I think the radiator leak if present is probably sufficient to explain your overheating.

If there is even a small leak at any location in the cooling system, the system will not hold normal pressure. Under normal operation the coolant goes above 100C at which water boils at atmospheric pressure. Two things keep the coolant from boiling normally - 1 the presence of antifreeze which raises the boiling point vs pure water, and 2 pressure above atmospheric (50-100% higher).

If your system does not hold pressure then coolant will boil somewhere within the system, if the engine is doing any significant work. Coolant as vapor will not circulate properly and does not have the capacity to absorb heat that liquid has. You don't have to lose much coolant, though loss makes the situation worse (and 1/2 gallon is a lot).

Possibly your mechanic did not put the car under enough load to overheat it.

You can diagnose a stuck thermostat - if the thermostat is stuck closed the upper radiator hose will not get very hot even after the engine is very thoroughly warmed up - because the blockage prevents coolant from flowing from block to radiator. If stuck open the car will most likely run too cool during highway driving and the heater output will be very weak.

With much older (usually non-European) cars than yours where the cooling system was not sealed or was only semi-sealed, the operating pressure of the system was lower and the above would not apply.

The cooling system is 2 gallons, 8 quarts. If you were down 2 quarts, due to a leak, the thermostat and ECT would be exposed to hot air (steam) and read high. If the mechanic filled the system to proper levels, it will not overheat until the cooling system again leaks enough to expose the thermostat to air.

Tell your mechanic to order a new radiator AND hoses, ECT, and thermostat. I suggest using OEM parts.

Turning on the cabin heat blower acts as a small radiator and helps cool the antifreeze. This should always be turned on by anyone who notices a hot temperator gauage, regardless of the outside temp. Roll the windows down and turn off the AC while you are at it.

BTW, have your mechanic 'clean' the cooling fins on the condenser while the radiator is out. It gets pretty grimy in there.

Klaus

If the mechanic topped up the coolant it should be good until it gets low again. If its a slow leak it might take more than a few hours running to leak out 1/2 gallon. Also, IIRC the expansion tank is like a quart, which means your upper hose would probably be empty, I don't think the water pump would be starved (it probably wasn't operating at full capacity though), but thats another possibility with the low coolant and the pump being a the top of your engine. The heater core is lower in the system meaning that more fluid will go through that when its low, making it a good idea to blow heat full blast if your car is overheating..

I'm going to assume that your gauge (and sensor) are correct, there's no reason to assume a failure there. So that leaves these options: a) the 1/2 gallon being low prevented the system from circulating enough fluid to get enough heat out. Remember the thermostat requires enough force to open, low fluid could force higher temperatures to build the pressure to open the thermostat. b) the radiator is old/gunked up which not only caused the crack but also its not as able to reject enough heat to the atmosphere. c) faulty thermostat. d) obstruction in the line.

Thats not terrible on the options, only 4.. Do you know the last time your system has been flushed? It should be done every 2-3 years. Also, is your thermostat original as well as OEM? Thermostat will run you $12-20 depending where you get it. Dealer is obviously the most expensive, but they'll also have the part. Get OEM, aftermarket tends to fail in closed position. This is much less than the radiator, and should be replaced if you replace the radiator anyway.

My guess is you'd have no problems with this car as long as you monitor the coolant. Keep it full and it should be fine, however eventually you'll spend more on coolant than you would on a new radiator!