Rising Temperatures

I have a 2003 Bayliner 245 with a Mercruiser 5.0 and an Alpha One, gen two. Last year the temperature gauge went from slowly normal to the bottom of the range several times and then I got an engine warning horn. The engine failed with warped heads and block indicating there was an actual over temp, although the gauge never showed an over temp. The mechanic replaced the engine, thermostat and outdrive water pump. Everything was normal for approximately 25 hours when the temp gauge started repeating the previous fluctuations. Have you ever seen this problem before? I don’t believe it’s a gauge problem, because there was an actual engine failure. The mechanic said it was probably a failed head gasket that pumped air into the cooling system.
Howard Malloy, via BoatingWorld.com

I haven’t seen that problem in a marine engine, but had it happen in the only Chevy I ever owned when the end dropped off the radiator and there was no coolant to measure and, therefore, no warning via the temp sender. I’d accept the mechanic’s theory, but why is the problem apparent for the second time? Did the engine fail again, or were you able to prevent it? I think lots of questions are in order: Was the replacement engine new or remanufactured, or was yours rebuilt? Do you have a closed or open cooling system? If closed, was the system checked for leaks? If a failed head gasket pumped air into the coolant, then there should be evidence of coolant in the oil, not to mention a poor-running engine and low compression on at least one cylinder. If the problem has occurred again, did the evidence from the first failure present itself in the same fashion or place the second time?

Unless it took several years for the problem to show the second time, there may be some responsibility on your mechanic’s part to join in the expense reduction with you. — GM

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