Well I burned up my number 7 cylinder by what looks like an overly lean condition that I am being told is caused by mercury's poor factory fuel pump. The dealer is telling me I should go to an aftermarket fuel pump. I know from doing a search on the subject that this is a fairly common problem on the 500's but could not find what the cure is. What aftermarket fuel pump should I go to? Should I install a fuel pressure gage and maybe a water pressure Gage (I thought they were really only needed for higher horsepower engines)? Any other ideas on correcting the problem or increasing HP if I'm having to do some work anyway?

Try using merc's water pump/ fuel pump combo. Merc's water pump # is 46-807131a31 and fuel pump is # 842429T. It will help boost fuel to the fuel filter. Change the electric fuel pump and regulator too.

I believe there can be arcing on the plug wires that can cause major problems on the #7 Cyl too do a search for it maybe this could be a contributing factor too.

If #7 was overly lean it points to the injector for that cyl. The injectors are batch fired with 4 injectors firing at once- 4 on the even banks, 4 on the odd cyls. Was there a hole in the piston or were the cyl. walls scored? The rear cyls , 7 on the port side, 8 on the stbd. are the rear most cyls and can be affected by overheating due to low water pressure. Steam pockets can develop which causes poor heat transfer even though the temp looks ok on the gauge (temp sender on stat housing). You need at least 16-20 psi at wot water pressure-20+ for blower motors.

Regards,

Mark

Thanks for the help. I had some computer problems and have not been able to respond. The head is out to be machined and they are replacing two valves, #5 and #7, everything else they say still looks new. They are telling me that it is a problem with the fuel rail that caused the lean situation and they are suggesting some kind of baffle to prevent the problem again. I know others have had this problem and wondering if this sounds like the correct solution . I bought a boat with the 500 EFI's to run trouble free and would like to get this solved before I grenade an engine.