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Originally Posted by cdru
The best way would be how shops to it. You hook up a vacuum pump and suck out all the air, then open a valve and let the vacuum draw coolant into the system. Mityvac makes one but it costs around $80 I beleive.
To do it by hand, it take a while. Open up both bleeder screws. Fill up the radiator as full as you can then close both screws. Turn on the vehicle and let it run to normal operating temperature. Open the screw just after the thermostat housing ever so slightly to bleed off any air that may get trapped over on that side. Turn off the engine and let the van cool. Repete several times and the air should be forced out.
As the system heats up, the air becomes pressurized. The radiator cap will hold 15 psi. After that, it will start to vent to the overflow tank. This is normal and you want this to happen. When the system then cools, a vacuum is formed and the radiator cap then opens, drawing coolant from the overflow tank. By doing this several times, you "pump" out the air and then suck back in coolant. If your radiator cap isn't working correctly though, it may never allow your coolant to become pressurized or to allow coolant to be drawn back in.
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Thanks for reply. so is there a way of knowing if cap is working or just get new one and go from there?