Only the Strong Survive
We own a '73 454 T-top coupe with 84,000 miles on it. We just had the A/C repaired. It was not that cold after repairing it. Then we had the radiator replaced. Our mechanic put in a heavy-duty one. The car now is overheating when the A/C is on and our mechanic can't figure it out.
Linda Stygar, Florida

In defense of your mechanic, Corvette overheating issues are difficult to diagnose and in a class all by themselves. Every other GM product with a 454 had a large engine compartment that let the hot air out. Your '73 has limited air flow, keeping an already hot situation even hotter.

To start with, have a combustion gas test performed to make sure the head gaskets aren't leaking combustion gas into the cooling system. A combustion leak tester kit is used, and this kit comes with a ball, tubes, test fluid, aspirator bulb, and engine adapter (cone-shaped device you place in your radiator filler cap). If there's exhaust in your coolant, the test fluid changes color. If the liquid changes color, the combustion gases are entering the cooling system, and no radiator or fan will cool it properly. Major work is required, but it's best to know the status of the head gaskets up front.

Now a performance check of the engine is required to make sure it's running well with correct fuel mixture and ignition timing. Late ignition timing will create extreme engine and exhaust manifold heat, making the engine compartment toasty. With today's fuel, we usually have the timing set late to prevent spark knock with all that iron used for the block and heads. Another reason to set the timing later than the factory specs is starting issues, bad battery cables, weak batteries, and worn starters make for hard hot starting. We recommend fuel additives to boost octane in the big-block Corvettes, even with the '73's relatively low-compression engine. This allows proper ignition timing dependant on engine horsepower configuration. You need to make sure the engine is not running lean on fuel from a dirty carburetor or clogged fuel system

Once the engine performance is corrected (if necessary), the cooling system itself can be checked. You mentioned a large radiator was installed, and since the problem is still there that may have made the situation worse. Air must flow through the radiator core to cool the coolant. Stepping up to a 11/42-inch-tube, four-core radiator from a three-core was the norm for many years. The additional air resistance has been found to negate the benefits of the additional row of tubes because air doesn't flow as well through the bigger radiator. Today's radiators are mostly aluminum and use large 1-inch tubes for more surface area. The aluminum has better heat convection to also add cooling, although the initial cost is greater. They also last longer. I can understand that you may want to keep the original look of the cooper/brass OE radiator, but in this case you may want to step up to the aluminum radiator.

Another area that is often missed when diagnosing cooling system woes is the direction of air flow, or at least containing the air and funneling it through the radiator. The radiator and shroud originally had seals around the core support and across the top of the core support to ensure any air passing through the front of the car was directed through the radiator. This is especially important at speeds above 40-45 mph. Below that speed, the cooling fan has to maintain air flow. This is where the fan shroud seals come in to play. If they're missing, the fan will draw air from around the radiator, not through it.