problem isolated to the diaphragm pump but would like to confirm how much fuel pressure it should be producing. I am seeing less than 4 psi on the test gauge. I was expecting 15-20 but I do not the have a book for this particular motor. Can someone give me the psi I should be seeing?

Thanks,
Jeff

 

[alldodge]

Unable to say with 100% accuracy but finding it should be close to 42 psi

 

[Jeff J]

42 psi range would be for the high pressure pump sending fuel to the injectors. I am looking for the pressure going into the VST. It’s 19 psi on my 70HP Yamaha.

 

[alldodge]

VST on Mercruiser is 4 to 7 low side. Just needs to be enough to keep the tank full, similar to carb bowl

 

[boscoe99]

42 psi range would be for the high pressure pump sending fuel to the injectors. I am looking for the pressure going into the VST. It’s 19 psi on my 70HP Yamaha.

Where are you seeing a value of 19 psi? In a service manual or from an actual measurement?

 

[Jeff J]

The Yamaha F70 service manual.

 

[Jeff J]

I am sending the boat back today. The new mercury lift pump is putting out 3-4 psi which matches the old pump except for the needle on the gauge is steady now.

I attempted to order a manual for this motor through ebay but it got cancelled an hour after I paid for it. The vendor’s canned excuse was “out of stock or damaged”.