Mercruy 125 Temperature Alarm

[Mad-jack]

I am considering a purchase of a 2002 Merc 125 Saltwater ELPTO. The boat was kept in a shed with only a monthly start and flush being done. I have test drove it twice and there is a problem. It runs fine under 4200 RPMs. When you get to 4200 rpms, after about 1 minute, the temp alarm goes off and stays on until you shift to neutral and wait a few seconds. After the first test drive, the current owner took it to the Merc mechanic and he installed a new water pump kit, new thermostat gasket, and he changed the lower unit oil and oil seal. After reading through this forum, I asked him about the thermostat and he says the thermostat seems to be opening and closing like it is supposed to. After the repairs, on the 2nd run, the boat still runs fine under 4200 rpms for an extended period of time but the alarm still goes off when the rpms are 4200 rpms or higher. There is a steady stream of water from the Pee hole and the motor does not seem to be be getting too hot to the touch. The mechanic seems to think there is a coating of slime or something inside the water jacket so it's not removing the heat like it should be doing. I am open to any suggestions at this point because I still think it might be the thermostat. Do you all think this is something that is more serious going on here? The rest of the boat is in really good shape and the maintenance looks like it has been done by the owner.

 

[capttwb136]

Re: Mercruy 125 Temperature Alarm

I am considering a purchase of a 2002 Merc 125 Saltwater ELPTO. The boat was kept in a shed with only a monthly start and flush being done. I have test drove it twice and there is a problem. It runs fine under 4200 RPMs. When you get to 4200 rpms, after about 1 minute, the temp alarm goes off and stays on until you shift to neutral and wait a few seconds. After the first test drive, the current owner took it to the Merc mechanic and he installed a new water pump kit, new thermostat gasket, and he changed the lower unit oil and oil seal. After reading through this forum, I asked him about the thermostat and he says the thermostat seems to be opening and closing like it is supposed to. After the repairs, on the 2nd run, the boat still runs fine under 4200 rpms for an extended period of time but the alarm still goes off when the rpms are 4200 rpms or higher. There is a steady stream of water from the Pee hole and the motor does not seem to be be getting too hot to the touch. The mechanic seems to think there is a coating of slime or something inside the water jacket so it's not removing the heat like it should be doing. I am open to any suggestions at this point because I still think it might be the thermostat. Do you all think this is something that is more serious going on here? The rest of the boat is in really good shape and the maintenance looks like it has been done by the owner.

When the alarm goes off remove the cowling and feel the water cooling jacket, if it feels cool to the touch you know the cooling temp is fine. Since you have a good tell tale stream it most likely is not the thermostat. I would suspect the over heat sensor. Could the alarm be the low oil alarm?

 

[Texasmark]

Re: Mercruy 125 Temperature Alarm

The tell tale is plumbed off the exhaust manifold. Pop the cowl and follow the tube. This is a separate cooling circuit due to the fact that the exhaust must be cooled by fresh water as long as the engine is running.

The stat and a high speed pressure relief (pop-off) are located on the upper left corner of the rear of the block and are there to control the water temp around the cylinders. The stat controls engine temp below 2500 rpm. Over 2500 adequate pressure from the water pump unseats the popoff, a much larger, non temp controlled valve opens. I have a 90 and my service manual covers 75 thru 125.

US domestic water heaters, in the N position on the thermostat are set to 140F to give you a bench mark on what 140 feels like. The stat in my engine and surely yours too, being of the same series, just 4 cylinder vs 3, is stamped 143F which is the opening temp. So, at low rpms, after you give the engine time to heat the water, placing your hand on the block above the spark plugs should feel like the water from your hot tap at home.

Above 2500 rpms the popoff opens and there is no way that the stat can maintain any kind of temperature in the block as the larger popoff lets it out before it has had time to heat all that much...obviously the water picks up engine heat.

The over temp switch is set to 195 F opening and about 175 reseating. Water boils at 212F at sea level for another comparison as to what 195F feels like.

So, you can make a pretty good guess with your hand as to whether or not you have a cooling problem or a bad OT sensor....the tan or tan/lt. blue stripe wire on the rear of your block. Since the impeller blades are bent back at higher rpms and on that engine ram water assists in developing water pressure, I'd ensure I didn't have a bad OT switch before I tore into the LU and replaced the impeller.....but how long has it been since you replaced it?

HTH,
Mark

 

[Mad-jack]

Re: Mercruy 125 Temperature Alarm

The mechanic just changed the whole water pump kit out before I test drove it the second time. I also noticed on the ticket that he repaired the poppet valve seat (labor only). I am now more inclined to go with what you said- that it is the over temp sensor. Is there a way to test it with a meter- ohm it ?? Thanks for all the good info by the way.

 

[Texasmark]

Re: Mercruy 125 Temperature Alarm

Yes. Put it in a pan of water on the stove and get it up near boiling....not a rolling boil, but getting bubbles and some steam off it. Put your ohm meter leads on it before you start, case of the sensor to tan wire terminal which should be open circuit. Once the water gets where I said the meter should show a short circuit, considering the meter lead resistance....something less than one ohm. If you get the short circuit well before the water starts steaming and bubbling, get a new sensor.

Mark