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Overly simplified, but here are a couple of main reasons cars over heat:
1) Not enough air flow - this crops up its head at low speeds and idling. The engine isn't putting out much heat, but not enough air is flowing through the radiator, and the coolant travels through without cooling down. This is the problem if it overheats while idling, or moving very slowly, but cools down when the speeds pick up. Some older cars will even overheat at highway speeds if you are in the draft of a large truck. The cause is the fan not pumping enough air around at low engine speeds. When the car is moving the fan doesn't really do much as the air hitting the grille forces it's way through in sufficent quantity. Fix by increasing the airflow at low speeds - either an electric fan or an improved belt driven fan.
2) Not enough coolant flow - this will occur at higher engine outputs. The reduced coolant flow is sufficient for easy use, but when the engine starts working hard it can't pump enough coolant around to keep itself cool. So even on the freeway going 90 mph with air blasting through the radiator the temp gauge will slowly creep up. The causes can be a bad thermostat (won't open), the wrong thermostat (doesn't close the warm-up bypass valve in the Volvo's head), bad pump (never seen one, could happen I suppose - they usually leak and wobble well before the blades corrode or wear), pinched hoses, blocked radiator, or even a blocked block full of lime.
3) A wild card is something untoward happening in the engine, like a lean mixture, retarded timing, or a leaking headgasket. But those will usually create other symptoms of their own.
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