Overheating While Idleling - SOLVED!!!!

Where does temp gauge register, normal or hot while idling? If the gauge shows normal temp the radiator cap may be bad. Rarely do you have a water pump go bad and not pump, they may leak but they still pump water.

Does car run cool while going down the road? If it does the problem is likely the fan is not pulling enough air into the radiator at idle. Is belt good & also tight.
Are the side metal plates which direct air into the radiator installed?

If it runs hot at idle and/or in motion the radiator may be plugged up or the thermostat is malfunctioning.

With the engine off after you warm the car up, feel around on the radiator and see if there any cool spots. Plugged areas on radiator will be cool.

To check thermostat, take out thermostat, place in a sauce pan with water & heat on stove and see if spring plate opens up.

If the car has sat a long time with straight water or old expired anti freeze in engine the whole system may be clogged with rust, a repeated drain and flush of block may help along with a radiator clean. There are several rust removers on the market.

If you have to, pull the head and run a wire coat hanger through all the water ports in the block and head while running a water hose to flush (pretty much a last resort).

Make sure the heater has water circulating through it (putting out heat) this is often overlooked when cleaning out cooling system.

Flush and drain until water is clear (no rust).

Also make sure the fan is about 3/4" to 1" from the radiator core, too much clearance here and they will overheat when the car is sitting still. The fact that it runs cool while driving points to not enough air movement thru the core when it's sitting and idling. The radiator mounts are adjustable fore and aft by abount an inch. Look for the bolts on the sides of the radiator support.

The thermostat should be used in the system. It controls temperature of engine but also controls water volume and the time the water stays in the radiator.

Without the thermostat the coolant at high engine speed flows through the radiator so fast the heat isn't removed. you still overheat with the system full of coolant.

Race engines use a plate in the thermostat opening which has a circular opening to control coolant flow. A different size hole allows a different temperature depending where you want the engine to run. This takes away the problem of the thermostat sticking shut and cooking the engine.

On a street engine the thermostat keeps the engine at operating temp, from the time you start up (keeps coolant in block for faster warm up) to any kind of driving you do.

Make sure you have the spring in the lower radiator hose or are using a reinforced hose as the suction of the water pump will collapse the hose shutting off the water flow, the higher the rpms the tighter the hose collapses.

Make sure you have the correct cap. If you have the deep filler neck, a modern shallow reach cap will leak..all the time.

Just to clear things up... Overheating when running with no thermostat is not because the coolant runs too fast through the radiator that there isn't time to cool it. Higher coolant flow will always improve cooling, but at higher flows other things start happening in the coolant to decrease the heat transfer like foaming of the coolant, cavitation in the pump, and coolant being forced past the radiator cap because of the increased pressure (cross-flow radiators get by this problem by having the cap on the low-pressure, inlet side of the pump). So, overheating with out a thermostat isn't strictly a matter of the coolant flowing too fast through the radiator, but the side effects of the high coolant flow throwing off the whole system.

Here's some nice links on how the cooling system works and what you can and can't do to improve it:
http://www.arrowheadradiator.com/14_rules_for_improving_engine_cooling_system_capability_in_high-performance_automobiles.htm
http://www.stewartcomponents.com/tech_tips/Tech_Tips_1.htm

Also, think of it this way... If you're slowing the coolant down through the radiator, you are also slowing it down through the engine. So whatever additional heat you lose through the radiator at a slower flow you also gain as the coolant stays in the block longer.