How The Motorhome Coolant Overflow Reservoir Works

[DriVer]

WORKHORSE TECHNICAL REPORT
from The Workhorse Technical Team

Technical Report No. 2

Note: This is the second in a series of Workhorse Technical Reports with information that Workhorse considers most important in helping motor home owners avoid potential problems and maximize the economy, safety and enjoyment of their driving experience.

Key maintenance check: How the motor home
coolant overflow reservoir works

Coolant overflow reservoirs were added to vehicles to make cross flow radiators more efficient. This is done by assuring that the radiator is full to the fill cap at all times during operation. To ensure the most efficient radiator operation and prevent any potential cooling problems, the operator should understand how the system works and see that it is regularly checked. Here is a brief explanation of the radiator-reservoir system and what to look for in checking proper fluid levels.

Coolant movement from radiator to reservoir
Coolant flow from the radiator to reservoir occurs when the coolant expands from heat. The expanding fluid opens the radiator spring loaded cap and flow occurs in a tube from radiator to reservoir. (The overflow tank is often called an EXPANSION tank.) This fluid movement occurs due to a pressure build up from expanding coolant. The pressure is NOT the result of boiling coolant.

If the radiator is low, the expanding coolant will build pressure in the radiator, but often insufficient pressure to lift the pressure cap. (Air is compressible—coolant is not.) If pressure exceeds the 15 psi cap, only air and not coolant will escape to the reservoir. Thus, the recovery system will not work when radiator level is low enough such that expanding coolant does not empty the trapped air from the radiator.

Coolant movement from the reservoir to radiator
When the coolant in the radiator cools, it shrinks in volume and creates a vacuum in the radiator. This vacuum pulls coolant from the reservoir back into the radiator through a one-way check valve in the cap.

If the radiator level is low, the normal pressure produced from normal engine operation will decrease and a sufficient vacuum will not be produced to pull the coolant from the reservoir.

Conclusions
* Coolant recovery system is inoperative when radiator is sufficiently low on coolant.
* Coolant tank can be at full mark at the same time that radiator is low enough to cause overheating.
* If an operator fills an empty reservoir, he must check radiator level to assure that it is full.
* If reservoir level does NOT CHANGE between hot and cold, radiator level is low.

Recommendations
* Radiator level should be checked (by removing cap) at every oil change (Caution: Open only when cold. Contents under pressure.)
* Reservoir level should be checked for HOT and COLD level at every fueling. (Cold level should be checked before starting a day's trip before start up.)
* Radiator level (as well as all fluid levels) should be checked before and after storage of one month or more.