Engine cooling systems have undergone significant improvement in recent years and maintenance requirements are remarkably reduced. The resulting lack of owner involvement may instil misgivings regarding proper cooling system care techniques. Antiquated conceptions might lead to incorrect assumptions or approaches for cooling system needs. The coolant recovery reservoir could be the only point of interaction between the vehicle's cooling system and it's owner.
The purpose of the coolant recovery reservoir, or antifreeze tank, is to allow for expansion and contraction of the coolant. Coolant, like most liquids, expands as it heats and this growth in volume must be accomodated. Before the recovery reservoir was in use, the engine cooling systems were designed to allow for some expansion, but little regard was given to losses through the overflow hose. Older systems required frequent inspection and adjustment of the coolant level.
On the Level
Coolant recovery reservoirs are marked with two levels. The lower mark is for checking the coolant level when the system is cold. Most systems are meant to be checked with a cold engine when coolant level should be at the lower, or "cold" mark. The higher mark signifies proper level at operating temperatures. The volume of space above the "hot" mark is provided for coolant expansion and not intended to store excess liquid.
Coolant levels above those required by the system may be expelled through the overflow hose or conduit plumbed to the recovery reservoir. Expelled coolant may contact hot engine parts and appear to be a system malfunction, or leak. Coolant could contact eletrical components and cause brief malfunctions or permanant damage. Puddles may form under the vehicle when parked, and raise false concerns. Slight overfills may be tolerated by the system with no noticeable consequence.
By the Book
A reservoir carrying more fluid than it was designed to hold may break loose at attachment points, over time, and extra coolant serves no beneficial purpose. Consult the vehicle owner's manual for precise information for your make and model. Remove excess coolant from the reservoir with a turkey baster, if compelled to remedy any overages. Do not reuse the baster for cooking. Visit your local dealership or repair shop to dispel any further doubts or concerns.