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Cooling system

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XBRUISER

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I would love some insight on where I might find areas of cooling system restriction, how to correct, and picture to guide me in the right direction..
 
SN#0B225205
mercury classic 50/45 hp

im getting an overheat alarm at 5000 RPM, it clears within secondes of dropping to 4500 and otherwise runs fine at 4800 as a "sweet spot".
Water Pump assay was replaced in march, I bought the boat just a month ago.
 
I seriously doubt the temperature sender is that sensitive or fast-responding. This is a very basic cooling system with no thermostat. So the faster you go, the more water pressure you get.

I'd bet it's either the calibration of the sensor or a problem with the alarm module giving spurious alarms. Probably a good idea to check all the connections, clean/tighten as necessary.

Something else you could do while running is feel the exhaust cover, see if it's getting hot at higher rpm. It should never do so, only lukewarm at the most, since water is constantly flowing thru the cover. So the only reason it would get hot is if there wasn't enough water flowing.

When you run up to 5000 is there any change in water flow out the telltale?

How 'bout the exhaust relief ports at the back of the engine? They should be dumping water out there like mad.

You can use a laser thermometer on the block/exhaust cover while you're running. You should be able to note any drastic changes in temp at higher speed.

You can check the action of the sensor by disconnecting it and measuring the resistance while underway. The resistance should change smoothly, if there's any spikes at 5000 rpm without a commensurate increase in block temperature, there's a problem with the sensor.

BTW I suppose it's possible for there to be some sort of obstruction in the block (such as silt buildup or a busted impeller blade), and if you find the block is heating up too much, you can remove the cyl block water jacket cover (the one over the spark plugs) and inspect the large cooling chamber within. You can even flow water up the copper water supply tube and flush out the motor (you'll have to pull the L/U to do that).

If you do end up pulling the cover, use extreme caution on the 1/4-20 bolts, if they're seized you'll break them with too much force. Use a hot torch (MAPP works well) to heat up the structure around the bolt and that should help release it.

HTH & let us know if you find anything........ed
 
Thanks Ed.

To answer your questions:

telltale is a solid steady stream.

Exhaust reliefs are spewing wing water solidly

i will check resistance while underway next

If it is in the module at the controller, how can I check that?

I'm beginning to think that the length of time that it has set without being used could be part of the issue, i.e. Module, silt, bad sensor.

The water pump was changed in march, but it had sat until a month ago when I bought the boat, maybe sitting that long caused the impeller to get brittle and it might be time to just drop the lower unit and start at the source.
 
An impeller new in March should still be in good condition, even if it sat for a while.

To check the resistance of the temp sensor you'd disconnect the wire going to the sensor and connect one meter lead directly to the sensor, the other to ground.

Actually I was wrong about the resistance, this is just a simple switch so it should be open when the temperature is low. The manual I'm looking at says 300 deg F for the switch to close, that seems way high and I doubt is correct. Other motors go off at around 190-200 deg F and that's plenty hot for any 2-stroke. Since your motor doesn't have a thermostat, I'd be really worried if any part of the powerhead was running at more than 140 deg F.

For the alarm module itself, another thought I had is to disconnect the temp sensor only and then run the motor, if the alarm still goes off there's something wrong with the module.

This is the steady tone, right? Which is the overheat; oil injection issues get the 'beep-beep' alarm. And the module does its self-test beeping when you turn the ign on, yes?

If the hi-temp alarm switch is going off at the wrong temp you'll need to replace it if you want to retain that function. Some of these switches are really expensive, others not-so-much. If yours is one of the expensive ones, I'd be more inclined to install a water pressure gauge, which is a more accurate indication of what's going on in the cooling system, anyway.

HTH........ed
 
The oiling system had been removed prior to my purchase, so the only sensor I have is temp. No it does not beep on initial startup. Maybe installing a manual gauge is the way to go. I will check temps when I take it out tomorrow.

Thanks again, I will let you know what I find out
 
Since it doesn't self-test (beep) on startup, it makes me wonder whether it's really working properly at all.

Note that if you ground the terminal for the temperature switch, it should cause the horn to go off (with the ign switch "ON").

BTW here's the diagram that shows the temp switch:

http://www.marineengine.com/parts/mercury_marine_parts/sn/0B225205/ 1045727/335_100

#36 on the diagram. The 260 deg F switch is $360.75!

The 300 deg F switch is only $68.00. IMHO, by the time the block is 300 deg, it's gonna be toast anyway! So all the sensor is gonna tell you at that point is that your pistons and rings are fried. Pretty much useless.

Heck, you could install an aftermarket water temperature gauge and a water pressure gage for way cheaper than the price of the 260 deg switch.

Or replace the $68 "meltdown alarm" and maybe it'll go off before the engine seizes!






 
If the motor should reach anywhere near 300°F, I guess even before seizing the motor would start running erratically since the mixture should ignite well ahead of its intended timing, knocking and sounding sick. Surely also with a loss of power.. as I unfortunately just experienced with my `86 50HP..:(
 
There is no module on this engine and the only oil injection offered was the Autoblend. The sensor screws into the cylinder jug next to spark plug and has to have the higher reading as its not in any cooling water. Sound like impeller has took a set so I would replace it first.
 
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