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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10898
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 2001 four winns with a mecruiser 5.0 mpi. The last

Customer Question

I have a 2001 four winns with a mecruiser 5.0 mpi. The last 2 years I been having intermitten stalling issues. The boat will run fine for a couple of hours then it will completely shut off. It will start up right away and run for a few minutes before it does it again.
Submitted: 2 years ago.
Category: Boat
Expert:  Tan replied 2 years ago.
Hello. Ok this could be a fuel starvation issue causing your issue. I would run the engine on a different fuel source. This will eliminate the possibility of a fuel restriction on the boat side fuel system. The fuel pick up tube in the fuel tank has a screen on the end of the tube. This tube can clog from debris in the fuel tank. What happens when you run the boat the debris start collecting on the screen till it clog enough to cause the engine to stall out. Then soon as the engine quits the debris falls off the screen then the process starts all over again. If no change then we have to check engine fuel pressure while engine is running to see if you are loosing fuel pressure. If you loose fuel pressure this can be a fuel pump failing or a relay failing. Since the engine start right back up I don't think this ids the problem. The next test will be to get a timing light and run the engine when it start acting up look at the timing light is the flash steady? There again I don't think this is the issue as it starts back up. Now the only thing that fits your scenario is the fuel starvation due to debris in the tank. Do these checks and post back with findings.
Customer: replied 2 years ago.
The boat has been seen by 4 different mechanics. The first one changed the fuel pump and it did not correct the situation. The second one could not replicate the issue. The third one couldn't replicate the issue until taking it out on the water and running for a few hours. He was convinced it was the ignition starter and changed it. The last one who saw took it out on the lake and couldn't replicate the problem but was convinced it was a corrupted ECM. I did notice this last time while driving the boat that the tachometer was spinning erratically, could that indicate an electrical issue?
It's hard to replicate the problem on muffs and only seems to happen under load. Once it shuts off, it will only run a few minutes and then it shuts off again until it sits for a few hours, could that Lao indicate an electrical issue such as something heating up?
If it is a clogged screen how would I correct the situation?
Expert:  Tan replied 2 years ago.
This requires removing the pick up tube from the fuel tank. This tube screws into the top of the fuel tank. The fuel supply line connect the the fitting. I would run the engine on another fuel source to be sure this is the problem. The other way to test the fuel supply is to connect a vacuum gauge to the fuel supply line and run the boat. If it exceeds 2" of vacuum then we know the boat side fuel system is the problem. It vary rarely is the ECM. Most issues like this are fuel starvation.
Customer: replied 2 years ago.
A pressure test has been done before, is this the same as a vacuum test?
Expert:  Tan replied 2 years ago.
No this is on the suction side of the fuel pump. The test you had done was fuel pressure test that the opposite side of the fuel pump. When you do this test the engine won't run correctly if it is pulling to much vacuum. It's like drinking soda through a straw. Pinch the straw in half it's allot harder to pull soda through the restriction. What you do is pull the fuel line off the fuel filter use a brass tee to connect the vacuum gauge, the fuel supply line and then connect to the fuel filter. The vacuum gauge can be connected anywhere on the suction side of the fuel pump. Here is an exmple of the test.
Customer: replied 2 years ago.
Did the vacuum test today. The gauge was consistently at 5". In idle it was at 5 and didn't move much on throttle. I didn't throttle up too much cause I was on the muffs not the lake, never got much more than 1200 rpm ( up to 1800 for a minute). The motor never cut out. Where do I go from here?
Expert:  Tan replied 2 years ago.
Ok the maximum you should have in vacuum is 2 inches. This means you have a fuel restriction on the boat side fuel system. This is either a collapsing fuel line, stuck or sticking anti siphon valve, clogged vent line or debris in the tank clogging the pick up tube. I would start with the fuel lines and work my way toward the fuel tank. The pick up tube will screw out of the top of the fuel tank. We now know the engine isn't the problem.

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