1990, 90hp Mariner fuel line loosing pressure

My 90hp Mariner is driving me crazy. The engine starts and runs well when running at higher rpm. At slow idle speed the priming bulb must be squeezed every several minutes to keep the motor running. I have performed the following and have the same problem:
checked the fuel tank vent for blockage
replaced the primary fuel filter (fuel/water separator)
replaced fuel line and priming bulb from primary filter to engine adapter
replaced the small see-through filter on the side of the engine
checked all fuel line connections on engine for good seal
I do see bubbles in the small engine mounted filter when the engine is running. I assume air is getting into the system somewhere!
I would greatly appreciate any additional non-invasive things I can do to identify & correct this problem.
my email is: bhwoodside@yahoo.com

Bill

Re: 1990, 90hp Mariner fuel line loosing pressure

You are correct in your assumption that air is leaking into the fuel system, enough so that at an idle you effectively run out of gas.

Use that clear filter or a piece of clear line and a hose nipple in different places in your fuel system, from the tank on. looking to find the point that the air is entering the system. Poor connection, cracked fitting, usually a poor o-ring on a quick connect fitting. Check ALL fittings.

Re: 1990, 90hp Mariner fuel line loosing pressure

As already suggested, using a clear hose to find an air leak is a good way to troubleshoot. I would only add the following comments:

-There is no pressure in the fuel line between the fuel tank and the fuel pump on the motor. The fuel pump sucks the fuel from the tank causing a vacuum in the line.

-Perhaps you have several concurrent problems. A weak fuel pump can also cause the symptoms you describe.

-Confirm that the fuel delivery system is working OK (after you are done fixing anything you found wrong) using a vacuum gauge. The FAQ section has a good article titled "HOW TO: Testing your boats fuel system" , which provides pretty good guidance.

Just my thoughts....

Re: 1990, 90hp Mariner fuel line loosing pressure

Thanks CharlieB & hkeiner,

I have checked all connections and will begin to test each connection individually early next week. I also plan to get a small portable gas tank and hook directly to my engine bypassing my entire system before the engine. This should isolate the problem to the engine if the problem still exists.

Re: 1990, 90hp Mariner fuel line loosing pressure

I had a similar problem who's driving me crazy last year. After changing mostly all components of the fuel system without succes, i nailed it down to the fuel pump cover that i overthightened when i replace the diaphragm. It was made of plastic, so tightenig it too much made it bend a bit and it wasn't perfectly sealed, sucking some air...

Re: 1990, 90hp Mariner fuel line loosing pressure

Alternative fuel supply is always a good place to start, as others have said so many times before. This will narrow your trouble shooting down to either the boat or engine side of your rig. The clear piece of test hose will show the leakage whick likely is causing loss of vacumn pickup....guessing this is your problem....just my 2cts