Chevrolet Repair: Engine Cooling, coolant temp, laser thermometer

Question
1994 Chevrolet Silverado C2500 2WD 5.7L V8, 220,000 miles.  Recently, I've had overtemp cooling problems.  The gage indicated into the red zone and check engine light illuminated.  After a couple of minutes, the engine temp would drop to the normal range then would be fine but I noticed that performance was not up to par.  After being parked for several hours then restarted, the temp would rise above 210 degrees then drop back into the normal range with no overheat indications.  It would do this sequence after an overnight cold soak.  I had to continually top off the coolant as it seemed to be forced out through the cap.  The system was checked by my shop and found to be tight with no leaks except for the radiator cap which was said to be weak and not holding pressure.  It was replaced but the problem continued.  I replaced the thermostat with a similar type recommmended by the manufacturer.  Now, the temperature is too low, the indicator never gets above 100 degrees and the engine seems to be operating normally.  I checked coolant flow with he cap off and see that the pump is circulating coolant.  I no longer lose coolant.  I also checked the temperature sensor and wiring which seems ok.  I measured 5 volts at the lead and the sensor itself reads resistance.  Interior heat is also low where before the thermostat replacement would produce plenty of heat.

Answer
Hey,

 you need to check coolant temp with a laser thermometer at the water outlet by the stat.  If you TOOK OUT the stat and didn't put it back in it sounds like that is what is happening.  If it has a stat in it and is regulating at a low temp proven by the thermometer you must have a bad stat.  Make sure the clutch fan is ok too.  But it still wouldn't matter if the stat was ok.

Wayne

 I was referring to the 'clutch fan'  It is a viscous clutch that is controlled by a bi metallic spring.  It is designed to couple when radiator radiant heat increases causing the spring to move and turn the clutch.  Make sure the fan isn't freewheeling.

w