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Old 04-22-2013, 05:18 PM   #427
Scraping Paint
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Okay, then we're back to where we were. If you noticed that kind of a coolant temperature drop then you've got a problem. The first thing one would think is that your thermostats, for whatever reason, are not doing their jobs. There is NO WAY a simple change of oil type should cause a temperature drop that a properly functioning, and properly rated, thermostat should not be able to compensate for.

Doesn't matter how new the thermostats are. I've known people to install a new thermostat and have it go bad within weeks or even be bad out of the box although in your case you say they seemed to be functioning properly at first since your operating coolant temperature remained the same.

Again, using our own engines as an example, we greatly increased the flow of cooling water through the heat exchangers to the point where the transmissions run significantly cooler than they used to. I don't mean a few degrees, I mean ten or twenty. But the engine coolant temps have not varied by a single degree because the thermostats continue to do their jobs.

There is a second explanation for your problem--- for that is what I think it is---- one which we have experienced on our own boat, and that is the sensor/gauge wiring. It takes very little corrosion or whatever to affect the signal from a sensor, or the circuit to the gauge, including the ground connection. We had a coolant temperature gauge on one engine suddenly begin to show a coolant temperature ten degrees less than normal. Turned out to be nothing more than a deteriorated connection to the sensor on the header tank. Cleaned it off and back to 180 went the gauge.

This has nothing to do with whether I think running synthetic oil in your engine is a good idea or not. I doubt synthetics make enough of a difference in these old, loose tolerance, slow-running engines to matter outside of the "promotes-oil-leaks" thing which may or may not have been cured with the newer oils. Depends on who you talk to.

I think what you're seeing with your temperatures has nothing whatsoever to do with the lube oil in your engines. I think it's telling you something's not quite right with the cooling systems. Whatever it is, it's interesting that it's occurring in both engines at the same time. Which may help rule out thermostats and start pointing toward a cause common to both engines--- instrument voltage, connections, I don't know.
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