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Steady Alarm as high throttle, stops when I slow down

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BaldPoem53

New member
I have a 1987 Mercury outboard, 50 horsepower, 3 cylinder, model number '50 ELPT' and serial number OB206166. The oil injection was removed by a previous owner. When I was out with it today I had a steady alarm sound 3 different times. Each time the alarm stopped when I reduced throttle. The tell tale had a strong stream at idle and stronger at high throttle.

I'm unsure what to look at to troubleshoot the issue and don't know my way around the engine real well. Also, google/other forums have confused me on what this motor has for sensors/etc.

One other note, I don't think the alarm came from the engine area, but from the throttle lever area.

Can the experts here give me some direction?
 
I doubt you have a high RPM "Pop Off" valve like higher HP Mercs like mine. Those devices open at about 2500 RPMs, bypassing the thermostat for increased cooling.....and you don't need a stat when running in upper RPMs. If it doesn't open, you will overheat and get a warning buzzer.

The source of the tell tale stream is important in helping to troubleshoot this problem. My 3 and 4 cylinder Mercs have a plug in the exhaust water jacket cover plate where it gets its water. The exhaust cooling is a separate circuit and has no relationship to the main block cooling. You might follow the hose from the outlet and see the origin of yours.

I have 2 suggestins: Pull the tstat and look at the bottom of the pellet for the opening temp. Should be somewhere between 125F and 143F. Carefully lift one corner of the plate and insert a string. Get a pot of water on the stove with a candy thermometer or a thermometer that can read these temps and insert it and the stat in the water, suspending the stat by the string. Fire up the burner and when the stat falls to the bottom of the pot, check the fluid temp. Should be within a few degrees of the pellet stamp.....if not, time for a new one.

If the stat passes the test then you must have clogged passages in the main block cooling loop. You need to open it up and check for blockage. With good flow to the exhaust the clog probably isn't between the water pump and the block. Easiest place to start is to pull the water jacket cover off the block.....spark plugs stick through it, and look for clogged ports.
 
Thanks for the reply Texasmark. There seems to be no thermostat for this outboard. Right now as far as I can tell the cooling just enters the bottom of the water jacket and comes out of the top and then goes to the tell tale.

Right now what I'm trying to do is put a temperature sender in place of a brass plug near the #1 (top) cylinder and running that to a temperature gauge. This should allow me to confirm if there is an overheating problem occurring.

Hoping to take the boat out this week and see what happens.
 
BaldPoem53 said:
Thanks for the reply Texasmark. There seems to be no thermostat for this outboard. Right now as far as I can tell the cooling just enters the bottom of the water jacket and comes out of the top and then goes to the tell tale.

Right now what I'm trying to do is put a temperature sender in place of a brass plug near the #1 (top) cylinder and running that to a temperature gauge. This should allow me to confirm if there is an overheating problem occurring.

Hoping to take the boat out this week and see what happens.
Click to expand...
You are on the right track....keep us posted.
 
Texasmark said:
You are on the right track....keep us posted.
Click to expand...
So I'm out on the lake now and did have the issue reoccur. The temperature gauge read like 132 when the alarm went off and maxed at 142 (if memory is correct). After slowing down to an idle and then picking up some speed it then came down into the 120s.

So thoughts on next steps?
 
BaldPoem53 said:
So I'm out on the lake now and did have the issue reoccur. The temperature gauge read like 132 when the alarm went off and maxed at 142 (if memory is correct). After slowing down to an idle and then picking up some speed it then came down into the 120s.

So thoughts on next steps?
Click to expand...
Re-created 2 more times watching closely. Shortly after the temperature gauge hit 131 the alarm went off. I'd have to throttle down to a near idle and then it would stop. The last time my son noticed that the water from the telltale was steaming when we were idling after the alarm went off. I did capture some pictures/video at this time (after a couple minutes of idling) if that would help anyone.

I did fill up with gas today and mixed at a rich 40:1 ratio.
 
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