Problem with over heating

[Wayne Maybury]

No, not my Cobra, it never overheats, but I do have a problem with my '66 Mustang.

I recently restored a '66 Mustang coupe that has a 289 and a C4. I had the engine rebuilt even though it was in good shape, changed the water pump, the thermostat (twice), the cap on the rad, etc. The engine now has flat top pistons, a mild Comp cam, an aluminium intake with a new 600 cfm Holley, and headers.

Here is the problem. The engine comes up to normal temp and runs at that level for a while (half an hour or more) then if I push it a bit harder on the highway or get stuck in traffic, the temp goes up by about 15 or 20 degrees and stays there. It does not come back down once the car is moving normally again. I figured that the thermostat was sticking so I changed it and went down from a 180 to a 160. Now it runs cooler but will still go up by about 20 degrees the same as with the 180 thermostat. I have the original temp gauge so I cannot say exactly what the temp is but the increase is quite significant.

The level of the coolant does not go down so there are no leaks and it doesn't boil over but it did puke out coolant when turned off until I changed the rad cap to a 16 psi. The radiator came with the car but it looks like it is new, the hoses and fan belt are new or in very good condition, and the pump was changed when the engine was rebuilt. The coolant appears to circulate well but of course I can only look when the engine is relatively cool. I have tried both a regular fan and a new clutch fan with no differences. It always does the same thing even if the outdoor air temp is relatively cool. It is no better nor no worse if the air temp is hot or cool.

Does anyone have any idea why this engine will run at two different temperature levels? Normally engines that heat up in traffic will cool right back down as soon as the speed picks back up. I have thought about buying a new aluminium direct replacement radiator from Summit but I am not even sure if that will help or not.

Wayne