14. Why does my engine run at 160 to 170 degrees (with a 140 degree thermostat), even after insuring that my cooling system is well maintained?

As engines age and cooling passages in the block and head become less than perfectly open (even after flushing), it becomes just as easy for water to flow up through the bypass loop as to flow through the engine. At this point, temperatures may tend to creep up, sometimes as high as 180 degrees.

At 180 degrees, the normal 140 degree thermostat is fully open and mechanically restricts the flow of bypass water entering through the top of the thermostat housing. In cases where cooling systems are reasonably well maintained, temperatures can be brought down closer to the thermostat's range by installing a partial restriction in the by-pass loop. - Updated: November 4, 2003