1984 Merc. 150 overheating.

[JRatliff]

I have a 1984 V6 Merc. 150hp that is overheating. I have changed the water pump and thermostats(Even tried to run it without t-stats). I have changed the head gaskets and have good compression. I have changed the exhaust plate gaskets, and I have checked the idle valve(poppit) for blockage(spring is good and plastic valve looks fine).<br /><br />It will idle and cool fine with water coming out the pisser and water pressure about 12psi. When I get to about 1500rpm the water pressure gets eratic and then the temperature starts climbing. If I accelerate out of the hole real fast, before the engine gets hot, the water pressure goes up to about 25psi and then the temp starts climbing.(Other engine at about 10-15psi) The water pressure on this engine is totally different than my other engine once I go above 1500rpm.<br /><br />I have high water pressure and an overheating engine.<br /><br />My next thought is to replace the powerhead gasket...<br /><br />Also, I have noticed on the overheating engine that water never comes out of the two rectangular exhaust holes on the back of the engine(below the pisser-above the lower unit). On my good running engine a lot of water comes out of these.<br /><br />Any suggestions would be greatly appreciated!!! The powerhead is coming off this weekend unless somebody has a better idea..<br /><br />Thanks<br />JR

 

[KennyKenCan]

Re: 1984 Merc. 150 overheating.

Hi JRatliff,<br /><br />Have you serviced the poppet valve?<br /><br />It is located on the stbd side of engine, all the way at the bottom.<br /><br />This valve only opens when the pressure gets to a predetermined pressure, and that essentially by-passes the thermostats allowing the engine to remain cool.<br /><br />Be advised that these motors are notorious for running hot.<br /><br />Don't be suprised if the temp goes up around 200 degrees, which is greatly affected by the temp of the body of water you are boating on.<br /><br />Hope that helps.<br /><br />Kenny

 

[CU2NITE]

Re: 1984 Merc. 150 overheating.

How hot are you getting? Do you get the alarm? Also if you need a new head gasket your plugs will be clean in the cylinder the water is leaking in.<br />CU

 

[JRatliff]

Re: 1984 Merc. 150 overheating.

Thanks for your replies...<br /><br />I have looked at the poppit valve and it looks fine, I did change the diaphram. I'm not sure what you mean by "serviced".<br /><br />Also the head gaskets were already changed...<br /><br />The powerhead is off and I am changing the two gaskets below it. Wish me luck. I couldn't wait, I am ready to go fishing.<br /><br />Thanks,<br />JR

 

[JRatliff]

Re: 1984 Merc. 150 overheating.

CU2nite,<br /><br />The temp. is climbing rapidly and at 200 degrees I shut it down to idle and the temp goes right back down. The other engine is running at 142, right where the thermostats are set.<br /><br />Thanks,<br />JR

 

[CU2NITE]

Re: 1984 Merc. 150 overheating.

Jr I find your problem very interesting. I have the same motor (85) and when idleing my temp stays low even at trolling speed it stays low, but at half throttle it will jump to around 175-180 at full throttle it will climb to around 195-200 thats when I come back to half and the temp drops. Early in the year I did't have problems but the water temp was around 50 degrees. Now that the temp is around 85 I have a hotter engine. I've done the same things you have impeller, houseing, stats, headgasket, poppit and still have the same results. I have talked to many and they tell me this motor just runs hot in warm water. I think I will just live with it until the water cools down. What interest me about your situation is that you have two motors and one is running at 145, I would think that one would get hotter than the termastat at full throttle. Please let us know what you found out. I'm same as you love to fish, I'm going to brave the 100 degree temps today and try to catch some walleye and wipers. Good luck.<br />CU

 

[JRatliff]

Re: 1984 Merc. 150 overheating.

CU,<br /><br />What bothers me is that when the water pressure goes up the temp. starts climbing and the tell-tale(pisser) stops. Since the tell-tale is stopping I know there is something wrong and the water pessure is going up makes me think is is not water but air pressure(Exhaust) at that point.<br />Also, you are probably right about getting hotter than the thermostat at full throttle. I haven't been able to get there very long at all.<br />My water pressure guage starts acting up at about 1500 rpm.<br />I am putting the powerhead back on tonight or tommorow and will let you know.<br /><br />JR

 

[JRatliff]

Re: 1984 Merc. 150 overheating.

The power head is back on and the motor is running a lot better. CU, you were right mine is getting warm like yours but now I have water peeing when I get up to the 200 degree range. <br /><br />We also found out that the right engine is running hotter than the gauge shows. We are changing the temp. sending sensor on it.(We really don't know how hot its been running)<br /><br />JR

 

[CU2NITE]

Re: 1984 Merc. 150 overheating.

JR<br />Good to hear it's running better, will you post back and let me know what your other motor is running at when you get the new sender on? I'm curious if it will run around 195-200 also.(this temp still makes me nervous!) Good luck.<br />CU

 

[BF]

Re: 1984 Merc. 150 overheating.

I've got the same engine but a couple years older. The part that concerns me is you say there's no water coming out the 2 rectangular exhaust openings when running! Like your other motor, on mine there is lot's coming through there at any rpm's above idle.<br /><br />Maybe it's changed now since you say you now have water coming from the tell-tale. So, what gasket was the culprit??<br /><br />Brent

 

[JRatliff]

Re: 1984 Merc. 150 overheating.

The last gasket I changed was the powerhead gasket and now it runs a lot cooler... Still more to go, I want to be able to run and not worry about slowing down to keep the engine cool!