Thermostat woes

[rperrone]

Not sure what happened? Westerbeke/Universal 35B with 400 engine hours. Coming into channel, of course, at about 6 knots,
running 2800RPMs for about 5 minutes. Prior to that 2000 RPMs for about 15 minutes, enough to douse sails. I look down at instrument panel and Thrmostat reading off the guage, to the right of the highest reading, It is the Westerbeke admiral panel guage. Overheated engine happened to me once before same boat diffferent engine comming out of Buzzards Roost Marina, SC about 10 yrs ago. I learned that the thing to do is to shut her down. Well, I am in the channel this past Saturday, and know I do not have much choices. So we shut her off. She was pumping water, I checked the Coolant Resovoir, it was filled. I looked in the engine compartment, no burning, steaming, riser was not throwing smoke. All w/in 90seconds. Ok, we start her back up no problem, she drops to 190 deg F. Still much hotter than her normal 165 deg F. Then she shoots up off the guage again. I pull into harbour, set the hook. Couple hours later, Start her again while at anchor, run her up to 3200 RPM, no load, she makes up to about 168deg. Could it have been that the intake at the thruhull was temp clogged? is it a faulty thremostat? is it the HEX that needs flushing? Next morning ran fine, but did not put her on
a high load. Need help?

[JWSutcliffe]

It's been my experience that marine engine overheating issues generally have multiple causes related to inadequate discharge of heat overboard. Thermostat problems may be an issue on raw water cooled engines, but are unlikely with a fresh water cooled engine. After all, how often do you hear about automotive thermostats failing? Or Kubota tractors? Simple answer -it's rare. What is far more common is loss of heat exchanger capacity because of fouling, compounded by some raw water impeller wear and growth over the inlet screen. Add some seaweed across the screen suddenly and you have a sudden overheat situation. If the HX was still like new, some loss of raw water flow wouldn't be as critical.

I went through this with my M25 for 2 years. Several sudden overheats, more often inability to run to full rpm. Replacement of the heat exchanger this year with a larger size ended it all.

[rperrone]

35B Manual Maintenance Schedule advises clean proffessionally at 1,000 engine hours ( HEX). Replace HEX zincs at 50 hr intervals.
we do that religiously. It seems like we should get a lot more than 400hrs of service from (HEX). Earlier post, I forgot to mention, we changed the impellar twice in the 400 hrs. So far each change has turned out to be not needed. Blades show only a slight curvature in the rotation direction. Very little wear on rubber blades.

[Ken Cave]

Does this engine have a mixer elbow behind the exhaust manifold?

Yanmar has problems with these as they clog up.

Just a thought.

Ken Cave
CD 28
Dragon Tale

[rperrone]

If you are referring to raw water and exhaust riser elbow after manifold?
yes

[Andy Denmark]

Bob,

When was the last time you looked at your engine zinc?

Could be it corroded away and a piece fell off and is lodging in the thermostat. Water pressure holds the piece in place where it prevents the thermostat from opening. When the engine stops and flow stops (or engine cools down and the chunk of zinc falls out) this allows water to flow again. Engine then runs cooler until the problem repeats.

Same problem could occur in a strategic restriction in the water jacket.

Just a WAG or two.

Andy

[rperrone]

Andy,
Aabout 6 weeks ago. I replace the HEX zinc and there was nothing left. Must have completely corroded. Much to my dissapointment.
Anyway, 2 weeks ago I checked the same newly replaced zinc and it has begun to corrode but not to the point of needign replacement. The Manual gives you about 5 diagrams indicating stages of zinc remain / zinc replace. So if your WAG is right, how can I get the sediment/zinc bits out of the Thermostat circuit?

[JWSutcliffe]

This engine is fresh water cooled, so the thermostat is in the fresh loop while the zinc anode is in the heat exchanger in the saltwater loop. No way for zinc bits to get into the thermostat.

[Andy Denmark]

Wrong at the concept level! Well, it was worth a guess.

Is the raw water strainer above the waterline -- wonder about an air bubble starving the water pump if the strainer is mounted too high?

Standard practice is to mount the filter so the water rises just above the top of the filter outlet (consistent with the hull waterline). This guarantees water to the pump without sinking the boat if the filter is opened up for cleaning or servicing.

Another WAG ............

[rperrone]

OK. Last week ( Thursday) had a diver dive on the boat. He found many of the holes on the port side raw water intake filled with mud.
Supposedly from Irene. He has cleaned them out.

Let's see what happens next?
Thanks for input.