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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11051
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have early (95) mercruiser 502 mpi engine, it will run for

Customer Question

I have early (95) mercruiser 502 mpi engine, it will run for half the day then quit like it is our of fuel. I added electric low presser fuel pump still does the same thing. I pulled fuel hose between tank and fuel pump and there is no fuel in the line. If I let electric fuel pump run for a few minutes before trying to restart it will start but dies out right away. the next day I can run for half the day again?
Submitted: 11 months ago.
Category: Boat
Expert:  Jason replied 11 months ago.

Hi Richard, my name is Jason.

Did you hook up a fuel pressure gauge in order to see if the engine actually had or did not have fuel pressure when it stalls/will not restart?

Customer: replied 11 months ago.
No I can see where to connect on this engine, my pervious engine the rail was exposed this model is not. If you can explain where the valve ( connection point) is at that would be a start.
Expert:  Jason replied 11 months ago.

I certainly can but I will need the serial number on the engine in order to pull the correct manual.

Expert:  Jason replied 11 months ago.

Did you have your serial number handy?

Customer: replied 11 months ago.
The serial number ( 0K005205 or 0K005005)comes up as a 5.7 because of the age
Expert:  Jason replied 10 months ago.

That would not be the correct serial number for that engine. In order for me to be able to tell you anything about your engine I would need the correct serial number for it.

Customer: replied 10 months ago.
That stamped on top of the engine, it a 1995 or 1996 502 MPI that's all we have to work with.
Expert:  Jason replied 10 months ago.

I don't understand how that can be stamped on top of the engine. What is stamped on the metal tag just above the engines starter motor?

Customer: replied 10 months ago.
Looking on line the only motor that has the my fuel pump assembly is the 1996 502 Mag MPI GEN VI,
Expert:  Jason replied 10 months ago.

Sorry, won't be able to help you with this one. Good luck.

Expert:  Tan replied 10 months ago.

Hello. My name is***** try something different. Since it is a early model MPI lets try running the engine with a vacuum gauge connected to the suction side of the fuel filter.

This will require a brass tee some fuel line and a vacuum gauge.

Here is a diagram of how to plumb the gauge in.

Run the boat and monitor the vacuum gauge it should never exceed 2" of vacuum. If it does you have debris collecting on the pick up tube causing fuel starvation.

Can you do this test and do you under the set up?

Your symptoms are of debris in the fuel tank clogging up the pick up tube.

Customer: replied 10 months ago.
I can do this test, however I pulled the pickup tube out and check it, the tube doesn't have a screen at the bottom and was clear. I pulled the high presser pump out and cleaned the screen which was very bad. prior to cleaning this screen the engine would gradually slow down to an idle now it completely quits.
Expert:  Tan replied 10 months ago.

You can still have something in the fuel tank causing this issue. We need to do this test to eliminate the boat side fuel system as a problem.

We also need to check the fuel pressure on the fuel rail. On this engine the fitting is at the very front of the plenum just to the stbd side of center. The fitting can be hard to see but it is there.

Now do you have the fuel cool or VST with fuel pump on top of the water pump?

Expert:  Tan replied 10 months ago.

The fuel fitting is under the cap # 14 in this diagram. # 9 is the fuel rail.

Customer: replied 10 months ago.
this has the VST with mechanical pump on water pump. I see a brass cap on the end of what I think is the rail?
Expert:  Tan replied 10 months ago.

That's the schrader valve remove the cap and screw the fuel pressure gauge on that fitting.

Do the vacuum test and fuel pressure test. Watch both when engine starts to act up.

Post back with findings.

Customer: replied 10 months ago.
thanks I will get back to you tomorrow.
Expert:  Tan replied 10 months ago.

Ok your welcome.