Overheating?

[cobra 3.0]

My temp guage shows very hot on my 3.0 OMC Cobra, but I'm not sure if it's just the guage that could be incorrect. I changed, the impeller, the thermostat, and the temp sending unit. It still shows as hot.<br /><br />How can I test to be sure?

 

[Striper Blues]

Re: Overheating?

remove the hose that feeds the engine water pump from the impeller pump, its about an inch and a quarter or so in diameter and connects at the front of the engine on top at the thermostat housing. Be sure you are getting water to the engine from the impeller pump. I had a similar problem and it turned out to be the impeller pump housing, its plastic and with indistinquishable warping it an keep pump from priming. If you do get water forcefully flowing from this hose, you might have a stuck thermostat, or possibly a bad sending unit. Be careful about assuming its a bad sending unit, as over heating the engine can cause all kinds of bad problems (I know), like melting the flapper in the exhaust or worse. Start by checking that you are getting water forcefully from that impeller pump, then work your way through the cooling system.

 

[Striper Blues]

Re: Overheating?

by the way, I should have mentioned that my engine worked fine on the muffs in the drive way, but wouldn't "pull" water when floating...

 

[cobra 3.0]

Re: Overheating?

hhhmmm...mine is working great on the muffs in the driveway too! My flapper in the exhaust is completely gone too! I just removed it and haven't replaced it...not sure if I will or should.

 

[rml]

Re: Overheating?

FWIW, All the folks that I have talked too, on this board and locally, have said that the exhaust flappers are pretty important. When coming off from plane quickly or if the engine "diesels", it is possible to have water end up in places where it doesn't belong (i.e. cylinder head). The flappers are not expensive and a cinch to change -- just make sure to put 'em in the right way <br /><br />Regards,<br /><br /> --rml

 

[EABowens]

Re: Overheating?

Three years ago or so, I had the same problem with and older Mercruiser. No Flaps involved, but cooled fine with the muffs in place. Turned out to be the raw water pump in the outdrive.Is this the impellar you replaced?

 

[cobra 3.0]

Re: Overheating?

Yes. I replaced the impeller only but not the outer impeller casing yet. Mine looks like the original casing. I will get a new casing and hope it fixes the problem.

 

[rybad]

Re: Overheating?

Check your impeller in the outdrive, and replace it (at least every two years, even if it looks fine) .. Trust me, its the best $8-15 you can spend!<br /><br />The exahust seals ("flappers") is an interesting debate. When Volvo took over the OMC outdrive division, the scrapped the exhaust seals on the later models, leading people to believe that they are not "really required". However, you have to understand that OMC obviously thought they were a good idea, and you have an OMC, not a Volvo - so I would replace them. Like the other guy posted, they are very easy to do, and they pretty cheap (about $20 each) and it can prevent water from back-filling your cylinders and causing expensive damage. Also worth noting, on the Volvo's that I have seen, the exhaust ports are physically different as well, which could be why Volvo did away with the exhaust seal method (probably not required with the redesign).<br /><br />My theory is, if it's original equipment, replace it.

 

[cobra 3.0]

Re: Overheating?

Well, I put in a whole new impeller kit housing( already changed thermostat and sending unit too) and all and it still shows 3/4 of the way to hot. At cruising speed, I sometimes see it lurch into the red for a second or so...then drop back down to 3/4 to hot. I'm wondering if there is anything floating around blocking things. The manifold seems to be the original and has a little crack in the belly of it where water seeps out a drop at a time. Could the manifold be clogged? When I flush the hoses with a garden hose everything seems to flow well... I think I will take off the manifold next!

 

[metwrench]

Re: Overheating?

Like one of the guts pointed out, what is the Flow like into the T-stat Housing ? Thats where to start...Have Flow there at Idle ? If so, Do you have flow out the exhaust, warm/hot water flow ? check this while IN the water where your having problems..Not on the Muffs !<br /><br />If you have flow, Feel the tops of the risers, are they warm ? are the equal in temp ?<br /><br />Lastly, My guess is still the raw water pump not feeding the T-stat.. <br /><br />Also, I might look at head gaskets,,is there lots of steam out the exhaust on the muffs when the engine is first started ?

 

[cobra 3.0]

Re: Overheating?

I have good flow to the thermostat. My Elbow/Riser is warm to the touch. No steam from my exhaust from what I can tell...Is steam that evident when the boat is sitting in the water?<br /><br />When you say raw water pump, are you referring to the impeller or the water pump at the front of my engine?<br /><br />I will remove the elbow/riser and check it for blockage before I remove the manifold. The fact that the needle suddenly jumps makes me think something may be floating around and blocking things.