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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3981
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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I have 1998 mercury mariner optimaxs. have replaced all fuel

Customer Question

I have 1998 mercury mariner optimaxs. have replaced all fuel pumps,floats,needles etc etc etc.
JA: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: Have replaced all diaphragm, injectors etc on fuel rails. fuel is good, fuel is not leaking anywhere. When we push the Schrader valve ontop of the stbd rail fuel comes out. when we press the Schrader on the port air comes out. cant get the motors to take off. they might chug but no idle. we have twin engines, both engines same thing
JA: Is there anything else important you think the Marine Mechanic should know?
Customer: not sure if we are supposed to get fuel out of both shraders port and stbd. perhaps one is just air. if that's the case then im stumped.
Submitted: 5 months ago.
Category: Boat
Customer: replied 5 months ago.
Spoke to mercury, they werent very helpful. Im not sure but if I understood him correctly it sounds as though the fuel is delivered to the stbd rail first. That would mean that although fuel line gos from fuel pump straight to the PORT rail, it must pass directly across to stbd rail, then supposed to travel back to the port rail?!?
Customer: replied 5 months ago.
We have bled both schraders on both sides p-u-lenty! Still same deal, fuel out of the stbd, air out of the port! These things should run, I have spent a fortune on all the replacements parts on fuel delivery and rails, all parts are new.
Customer: replied 5 months ago.
Engine serial numbers OG653408 and OG699973.
Expert:  Charles replied 5 months ago.

Hello, my name is***** will assist you.

Expert:  Charles replied 5 months ago.

you are correct on the schrader valves. One is fuel and the other is air. Speaking of them, have you actually tested the pressures on both sides to see what it is getting up to?

Have you tested both electric pumps on both engines. Since you have twins, you will have a total of 4 electric pumps.

Customer: replied 5 months ago.
As far as I know all pumps are working well.
How can I test the pressures of the air and fuel? I have a compression tester, is there another tool to test air and fuel from the Schraders. If so what is it called and what should the pressures of each be?
Expert:  Charles replied 5 months ago.

You should be able to rent fuel pressure testing gauge from an auto parts store. Make sure that it can read at least 100 PSI. The fuel pressure should read at 90 and the air pressure should be 80 when engine is running. Fuel pressure will go to 40 at only key up and then it will fall off when the pumps turn off. It should hold around 20 PSI or so.