50HP Overheat

[Terry Vandemore]

I run a '96 Merc 50HP tiller on a 16' Lund Pro-Angler. I use it at idle speed quite a lot for trolling. After about 15 minutes, the overheat warning buzzer has been going off. I immediately open the throttle and run wide open for a short distance and it shuts off. I have checked the water intake and it is pumping water. Is it possible that the water pump is not getting enough volume to the block at low speed? It has been fine in years past and just started this problem. Thanks for any advice.

[Hogger]

Replace your water pump impeller. If you still have a problem have the sensor checked out. You could reverse the process but your motor is old enough that I'd replace the impeller anyway so try that first.

[Stan]

As previously stated, replace the impeller. I had a 89 40hp (same block) and found the authentic mercury impeller provided more flow at idle speeds that a well known aftermarket brand. Whether this was enough of a difference to cause problem , I wouldn't guess, but there was a difference in the rubber quality of the impeler. Replace the upper pump housing ( and lower impeller plate) to insure the best performance. The additional cost (about $10-15 more) is not a lot.
Hope this helps.

[steve(IL)]

Check the hose that routes water to the water pump. I noticed diminished flow on my tiller. First thing I noticed was a clip meant to hold the hose in place caused a slight constriction. I removed the hose and tried blowing through it. It was very difficult. I used a coat hanger to push through - a variety of mud and grit came out the other end.

There was nothing wrong with my impeller. Water now shoots out the pee hole just like the good old days :-).

[Hogger]

The hose is overflow from the power head, not to the water pump. The impeller pulls water right from the lower unit. Maybe I misread your comment.

The water flow from the overflow hose is not necessarily a good pressure indicator, you really need both a pressure gauge and temp gauge to do that. Cleaning the hose out is fine, but damage to the impeller was probably already done. Sucking up sand and weeds can cause serious wear in a very short time. Impellers should be replaced at least every two years on high performance motors, and just as often on lower HP motors if run high hours.

[Terry Vandemore]

Thanks to all who responded, I appreciate the help.

[Ooops!]

Steve(IL) here - I meant to say discharge hose. The discharge hose on my motor had blockage and performed like the post described.

Water shoots out strong and hard from pee-hole at idle and WOT.
Some posts suggest how fragile impellers are - should I be concerned as long as flow is strong?