13. What would cause my engine to run at temperatures below the rating of the thermostat?

Engine “overcooling” is usually caused by one of three conditions: 1)water is getting through the thermostat (check for defective thermostat), 2) water is somehow getting around the thermostat (check for deep corrosion on the surface of the head, where the thermostat seats, or on the base of the thermostat housing), or 3) The indicating system is defective.

Sometimes, overcooling is the unintended consequence of adding too much restriction in the bypass loop, which may have been added (ironically) to cure an overheating problem. Excessive restriction in the bypass loop can overpower the thermostat (forcing water through or around it) even in engines with fairly normal thermostats.

In these cases, you simply have to reduce the amount of restriction in the bypass loop until the thermostat can regulate temperature normally. - Updated: December 11, 2003