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Symptoms of a faulty Thermostat

J

jasonious

New member
End of Novemeber and I'm still boating up here in Seattle! Gotta love a nice old wood sedan. :)

I'm chasing down the cause of an overheating issue on my starboard circa 1967 FWC poly M318B motor. What I'm seeing is what appears to be a very long warm up period, then a steady increase in operating temp at idle and low revs. When I increase revs to about 1500, the temp seems to settle and then slightly decrease. I have new impellers on the dual pocket pump and the raw water pump impeller is about a year old. Seems to have good water flow all the way around. The heat exchanger is warm on top and cool on the bottom and while warm, I can place my hands on the risers. Swapping out the tstat is not so easy as it is the ancient screw in style.

Would a faulty tstat cause these kinds of temp swings? What are the general sysmptoms of a faulty tstat? Does anybody have a source for these old screw in types? I know there is a conversion kit available, but it costs $$$, and I'd like to keep my motors original.

I am basing the overheating on the temp gauge, which has a new sender. The port engine never gets above the second mark on the gauge, while the port was swings all the to the center mark. I have installed temp alarms on both engines that go off at 200F. Have not set off an alarm yet, thankfully.

Thanks, and happy holidays!
 
That's consistent with a sticking t-stat, but those weirdo dual pocket deals have all kinds of tricks. There's a restriction somewhere in the retrun path to the pump that may have rotted out on you.

Jeff
 
Okay. The circulating pump is normally a centrifugal type which can be "stalled" (flow stopped) without a problem: it simply stops pumping. Not so the rubber impeller type; they are a positive displacement pump. Shut off the water flow and it either slips the belt, blows a hose, or wrecks the impeller.

So, when the motor is cold and the T-stat is closed, this dopey pump still has to pump water someplace (or else). Most of it blasts out through the exhaust system, that is not T-stat inhibited, but SOME coolant HAS to flow through the motor or hot spots and internal steaming (not good!) would result. A bypass orifice is provided somewhere in the motor recirculation circuit to keep the coolant flowing through the motor.

Now, what happens (I've been told) is that this orifice--that coolant ROARS through when the T-stat is closed--tends to erode away, especially in raw water cooled applications. When that happens, the T-stat is less effective since it is not holding back the coolant enough to speed the warm up process. With really cold water, the T-stat might never open and the motor remain cold.

Understand?

Jeff
 
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