67 gto 455 engine completely rebuilt runs hot 200-220 radiator fan shroud thermostat new removed thermostat and ran 180 at 60mph slow speeds heats back up to 200 Have checked everything possible Dan?

Answer 1

big block gm engines are known for sucking the lower radiator hose shut. make sure the lower hose has a spring in it or is a new hose

Also, since a 455 was not stock for '67 your 455 may be working the stock radiator too hard. Consider an aftermarket higher capacity radiator. Aluminum will help even more. If you are concerned about looking original... spray paint it black.

Pontiac engines are notorious for running hot. Especially the large bore 455. As stated above, make sure your cooling system is in excellent condition and run the largest aluminum radiator you can get. One other thing....what type of fan are you using?? Perhaps the best fan made is the 7 blade factory fan with an rpm/temp sensitive fan clutch. These are critical on Pontiacs. Those flex fans and factory 4 blade fans just won't cut it on a street driven Pontiac. Also use a 160 degree thermostat. One last thing to check is your fuel mixture. A lean car runs hotter than one set up correctly or even a little rich. Timing also has an effect on temperature. Good luck. if your engine is sitting higher then your radiator you might be dealing with air pockets a way to make sure,there is a tool that pulls vacuum through the cooling system from radiator filler cap it pulls 20 inches of vaccum at the same time it will fill your cooling system completely without waiting for thermostat,it pulls every bit of air from block. also make sure you have the heavy duty radiator 4 row made for big engines with a/c also retarted timing will cause engine temp to rise. ps the best fan you can get are the s shaped electric fans also make sure there is nothing blocking the air flow through the grill i have seen leaves between a/c condenser and radiator. there are also thermostat restrictor plates that replace thermostat all it is is a plate the slows the water a bit.driving with no thermostat is not recomended because the water run through the radiator to fast not allowing the water to cool enough

Good answer above, I would like to add a few things to it though.

1) If you are using a rebuilt or "new" water pump (read from China or Mexico), the space between the impeller and the divider plate is often far too large to allow the pump to work properly. The gap should be about .060, the plate can be bent to fit closer to the impeller, this will increase water flow.

2)The fan should sit about half way into the shroud for best airflow and cooling, that is that the blades should be half way out the back, and half way in the shroud.

3) An easy way to "burp" air from your cooling system is to drill 2 small holes (like 1/16") in the body of the thermostat, and when filling the radiator, jack the front of the car up, so it is higher than the engine.

4) Make sure you have the correct water pump pulley, there were different sizes over the years. A pulley from an air conditioned car will be a smaller diameter, and will spin the pump and fan faster in relation to the engine rpm.

Also have to make sure you get the right depth pulley, in late 1968 the pulley depth changed, so all pulleys must be of the same set, or you will have belt alignment problems.