Mercury 150 overheat Alarm
 

My Mercury 150 overheat alarm (constant) is going off and I would like a few suggestions on what may be causing the problem.

Here is what is happening.

I ran the boat for 20 minutes this weekend for the first time this year. Everything ran great until I got back to the dock. I pull up to the dock and as I am going up to the bow to tie off. The alarm goes off. I Look at the motor and it was still pissing. I turn the motor off and immediatley restart and the alarm starts going off again. So I turn it off. Then restart the motor a second time and no alarm.

The same thing happened at the end of last year so I changed the water pump before puttting the boat up for winter. Ran it once with no occurance and thought changing the water pump fixed the problem but apparently I was wrong.:work: LOL.

Any ideas or suggestions would be appreciated?

Did you change the thermostats and popit valve? Also does your motor have the fuel/water seperator on the right side of the engine? The reason I ask is there is a sensor on it to detect water and it is the same alarm.

When you changed the water pump what did you change? How did the plates look? any grooves?

I was having the same issue...changed the thermostat and the problem went away. Mine was on a 60 Yamaha but the problem is described the same.

I had that problem with my Yamaha. I had a good overflow stream but was over heating. I removed the foot and back flushed from the thermostate and got a wad of grass out. Once I did that I had no problem anymore.

Thanks for responding so quick. When I changed the water pump I am pretty sure there were new plates in the kit that I purchased. The motor does have a fuel water separator on the side of the motor and I also installed a filter/seperator before the motor. I checked that one yesterday and found no water in it. So I am pretty sure it is something with the temp.

I have not changed the thermostat or popit valve. Do you think that is what is causing the problem? Are the thermostats and popit valve east to change on most motors? I have changed the thermostat on a 25 yamaha and it was very easy to change.

Thanks again for the help.

Mercs are the easiest to me. The thermostats have 2 bolts that are easy to get to. The poppit is on the right side of the block. Some of the motors are easiest if you drop the cowling on the right side.

Man I never woulda thought about that. Might be a good idea to do that every year when I change the water pump? Just incase there's something hanging around in there...

Do the Thermos need to be open for the motor to piss? I always thought so......if so, and the motor is pissing, then how could it thermos?

Mine was stuck open, the engine would piss but you could've boiled noodles in the water that was coming out. I changed the thermometer and it's been working like a champ since then.

Were you running your motor In boiling g water? If not, then that's not possible......Thermostats open to cool the motor, if they're stuck open then motor would run cold and run fat......

I had a similar problem with a Suzuki 75, it was peeing good, cold water but the temp alarm keep going off. Changed the thermostat and it's working fine now. Whats the popit valve?

Dude I'm telling you. I had it on the hose and it was cool water where it was going in at the foot but hot as all hell coming out the pisser. Turned off the water, changed the thermostat, turned on the water and it was the same cool temp at the foot and out the pisser.

OK, there must be a bypass.......wonder if mine is the same way?

Guys thanks again for the help. I am going to try changing the thermostats and popit valve this week and try running it over the weekend.

Poppet Valve
 

This thing looks like a pain to change. I think I will try the thermostats first. LOL

It is number 15 in the diagram.

It doesn't look that bad...looks like it's only 2 screws, everything should stay together, looks like a few gaskets though, I would get some new ones to replace them when you take it off.

Are ya sure it is an overheat alarm.

A neighbor has had similar problem 2 times in the last year and called me to rescue him. It turned out to be low oil alarm. The merc has 2 oil tanks and the one in the motor ran out of oil. They just checked the external tank.

The alarm on it was a continuous beep also.

If it is overheat it won't let the RPMs go above around 1200.

Worth a check anywho's.

I figgered out how the Mercury oil injection system works, 35 miles offshore. If the cap on your big tank isn't screwed on tight, it will set off the alarm.
My Merc. 200 used engine case pressure to pressure up the big tank to push the oil into the small tank, which has the float switch in it.
I didn't have the big tank lid screwed on tight enuff.

I'm sure it is not the oil. I took the cowling off twice last year when it was going off and both resevoirs were full.

Oh... When it happened last year it would not let me get up on step. I didn't look at the Rpm's but it ran horrible.