From Phil (Terminator):
Losing coolant from the reservoir bottle
Also known as:
Overflow bottle is always empty
What is it?
The overflow tank of the coolant system isn't in fact an overflow tank - it's the expansion tank or coolant reservoir. When all components of the cooling system are functioning correctly the level of fluid in the reservoir will rise and fall a small amount as pressure in the system fluctuates. Once the engine has stopped and the water pump ceases to move water round the system and the forced cooling effect of the radiator ceases, there is a temporary rise in the fluid level in the reservoir bottle. As the system cools, coolant is drawn back into the system and the level of coolant in the reservoir will return to its original position, usually the max mark on the bottle.
If you are losing coolant, each warmup-cooldown cycle will empty the expansion tank further until it's dry.
Usually caused by:
The most common cause of fluid loss is a faulty radiator cap.
If you have a faulty cap it cannot sustain the pressure of the system and allows extra fluid to be transferred to the reservoir bottle. Under driving conditions the cooling effect of the radiator may be sufficient to counteract the reduction in pressure caused by a minor fault in the cap. However, once the car is stationary the radiator may not provide enough cooling to prevent the coolant level from getting so high in the reservoir bottle that some coolant is vented off from the reservoir cap. In a hot engine with the stock fan running any released coolant will fall onto the under tray and will quickly evaporate. When the ignition is switched off, no further forced cooling is possible with the stock fan. With a faulty radiator cap and the vehicle park on level ground or facing up hill, any vented coolant will not be visible as it will fall on to and be absorbed into the tray.
With a mildly faulty cap this venting of coolant will continue at each period of use until the cap is replaced. If the cap is not replaced the coolant the in reservoir will fall to a level that is below the mouth of the return feed to the radiator and air will be dawn into the system. However if the reservoir is checked when the vehicle is warm the level of coolant visible may look normal as it takes quite some time for the system to cool sufficiently to draw coolant back from the reservoir. Left unchecked the system will continue to vent fluid when ever the pressure in the system becomes higher than the faulty cap can sustain. Visual checks of a hot may lead the owner to mistakenly believe all is well.
How bad is this problem?
If you overheat your car due to lack of coolant, it can reach new-engine-time levels of seriousness. However, just having an empty reservoir tank isn't the end of the world. No damage is done until the car actually overheats, so fill it back up and replace the rad cap sooner rather than later.
Other possible causes:
There are other more serious reason for the car to lose coolant, a leak at the head gasket or a failing or loose hose connection. By checking your cold coolant level regularly you will soon spot a sudden change and prevent more potentially serious engine problems.