Engine Running Hot

[DJ Sailor]

I have a 1984 Hunter 34 with a Yanmar 3GMF 22.5 HP. Long story in a short form, while changing my batteries from flooded to AGMs, I had to change the hot water hoses from the engine to the water heater to clear the new alternator (you know how it goes, change the batteries, need new alternator with smart regulator, end up taking the cooling system apart and ....)

While doing so, I also changed out all the other engine hoses, cleaned out the heat exchanger (used a gun barrel cleaning brush), and put in a new thermostat along with new coolant. The engine exhaust mixing elbow was changed out about two years ago and I have not run the engine much this past year.

I noticed when I did get everything together (8 months later) that the engine was running much warmer, about 210 deg F (I put in a temperature gauge on my boat years ago) but the engine overheat light does not come on. I figured it might be that the new thermostat could be bad, so I took it out and ran the engine. The engine temperature was down to about 160 deg while running under load.

Testing the new with the old thermostat off the engine with a digital thermometer, I found both opened about the same time at the correct temperature (if I remember right, the thermostat are set to start opening at 160 deg F and fully open at 185 deg F). I put the old thermostat back into the engine and again, it was running hot, about 210 deg F. The engine use to run about 190 to 200 deg F.

Anyone have any thoughts on why it might run hot with the thermostat in but not while there is no thermostat?? I thought about some type of air entrapment but figure that when the engine is run, allowed to cool overnight and run again, it should clear out any air.

Thanks in advance

 

[Steve Dion]

Assuming that you have checked the strainer, raw water intake and impeller, I would also look at the mixing elbow.

 

[menestheus15689]

Sounds like air trapped to me !

 

[Allan12210]

Was the boat in the water for the entire 8 months? If so, maybe the intake through hull has gotten growth in it reducing the flow rate of the sea water through the heat exchanger. Check that the intake hose from the through hull isn't partially collapsing when runnning at high rpm. This will also lower flow rate. Checking the mixing elbow is a good idea if you haven't checked it in a while.

Finally when I couldn't seem to find the source of my running hot (like yours) I pulled the front cap off the heat exchanger after draining down the seawater and antifreeze from the two spigots. That cap comes off easily and I was able to put it back with out replacing the internal gaskets. The 24 tubes you will see are just over 1/4" in diameter. So I bought a 16" long by 1/4" drill bit and chucked it in my Dewalt drill. I reamed each tube out with the bit, running it in a small amount then pulling it back to clean off the flutes. You'll feel the bit emerge into the rear cavity. If you hit something solid, then stop and go to the next tube. Actually, its pretty obvious when you reach the end of the tubes. A lot of gritty material came out (saltwater deposits). After putting the cap back on and refilling with antifreeze the engine fired up and stabilzed about 20 degrees cooler.

Good luck,

Allan

 

[DJ Sailor]

Sounds like air trapped to me !

That was one of my thoughts but when I removed the thermostat, the temperature was, if anything, lower than it should be.

Any suggestions about getting rid of trapped air??

 

[DJ Sailor]

Allan,

Plenty of cooling water flowing through the heat exchanger. I cleaned the heat exchanger with a gun barrel brush (a bit safer than using a drill bit IMO), replaced all the cooling system hoses including the raw water hoses. I've used high quality hose and there is no sign of collapsing.

Again, no thermostat, cold engine temperature. With thermostat, high engine temperature. Both thermostats (old and new) checked out and I've swapped out both again in the engine.

 

[CalebD]

Left field chiming in here.

You changed the alternator, right? Is it a larger amperage alt than your old one? Could the new alt add extra load to your engine to make it run hotter, all other systems being essentially the same? Alternator bearings working smoothly?

I suppose you could put the old alt back on the verify that the new alt is not the culprit.

You've already confirmed that the T stat is not the problem.

Where is MaineSail when you need him?

 

[NYSail]

Air in the heat exchanger for the hot water heater sometimes is a bear to remove but wouldn't any air in system just make it overheat? I would check mixing elbow. My 3HM35F runs at about 145 degrees. My old westerbeke ran at about 180