Whaler Central - Boston Whaler Boat Information and Photos

Posted by FreeSpirit on 03/27/15 - 7:00 AM

Need advice: 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. Merc dealer checked motor by running motor on hose at idle for 15 min. Trouble not apparent until running for an hour at 5,000 RPM.

Posted by wing15601 on 03/27/15 - 7:22 AM

After all that I would guess something electrical is breaking down when it gets hot enough. If you run the boat at midrange rpm's while avoiding full throttle does this still happen? It seems with your description of the problem, the mechanic is wasting your time (not his, he's getting paid) trying to recreate the problem in the shop.

Posted by butchdavis on 03/27/15 - 7:24 AM

It's difficult for me to imagine running an engine at that speed for that long. I guess if one is commuting a long distance it would not be unusual.

If the cooling system is not in perfect condition I would expect the engine to be running at a temperature above optimum after operating at such a high RPM continuously for an hour. I read that the impeller was replaced but was the wear plate also replaced?

If it was August I would suspect that warmer cooling water temperatures could be a contributing factor but I believe water temps there are around the low to mid-seventies.

Have you tried pumping the squeeze bulb in the fuel line when the RPM drops? If that works you will know you have some kind of fuel flow problem.

Posted by 12fish74 on 03/27/15 - 4:16 PM

Check all grounds on the motor. I had an older merc 2 stroke a few years back that did almost the same thing, took me weeks to figure it out. I thought it was a fuel problem also & did about what you did in fuel lines & plugs etc... I talked to a mechanic buddy of mine, first thing he said ...did you check all grounds to the motor! Sure enough I did & there was one that went to the block that I could tighten by hand. Couple turns on the wrench & problem solved! Not saying it is that, but might be worth looking into.