8. My engine was running fine then started overheating. There is white billowing smoke or vapor coming from the exhaust. The engine seems to run smoothly but the smoke (vapor) worries me as well as the overheating. The raw water strainer is clean. Could it be the thermostat, and how can I check it?

It appears that the total flow of water through your cooling system has greatly reduced.

When the total flow of cooling water reduces, in addition to the engine overheating, the water sometimes turns to steam when it enters the hot section of the exhaust system. A normal amount of water will overcome the temperature gradient and keep the exhaust piping cool in the immediate area where the water enters the system; small flows will not be able to cool the exhaust enough to prevent steam.

Things that can cause low flow include the water pump itself, a restriction in the discharge of the pump or a restriction in the area of the rear fitting on the manifold. In general, you're looking for a restriction in any location where total flow would be affected.

A bad thermostat could cause overheating, by not allowing enough cooling water to go through the block and the head. However, a faulty thermostat would not affect total flow, since water would still be free to bypass over the top of the thermostat and leave through the exhaust system (assuming that there were no restrictions to total flow). - Updated: November 13, 2003