Engine Overheating

[The_Nuts]

I have a 1963 TBird with the stock engine which has been overhauled some time ago. The dash temerature gauge indicates the car runs cool (but in the acceptable range) for about 20 min of highway driving. Then the gauge starts jumping around, from cool to hot and back again. The jump cycle is very visable, taking only a couple of seconds. Each jumps however seems to get higher until the guage actually spends a brief time in the "overheat" area, before falling back into the "safe" range of the gauge. I have avoided overheating the engine at this time and returned quickly home when this happens. I have replaced both the thermostat, top radiator hose and the temperature sender - with no change to the problem. Have also confirmed that the thermostat is seated properly. Any suggestions???

[Harley Eaton]

Sounds like you need to replace the instrument voltage regulator or find a ground in the wire to the temperature switch.. Otherwise check the REAL engine temperature when the gauge shop too hot.