Radiator cap is so inexpensive relatively to a tester ($79 at Harbor Freight Tools and they should be the cheapest). When you change your thermostat, change them too/ If your radiator cap is the problem, the anti-freeze should be boiling when it reaches your reservoir. Also if its time to change your antifreeze, remember that that is about a gallon of water (if you flush the engine) in the block. Bear this in mind when you are mixing the anti freeze solution