HELP! Mercury 2002 150XR6 Overheat alarm going off at slow idle speed

[jherrman]

I recently purchased a new bass boat that has a 2002 Merc 150 XR6. I've had a out a few times now and I've noticed a issue. The motor runs strong, but when I idle at low speed for more than 2 or 3 minutes (such as through a now wake zone) the over heat alarm starts to sound. (one long constant buzz). This doesn't ever occur when just idling at the dock or when cruising at normal speed. It will stop if I shut the engine off for a minute or 2 and I've noticed it will also shut off if I start running at speed. When idling through these no wake zones when it goes off the water pressure is usually around 8psi according to my gauges and at cruising speed closer to 12-13psi. Any idea what the issue could be? I'm wondering if there's some kind of thermostat on the engine?

 

[sullmate]

Sounds like the poppet valve needs replacing. I had the same problem on a Merc 135.

 

[Texasmark]

The poppet is for high speed bypassing of the tstats, manual says when operating above 2500 rpm. Sounds to me like a thermostat problem which control engine temp at low speeds. There is one in the top of each bank of cylinders. Probably can find them by the ⅜" water line connecting their outputs together and sending the water to the Pee (on some Vs) and on down to the midsection for discharge. While you are at it, if you haven't replaced your water pump/impeller in awhile might add that too. If the stats check good I'd replace anyway but that would be an indication that a new impeller is in order unless you have more problems like silt and sand in your cooling channels.

 

[CharlieB]

Purchasing any used motor it is highly recommended to service the water pump. Without copies of complete service records it's near impossible to be certain of the water pump condition. Even simply sitting unused for a period of years the impeller vanes will take a curved 'set', greatly reducing their pumping efficiency.

Replacing an impeller may not be the total solution. While the pump is apart carefully inspect the SS lower plate, if there is a line worn into it looking like a perfect 'ring' near the center hole then the plate is worn and must be replaced. This is caused by the raised sealing line on the side of the impeller. Also look for this same wear ring inside the SS cup that rubs against the other side of the impeller. All parts are included in the complete water pump rebuild kit which is recommended far more than simply purchasing an impeller.

The poppet valve can warp, either the valve or the valve seat, causing a failure to seal.

I would check the water pump assy first with every new-to-me motor.

 

[Texasmark]

Purchasing any used motor it is highly recommended to service the water pump. Without copies of complete service records it's near impossible to be certain of the water pump condition. Even simply sitting unused for a period of years the impeller vanes will take a curved 'set', greatly reducing their pumping efficiency.

Replacing an impeller may not be the total solution. While the pump is apart carefully inspect the SS lower plate, if there is a line worn into it looking like a perfect 'ring' near the center hole then the plate is worn and must be replaced. This is caused by the raised sealing line on the side of the impeller. Also look for this same wear ring inside the SS cup that rubs against the other side of the impeller. All parts are included in the complete water pump rebuild kit which is recommended far more than simply purchasing an impeller.

The poppet valve can warp, either the valve or the valve seat, causing a failure to seal.

I would check the water pump assy first with every new-to-me motor.

My manual specified the dept of the groove that would pass inspection with no replacement. Don't have that number handy.

I would think that any lack of sealing of the poppet would allow water to bypass water that would otherwise be blocked by the thermostat allowing the block ambient to be cooler and if it was bad enough the stat may never reach 143F and open, like happens at speeds above 2500 rpm when the poppet is in control of engine cooling,