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Another overheating problem

B

backbays

Member
I could really use some guidance on this one. It is a 1996 Mercury Offshore 175hp that runs strong and has good compression (137 PSI) in all cylinders. It has always been used in salt water on Long Island, NY. About a month ago it started running warmer than usual. At very low speed (5MPH ZONE) it would between 1/2 and 3/4 on the temp. guage and at high speed the temp. guage will go betwen 3/4 and Hot. At mid range I can get it to run about halfway on the temp. guage. New thermostats. temperature guage, poppit valve, water pump kit, blew out and flushed every hose and fitting I could with a garden hose. I tried running the boat without thermostats but the results were the same. I get a flow of water out of the tell tale and a thermo scan confirms motor is running as the temp. guage says, but never an alarm which does work. My million dollar question is how to I go about troubleshooting the motor to determine why it is running hot before going on a witch hunt of pulling power head to check adapter plate gaskets or blockages, heads for head gaskets, etc.? Is there any way to isolate the probable cause or is their a typical failure one would expect to find on a motor of this year and usage? The history of the motor based on the prior owner (and I do believe her) was that the motor does not have a ton of hours on it and like I said it runs strong, starts easily, and idles great. Thank you in advance for any assistance. Kevin
 
If I was guessing, I would think you are maybe getting some salt build-up in the water channels from running in saltwater. That would not be totally uncommon.

Even if you freshwater flush after each time out you can still end up with salt scale and some of the channels are kinda small, so any restriction can lead to increased temps.

In smaller motors I typically turn them upside down and fill them up with a solution of 50% freshwater/50% white vinegar and leave them for about 48 hours - being upside down keeps the mix from "leaking" out anywhere and it disolves not only salt, but does a pretty good job on calcium and lime (poor mans CLR with zero negative effects on the powerhead itself).

With a motor the size of yours that would be near impossible, but if you tilt the motor to the full trailer position and drop the nose of the trailer itself as low as you can get it, you could keep a product in the powerhead for a day or two.

You could try my vinegar/water trick (less than a buck for the vinegar) or go with one of the commercial (salt off) products.

That may not be your issue, but it's a good place to start since it sounds like you have addressed any of the likely mechanical possibilities.
 
Graham- Thanks for the speedy reply. The boat is in the water and taking it out is not simple but I'll try your suggestion, perhaps with the boat in the water. Would pouring in the solution to, lets say, the thermostat openings and blocking off the intakes in the lower unit suffice and letting it soak for a few days be enough? The wisdom of age always makes me try the simpler solution before attempting more drastic approaches.
 
Here is a method to consider. Run the engine until the thermostat opens. Tilt the engine to drain and connect a set of ear muffs (use the muffs that connect w/a thru bolt) tightly to the water intake w/a short hose. Prime the hose w/the vinegar/water solution and connect the other end of the short hose to a five gallon bucket of the solution. Run the engine at idle until the can is near empty and shut it off. Leave the hose connected a few days w/the open end above the powerhead. Drill a hole in the bottom side of the bucket and connect a fitting for the hose. You may have to raise the bucket above the engine for gravity to help. Just something I thought of.
 
Thank you guyig. I have a home made five gallon bucket with a shut off valve I used to winterize my Mercruiser I/O that will do the trick as you described. At least i know what I'll be doing this week. I really appreciate everyone's suggestions and I am keeping my fingers crossed that it will lower the motor's operating temperature. Hey Graham. When I visit my family in Ottawa, Can. I never miss having the chips and vinegar. Does it have to be that great Canadian white vinegar? LOL
 
Given that this is the first time trying it I'm not sure if straight vinegar would be too caustic to some internals of the motor. The general opinion is 50/50 vinegar to water. I am open to suggestions and oinion though. I am leaving now to do some clamming and general hanging out.
 
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