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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10594
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1988 Mariner 75 HP motor. The motor always starts

Customer Question

I have a 1988 Mariner 75 HP motor. The motor always starts just fine. Sometimes while I'm running it the RPMs will drop and the motor will die. I check the bulb and there is no pressure in the system. Then I can't get the bios to reprime. If I tilt the motor all the way up then all the way back down I can get pressure in the system again. Also sometimes when priming the bulb I can't get it all the way hard. There is still a little air in the system.
Submitted: 4 months ago.
Category: Boat
Expert:  Tan replied 4 months ago.

Hello. My name is Tan.

If the primer bulb will not get hard you have either a fuel restriction or a leak in the fuel system on the engine.

What we need to do is run the engine on a different fuel source to eliminate a fuel restriction issue. If the engine runs better then we know the boat side fuel system is the issue. If no change then we know the engine is the problem.

Now when you get the other fuel source connected to the engine squeeze the primer bulb does it get form now?

Can you do these checks?

Customer: replied 4 months ago.
Hi Tan,I do not have another gas tank. At the beginning on the day it always pressurizes properly. It's after running for a while that it sometimes loses pressure. The fact that when I fully trim the motor up then back down that I can get it to repressurize - does that suggest anything?
Expert:  Tan replied 4 months ago.

The primer bulb only gets firm in the initial priming. Once the engine is running it only has fuel flowing through it. The primer bulb is on the suction side of the fuel pump so it only pulls fuel through the bulb.

Do this take the boat out run it till it starts acting up then squeeze the primer bulb real hard. Does engine pick up and run better?

Customer: replied 4 months ago.
before I figured out raising and lowering the motor would help i did have my passenger keep pumping the bulb when the motor sounded like it was going to die and the motor would continue to run for a few minutes then they would have to pump the bulb again.I should clarify that when the engine would die I could not get the bulb to reprime until I raised and lowered the motor then I was able to get the bulb firm again. I would squeeze and squeeze the bulb but it would not become firm. The motor would run for a minute or two liking from the gas I was pumping into the system but then it would die again. Then I accidentally figured out that raising and lowering the motor I was able to get the bulb firm again. Then the motor would run again just fine for a few hours. Which was usually the rest of the day on the water
Expert:  Tan replied 4 months ago.

Ok somewhere you have a hose that is kinking. This usually occurs near the motor. Check all the fuel lines be sure they are straight with no damaged or kinked spots. If all is good then you have either a failing fuel pump or the fuel tank is clogging the pick up tube not allowing fuel to flow to the engine. These are the only possibilities you have causing you problem.

Post back with questions and findings.

Customer: replied 4 months ago.
One more point that I forgot - this didn't start happening until I accidentally ran out of gas. I suppose it's possible I pulled some crud up from the bottom of the tank and clogged something?
Expert:  Tan replied 4 months ago.

Yep this was the purpose of running the engine on a different fuel tank. This is how we determine if the boat side fuel system is the problem. This also separates the engine from the boat side fuel system. This always points to either the engine or the fuel tank.

Where the engine fuel supply line connects to the fuel tank is a fitting that screws into an elbow on the top of the fuel tank. This fitting should screw out. This will be the pick up tube. If it has a screen on the end of the tube this is what clogs up with contaminants.

Remove the screen and throw it away. Any contaminants should be caught by the fuel filter.

Do this check or run the engine on a different fuel source to determine if this is the problem. You could also have a bad fuel line that is collapsing internally and when you tilt the motor is open the collapse back up.

Does all this make sense?

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