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Volvo MD11C - more cooling details
Several years ago I took everything apart, had the cylinder liners pressed out, and did a full de-scaling by hand. It ran well for a long time after that, but the back cylinder always ran hotter to the touch. Now wishing I had picked up an IR thermometer years ago, I can measure temperatures just over 200F where the rear head and manifold meet, but only 150-160F on the front.
The manifold on this MD11C had a previous problem that I haven't seen on any boards before: one side of the thermostat's yoke-shaped frame came loose so it was hanging on one mount, vibrating against the iron manifold. The lower temperature was masked by the temp cycling previously described so by the time it was discovered the thermostat had worn a hole clear through to the lower manifold chamber. A patch made with JB Weld is still holding, but makes me consider replacing the manifold to protect recent investment in a new head.
The old head, incidentally, suffered a corrosion hole between the exhaust port and water jacket. I'm told this is a common way for the head to fail (it lasted 30 years..). It seems as though early in the leak, small amounts of water were corroding the valve guide and starting problems there before bigger symptoms like lots of steam or a hydrolock cropped up.