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Merc. 150 Alarm Issue at high speed

K

Kingfish22

New member
Hoping to get some help as I'm new to this forum as well as the engine!

I have recently purchased a 1998 Mercury 150 Outboard (Offshore). I have a long (continuous) alarm that will go off after running the boat at 3 /4 – full throttle for longer than 5-10 minutes. I can run the boat at low speed (trolling speed) for well over 2 hours with no problems whatsoever. When the alarm goes off, I put the boat in neutral and within 10 seconds the alarm goes off. It also seemed the alarm would shut off when I took the Oil Reserve Tank cap off (I’ve checked the sensor in the oil tank and seems to be working correctly). Then again, that could be because it takes me longer than 10 seconds to take the cowling off and get to the oil reserve tank.... I've read that the short beeps are oil and the longer (continuous) beep is a temperature issue. Is this correct?

Is there a reason why at low speeds I have no issues and at ¾-Full throttle it alarms (again after 5 min. or so of running).

I’m thinking I need to replace all the hoses from the oil pump that seemed old and not terribly pliable (which may be causing a vacuum leak when run under full load) not allowing sufficient oil when running under high speed? Other friends have suggested it may be a temperature sensor that may need to be replaced.

I’ve bled the oil pump lines, which did not help resolve the issue.

Water seems to stream out the back of the engine steadily – it also appears the water box/impeller is working correctly. Again, the issue seems to be only when run hard -- when running at low speed I have no problems. Any thoughts, suggestions on what to look for to resolve the issue would be very much appreciated!
 
It could be a heat alarm.

Even with water coming out of the tell-tale it may not be getting enough through the powerhead fast enough to keep it cool when running near wide open.

These motors have a poppet valve as well as a thermostat that controls the cooling system.

The thermostat takes care of things at lower rpms, the poppet kinda takes over after the rpms hit midrange.

The poppet is a spring based (valve) that is "supposed to" open further (restrict water less) as the water pressure increases (higher rpms = higher water pressure).

You can find the poppet (assembly/cover) if you are standing behind the motor (so looking at the prop) located on the right hand side, down near the bottom of the block/near the lower cowl, just forward of the (swell) of the bottom cylinder on that side.

There is a little (cover) down there - basically round with with a smaller round cover connected to it. The lower part has a fitting and some tubing connected to it. It all comes off as one piece. Once it's removed the poppet assembly itself is inside - kinda looks like a multi-piece thermostat.

You can get the parts individually (valve, spring, seals etc) but a whole kit runs about 30 bucks or so and it's worth doing everything if you open it up.

If I was betting I would put money on a toasted poppet and not an issue with the oil.
 
galamb, I think you might right on this. I went down to the boat to see where the poppet valve was located. Going to order the part and replace. Hopefully it will solve my issue. It appears I will need to remove the lower cowl on the engine. Pretty easy process? Can't seem to locate any info on how to do it. Only see picture of parts from manual. Curious if you had any good links or videos you've come across? Really appreciate all your help!
cowl.jpg
 
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