(Not actually on top of the radiator!)
Pictures & diagrams ...
Radiator cap - 'small Nissan' (left) v. Toyota
Radiator cap operation (Dave Mason)
Radiator cap, a perspective (Mark Adams)
Radiator cap, closed (Mark Adams)
Radiator cap, cooling down (Mark Adams)
Radiator cap, venting (Mark Adams)
As the system heats up the coolant expands. Pressure lifts the lower, sprung, seal in the cap and coolant/steam (preceded by any air in the system) enter the expansion tank.
As the system cools coolant gets sucked back through the one-way valve in the centre of the lower part of cap - provided the upper seal of the cap does not allow air to leak in. (See diagrams) Coolant won't suck back either if there's an air leak in the pipe down to the expansion tank, or the pipe under the cap of the expansion tank isn't reaching the coolant in it.
The cap must be removed frequently to check the coolant level and it must be in good condition. Vaseline on top of the filler neck can help reduce wear on the upper seal. It's worth having a spare around for the day when you finds the seal is split.
Toyota have recently started supplying 16401 64501 (a "Denso" part, £11.75 incl. VAT) for diesel Townace/Liteace 1990 onwards. The refill valve, in the middle, has a plastic "float" rather than a metal disc with a spring holding it shut. The radiator will refill without the vacuum having to pull open the refill valve against a spring, so in theory a slight coolant leak shouldn't result in the system slowly filling up with air. It will also purge air from the system on each warm up before any pressure is built up at all. Further, it fits like a glove, is as smooth as silk, and the seals don't harden in the heat. As yet (Aug 2002) owners have yet to build up experience with this, or to understand why Toyota only sugest it for diesels 1990 onwards. Toyota say 16401 63010, which has a sprung metal refill valve, for earlier cars.
Otherwise Halfords type HRC607 at £4.29 is cheap and cheerful but stiff to remove so the upper seal wears out quickly. Or ask for one for a Toyota Camry Turbo Diesel, or "small Nissan". There are other references such as FC64, ARC131.
ADL Blueprint's ADC49902 (a Colt/Mitsubishi part £5 + VAT) does not have the plastic float so is similar to the Nissan/Halfords "old" type.
Pressure typically marked as 0.9 (kg/cm2) or 13 lbs. (Toyota specify 0.75 to 1.05 kg/cm2, equals 10.7 - 14.9 psi).
Dave Mason, editor, Sep 2002
How to check the draw back mechanism
Never remove the radiator cap from a hot engine. Severe scalding may result. Start with a cold engine.
1. Remove the radiator cap, and fill to the top of the tube, then replace the radiator cap.
2. Check the level of coolant in the expansion tank is at, or just above, the low mark when the engine is cold, and place a mark on the side of the tank. This is your cold point.
3. Start the engine and allow the temperature to reach normal. Then check the level of coolant in the expansion bottle, it should have risen to near the full Mark. Now mark the level on the side of the tank. This is your HOT point.
4. Allow the engine to cool off. A couple of hours or more is required to allow the coolant to cool off enough to draw back most of the coolant in the expansion bottle. The longer time you allow the more coolant is drawn back.
5. Check that the level in the expansion tank. It should have dropped from the hot point to somewhere near the cold point.
Results
If the level has not dropped from the hot point, then the draw back mechanism is not working.
What can cause this is a faulty radiator cap, leaking rubber tubes that connect the expansion tank to the filler neck, or the rubber tube in the expansion tank has become disconnected, or is leaking, or other leaks exists somewhere in the cooling system.
Do not suck on the rubber tubes, Antifreeze is poison. Even a small amount can make you very ill.
First, try to twist each tube at the connection point. If they turn fairly easily, then they are loose enough to allow air to be drawn in to the cooling system rather than coolant. Adjust the position of the clip to reseal the joint. If that fails to resolve the problem, remove each rubber tube and visually check it for cracks or splits. Check the ends to make sure they are neat and clean. If bits broke off when it was removed, its best to clean the fittings up and replace the tube with a new one. This also applies to the tube in the expansion tank.
If all the tubes are good, then the radiator cap may be faulty. Fit a new cap and perform the test again.
It is worth noting here that the radiator cap can be affected by overheating. I myself fitted a new cap one Friday, only to find that when the coolant burped off a bit of steam one week later, the new cap was incapable of drawing back the coolant.
If the problem persists then there will be a leak elsewhere in the system, and it will need to be found. Leaks in the cooling system will reduce the pressure and therefore the boiling point of the coolant. This will result in overheating and loss of coolant.
Dave Bright, 25 Sep 2003.
If the level has dropped to somewhere near the cold point, then the drawback mechanism is working.