2010 Mercury outboard loss of power at WOT after running more than one hour

Need advice please: I have a 2010 Mercury 40 HP EFI 4-stroke ELPT4 outboard with 70 hours that looses power after running for more than one hour. Starts up quick, idles fine, and runs up to 6,000 RPM. (Motor matched to 13' Boston Whaler by manufacturer) After an hour of running a combination of 3,000 to 5,500 RPM, the engine WOT drops to 4,000 RPM, and no more. I turn the ignition key off and turn back on (starts up instantly) and it then will only run about 5,000 RPM WOT. I have drained 6.6 gal portable fuel tank and filled up with major brand 90 octane "R/V" gas and added "Sta-Bil 360". Replaced spark plugs and gapped plugs to specs. Replaced fuel line gas tank-to-engine, and bulb. (All connections are tight with new clamps). Replaced fuel filter. VST just serviced and water pump replaced by marine mechanic. Lower unit serviced, and motor oil/filter changed. Motor flushed after every use and "pees" great when running. Any advice greatly appreciated

prolly best bet is to let a merc dealer get a computer readout..if it is a fuel pump be prepared for price shock...

You're probably right about the fuel pump since it is a water-cooled pump (I think). The engine takes an hour or more of running to start losing power at WOT, and the Merc dealer didn't see any error codes just running engine on a hose for 15 min at idle. He checked out motor and cleaned out VST. I guess I will either need to wait for the trouble to worsen in order for an error code to show the fault, or swap out the fuel pump and take an expensive chance. Thanks for your input.

did you try pumping the bulb at time of failure?

Good idea, and I will try pumping the bulb next time I experience loss of power. I suppose that by pumping the bulb, fuel will be forced into the fuel pump, thereby proving the pump is breaking down when hot? I wonder if the fuel pump is not cooling properly, given the loss of engine power takes over an hour of run-time before the problem shows up.

Those motors should not have a squeeze bulb, as they are equiped with a lift pump in the fsm squeezing the bulb will overfill the fsm and make the pcm think there is a fault. Are you getting any beeps from the control box?? What kind of battery are you using...deep cycle or cranking??

dealers are installing bulbs...i am running a 2011 60 hp merc...i would not replace a fuel pump until more diagnostic work is done..
after an hour of running you have lowered the level of fuel in the tank....make sure that tank is venting..i would crack the cap a bit as i know some of the newer tanks do not vent well with the vent knob or whatever..i have a moeler tank that i have to do that to...also is the fuel level is low you may have a crack or something in the pickup tube in the tank...bottom line is to try another tank....i also would go with whatever octane gas the motor calls for..mine is 87....the flashpoint of 87 is actually lower than 90...

Thanks for your response. Here is the configuration: The Mercury 40hp outboard (manufactored date 2010) on a 2011 Boston Whaler 13' all factory-equipped with a portable Quicksilver fuel tank, fuel line with an in-line squeeze bulb. The Mercury dealer just replaced the fuel line and in-line squeeze bulb, thinking that the inside lining may be breaking down. This is not the new EPA type fuel tank and line required after Jan 1, 2011. (I don't know what a fsm or pam are.) The battery is the original marine cranking type and takes a full charge. All connections are tight and clean. Now you have me thinking that the battery may be breaking down due to age. I will replace the battery next. The other thing is I read is that a marine engine should not be run over 80% of WOT for more than 1 minute. Maybe I need to keep it under 4,400 RPM (WOT rated at 5,500-6,000). Thanks for your input, much appreciated.

thanks, and last trip I had a full tank of fresh R/V gas, no alcohol, 90 octane, with Sta-Bil, vent fully open. Specs call for a minimum of 87 octane and up to 10% alcohol is acceptable. Had problem with 87 octane as well. I will check the pickup tube in the tank. Thanks for your input.

Hello, sorry to dig up your old thread... do you mind sharing the outcome of this? I have a 2010 115hp 4s Mercury with very similar issues!

Since they likely won't respond after this long you should start a new thread and get some current help.