1997 Mercury 150 hp EFI overheat alarm

The overheat alarm sounds after 5 minutes or so. There is a steady stream of water from the test nozzle at the rear of the engine. If the alarm sounds and I am getting water from the test opening would it be the impeller?

Or could it be the thermostat and what is a good test for that?

Or could it be the sensor itself and is there a simple test for the heat sensor.

Then I've read about an engine block by pass. Could it be that and how does one know?

S/N OG558922

Re: 1997 Mercury 150 hp EFI overheat alarm

I think the block bypass you are talking about is the poppet valve. Thermostat sounds like it is operating if you have flow but should be inspected and tested. Could be a weak impeller/water pump. and should be inspected if it has been a long time since it was replaced, etc, etc. Do you have the service manual? If not, post your engine s/n and PM me with an email address and I will forward a copy of the manual.

Gerry

Re: 1997 Mercury 150 hp EFI overheat alarm

Seeing a good stream of water from the tell-tale indicates that at least one of the thermostats is opening. If one of the thermostats is not opening, you would see water at the tell-tale, but still get an alarm. So, check/change the thermostats.

If the poppet valve (bypass) was constantly open, you'd be getting full water flow through the whole engine, and shouldn't have an alarm.

Chris......

Re: 1997 Mercury 150 hp EFI overheat alarm

Thank you veterans of the wrench and wakes. I have heard and heeded your wisdom and my engine is once again digging through the waters of Lake LBJ in search of the wily bass and catfish. There will be a fish fry soon.

Re: 1997 Mercury 150 hp EFI overheat alarm

Very good but what did you find?

Gerry

Re: 1997 Mercury 150 hp EFI overheat alarm

It seems the water pump that was put in three years ago was in to tight. At that time the engine would just quit. I figured it was the impeller at that time because the boat, engine, and trailer sat on a dealer lot for ten years. The price was right. Over all it was in good shape but the dealer did not make some need maintenance on the engine or trailer for that matter. The tires need replacement and as I mentioned, probably the impeller was pretty stiff. So I had a certified Mercury mechanic replace the impeller in Croton Michigan. He has done a lot of work for my family but maybe an employee did this work. Looked like it was all put back together with an impact wrench. Anyway, I put in a new impeller, housing, and gaskets and it is doing very well. After I do some work on something mechanical I always hold my breath for awhile before I trust my own work. My wife wonders why I am blue most of the time. Thanks for your help and the manual.