Weird Temps in 120 Merc

[trm]

120 Mercruiser<br />Strange thing begining to happen. Anything over 1800rpm (in gear)and temps are normal. Under 1800rpm and she heads for the redline (and a little beyond if I let it). When it hits the redline, either in gear or neutral, I go above 1800rpm and the temp returns to normal. <br /><br />What's going on??????????? Pro's installed new bellows and water shutter in June.<br /><br />Thanks for any and all suggestions on where to start looking.<br /><br />Swabbie

 

[B.E. Anderson]

Re: Weird Temps in 120 Merc

Swabbie,<br /><br />Sounds like you've got a bad water impeller. You should take care of this immediately, or the temperature fluxes you describe are going to lead to expensive engine failure.<br /><br />Brian

 

[trm]

Re: Weird Temps in 120 Merc

Thanks B.E.<br /><br />I thought that would be the response. But what bothers me is that with a bad impeller (new with less than 15 hours on it) that the heat increase/rpm would be the opposite of the current experience; i.e. hotter with higher rpms.<br /><br />By the way I watch the temp guage very closely and when it gets to the warm side, I rev it up and take the temp back to normal. Has not yet sustained high (red on guage) for more than 60 seconds yet.<br /><br />Does this make sense, or is everyone in agreement with B.E?<br /><br />Thanks in advance,<br /><br />Swabbie

 

[oldboat1]

Re: Weird Temps in 120 Merc

Assuming this is not a closed system (actually, maybe even if it is), I would also be suspicious of the impeller. If professionally installed, I would check back with the installer. If you check it out yourself, I would make sure that the impeller is intact and installed correctly (key in place on shaft, wear plate properly seated and sealed, etc.). Before doing anything, it would be a good idea to check the intake screens for partial blockages.

 

[Tacklewasher]

Re: Weird Temps in 120 Merc

I had the same problem but it turned out to be a loose fan belt. Maybe check your water pump on the front of the motor.

 

[boat doc]

Re: Weird Temps in 120 Merc

I had the same problem but it turned out to be a loose fan belt. Maybe check your water pump on the front of the motor.<br /><br />I have gotta see this one, does that fan work to draw fresh air into the radiator........ lol, <br /><br />it could be a bad impeller housing, blocked pick up. I have seen rocks get into the impeller house, and destroy them, or even an over heat destroy them. Did your guy charge you for a complete housing and impeller. A gasket could have been left out causing the housing to leak. There is also a seal that sets atop the imeller housing that if not set right, it will burn up and fail after a few hours, this causes the housing to leak. All of these will cause overheat at idle, because not enough water is being pushed through your cooling system.<br /><br />This is for later model ALPHA drives b.t.w.

 

[B.E. Anderson]

Re: Weird Temps in 120 Merc

Swabbie - What it boils down to (no pun intened) is poor water circulation. By increasing RPM, you up the circulation, thus bringing the temp down. I have seen this exact behavior three times now in a 20 year old mercruiser, and it has been a worn out impeller every time. I would heed the directions others have given you since you say the impeller has been replaced recently. You are either not picking water up efficently, or something is restricting the flow. The other experts here could advise you better, but I do know that it isin't good to get your motor hot @ idle, then rev. it up and "shock" it with a surge of cool water.<br /><br />Brian

 

[trm]

Re: Weird Temps in 120 Merc

Thank you all.<br /><br />I may add that the trouble began when the lake turned over and had to stick it in reverse to clean some weeds from the leg. Bet some of the finer stuff is in the impeller housing. I assume that there is no way to clean it out other than to go through the motions to replace the impeller and therefor, while in there, replace all the parts anyway. Unless you know of a way? Since I have indication water in the downleg oil, I'll be in there anyway with seals in hand as well.<br /><br />That brings up another point. Is it more difficult to renew seals in the upper end than the lower end? Will putting the leg in a 55 gallon drum of water and adding light air pressure in the leg reveal the location of the leak? Any other thoughts about locating the leak?<br /><br />Thanks,<br /><br />Swabbie

 

[boat doc]

Re: Weird Temps in 120 Merc

try this, get ya some mickey mouse ears, and run the boat under water hose pressure, this will flush anything small (half pea size) through, and you will be able to see if it cools while being force fed water.