Mercury 135HP 8 years old questions

I have a mercury outboard 135Hp, about 8 years old. It has been regularly serviced.

Recently when I was using it I got an engine overheating warning on the VesselView mobile app that was linked to my engine. I got it home and rang my marine mechanic who had recently serviced it. He said that the problem was very likely a thermostat, and as he didn't have a spare one available and I wanted to go boating the next day, he came over and cut the centers out of the 2 thermostats using a grinder, effectively disabling them. With a plan to replace them later when the parts were available. The water doesn't get very cold here, so I wonder if thermostats are required at all anyway.

I took the boat out the next day and it seemed to be fixed.

The mechanic said he replaced the impeller during the last service.

When I now run the motor, no water is coming out of the tell tale, so I am concerned that water is not circulating. I am using the muffs, with a tight fit and high water pressure. I ran the motor for about 2 minutes but no water out of the tell tale, but also no alarm or any sign of overheating. I didn't want to risk overheating or damage, so I stopped the motor.

The mechanic is not available at the moment, any ideas?

Any cooling water coming out the prop exhaust? With my engine(Suzuki) the tell tale is only a visual to let you know that the water pump is in fact spinning, the tell tale has nothing to do with engine cooling, not sure if Mercury is designed the same. Outboards run cooler than automotive engines so without a thermostat yours is running even cooler than Mercury designed it to run. Your mechanic isn’t too worried about running without a thermostat in the short term. I don’t know what the short and/or long term implications are of running an outboard without a thermostat so I can’t comment. Sorry I couldn’t be more help but I’m sure another forum member more familiar with your engine will chime in.

William01 said:

Any cooling water coming out the prop exhaust? With my engine(Suzuki) the tell tale is only a visual to let you know that the water pump is in fact spinning, the tell tale has nothing to do with engine cooling, not sure if Mercury is designed the same. Outboards run cooler than automotive engines so without a thermostat yours is running even cooler than Mercury designed it to run. Your mechanic isn’t too worried about running without a thermostat in the short term. I don’t know what the short and/or long term implications are of running an outboard without a thermostat so I can’t comment. Sorry I couldn’t be more help but I’m sure another forum member more familiar with your engine will chime in.

Click to expand...

Yes, plenty of water is coming out the exhaust. Thanks for your input.

That’s the important part but I would still think that water should be coming out of the telltale irregardless of thermostat condition. The telltale doesn’t stop flowing when the thermostat opens?

Have you tried some wipper snipper cord or compressed air to unblock the tell tale?

XS POWER said:

Have you tried some wipper snipper cord or compressed air to unblock the tell tale?

Click to expand...

No, I have not tried to unblock the tell tale, I will try with whipper snipper cord as you suggest.

I also have the screw in fitting to attach a garden hose directly to the motor. I asked the marine mechanic about trying that either instead of or in conjunction with the muffs and he said that hose fitting was for flushing the motor without running it, for example in a situation where you did not want to make a lot of noise. Does everyone flush using only the muffs or can that direct hose fitting be used too?

William01 said:

That’s the important part but I would still think that water should be coming out of the telltale irregardless of thermostat condition. The telltale doesn’t stop flowing when the thermostat opens?

Click to expand...

Yeah, I thought the purpose of the tell tale was to indicate if water was circulating correctly in the motor, my assumption being that if the tell tale was not flowing constantly the motor should be stopped to avoid overheating and damage. I have often wondered why the expected minimum flow rate for tell tales are not published by manufacturers. i.e. 840 mL per minute at idle. This would allow for an easy test of partial blockage or impeller deterioration.