Overheating troubles

[DONTGAF]

We have 1986 Mercruiser 5.7L 260 and we need some help. After starting the engine in the lake the Temp rises to about normal and then drops down slightly. (I assume the thermostat is now open) The probablem is that the temp will continue to rise until it gets just under the red. Our gauge doesn't have numbers on it just low high and normal. Now with it on the warm side if we take it out and run the boat sometimes it will come down in temp sometimes it wont. Sometimes it will come down and then go back up.
I guess my question is what is the best most logical plan of attack to solve this problem. We dont have a trailer so pulling the boat out is somewhat of a problem. We would greatly appreciate any help you can provide.

Thanks

 

[Don S]

Re: Overheating troubles

Since you are in the water, I would think a pump output test would be the first step. What drive do you have?

 

[redjmp]

Re: Overheating troubles

The bulk of overheating problems can usually be traced to a worn out impeller/ water pump.
The impeller should be changed out every 2 years and if there any grooves in the pump housing or plate, then they should be changed out as well.

That is where I would look first.

Risers also should get changed every 5 years or so as they will also rust and clog up.

 

[Don S]

Re: Overheating troubles

The bulk of overheating problems can usually be traced to a worn out impeller/ water pump.

That is where I would look first.

Granted, but since he can't pull the boat out of the water, the output test checks the impeller while in the water.

 

[redjmp]

Re: Overheating troubles

Granted, but since he can't pull the boat out of the water, the output test checks the impeller while in the water.

Sorry Don I didn't read that far!

But something tells me if it doesn't have a trailer and is difficult to pull out of the water, that the impeller probably hasn't been changed in a few years and needs to come out of the water anyways to do so as regular maintenance....

 

[GLENN M]

Re: Overheating troubles

sounds allmost like thermostat problem.when last time you took boat out of water,might be time for some overdue maintanence.

 

[Bondo]

Re: Overheating troubles

But something tells me if it doesn't have a trailer and is difficult to pull out of the water, that the impeller probably hasn't been changed in a few years and needs to come out of the water anyways to do so as regular maintenance....

Ayuh,... Pretty hard to argue with that logic....

 

[Brentathon]

Re: Overheating troubles

My order would be:
Impeller (sounds like a problem if you can't get boat out of water)
T-stat & plastic balls (easy and cheap)
Verify gauge is working (Infra red thermometer, borrow/swap gauge, resistance check, etc.)
Elbows (common cause of overheat, need to change every few years anyway)
Manifolds (last longer than elbows, but common overheat cause)
T-stat housing (I'm on my 3rd in my 1991 Alpha)
Enging Circulating pump (also standard preventative maintenance every few years)

 

[DONTGAF]

Re: Overheating troubles

About 4 months ago we had replaced the entire Upper and Lower drive unit. Along with Gimble bearing, bellows, shift cable ($2000 worth)

How is the pump test Don refered to performed?

 

[Brentathon]

Re: Overheating troubles

The "pump test" is service bulletin #82-4, but I highly doubt that it will be useful. It even says that it may not detect "marginal conditions" (like I had), or a "high speed pump output" problem (like I had).
Simply pull the hose from the O/D and look at the flow first.
Then, you should summarize the age, condition, etc. of all the cooling components (like I listed), for people to get an idea of possible culprits.......especially since your power package has many years on it.
This is my opinion.

 

[Don S]

Re: Overheating troubles

The output test will tell you if your pump, passages, and hoses are good or boarderline. here is a link to the Service Bulletin 82-4 so you can see how the test is done. Yes it depends on how accurate you measure it, but if your output is well over the MINIMUM output shown in the bulletin, then you can bet your problem is somewhere inside the boat.
I would also do the test at full down trim and turned to port and starboard. The water hose may very well be kinking when turned or trimmed down all the way, causing the odd times it does and doesn't overheat.

 

[DONTGAF]

Re: Overheating troubles

Thanks for the help. I will try the Pump Test this weekend and let you know what I find. Thanks again.