89 Merc 50hp overheats at full speed, temp buzzer goes off

[johnmoeller]

my 89 50hp merc 3 cyl over temp buzzes at full speed, If I pull the throttle back to idle, it goes off imediately. I get great water thru pee hole, don't seem to have a thermostat, took the water plate off and inspected it, and put it back together. I must be pulling air thru the water pump or something., Any Suggestions?

 

[KCLOST]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

When was the last time the water pump was serviced??? (replaced impeller, gaskets, by following the manual) A good water stream, doesn't necessarily mean a properly running pump. Especially a higher rpms.. That's when the impeller is stressed the most...<br /><br />What do you mean by "water plate"?

 

[johnmoeller]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

I replaced the whole waterpump two months ago and it ran fine for two outings, about an hour each. I purchased a whole kit including the bottom and top plastic housings and used the instructions to put it together with.<br /><br />The water plate, water baffle, ie. the plate that seperates the exhaust from the water flow that is on the linkage side of the engine. I opened it up looking for obstructions and the thermostat. It has a place in there for a thermostat, but none was in there. I assume it's an extra part for cold water use.

 

[johnmoeller]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

I should also say that while this is an 89 model motor, I doubt it has 30 hours on it total. It is garage kept and is used about 5 times a year at most. The water baffle, water jacket, exhaust plate all looked brand new inside.

 

[KCLOST]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

Is it possible that the motor is too high on the transom, causing the water intakes to be above the water surface while at WOT...

 

[johnmoeller]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

could be, though I have never moved the motor from it's present position. It does not have trim/tilt so it's either all the way up or all the way down. I took off the thermo sensor and am testing it. It was closing at a little over 100 degrees, obviously way to low for real use. I then ran it under real hot water, and now it doesn't seem to close at all at any temp. I've tested it up to 190 degrees now and haven't gotten even a chirp. I've probably melted the insides of the little thing now. <br /><br />Note to all. To test a thermo probe; attach a ground wire to the probe side. using a candy thermometer, heat up some water in a coffee cup, test the temp and then submerge the thermo probe into the water and listen for the chirp...

 

[KCLOST]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

That thermo switch is believe is rated for 260F..<br /><br />87 - 96829A 1 TEMPERATURE SWITCH ASSEMBLY (260 DEGREES)

 

[johnmoeller]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

I took the boat out for a test today and still have the over temp problem. I have pulled the powerhead off the unit and am inspecting every inch of it for anything I can find. I did find that there is a hose at the botom of the motor on the starboard side that pulls water from the intake and imediately dumps it out through a hole in the bottom plate. I can only assume this is a pressure relief of some sort, but it is removing cooling water from my motor. I am going to put it all back together as soon as new gaskets arrive and I will be closing off the "relief" hose to see if that makes a difference.<br /><br />I have blown the whole motor out with air and water today and all looks well.

 

[johnmoeller]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

I have read lots about Poppet's Does a merc 50 hp have one, I can't seem to find anything at all in my repair book about if the 50hp has one.

 

[KCLOST]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

It doesn't have a poppet valve, but that hose you are refering to should not be modified.... It doesn't pull water from the intake, rather it release excess water during higher rpms... If it does not, you could have boiling water around the head because it may not circulate out quick enough....

 

[johnmoeller]

Re: 89 Merc 50hp overheats at full speed, temp buzzer goes off

a new update.<br /><br />I took the head off the motor and found two hunks of old impeller stuck in the area. If they were floating loose in the head area, wouldn't they float up to the top, the very area that the water is trying to exit out of? I will put it all together on Tuesday and let everyone know whats up.<br /><br />Thanks for all the advice from all of you so far. It's nice to see the there ar still friendly people in this world.