The coolant recovery system is an important part of a vehicle’s cooling system. Low-profile crossflow radiators do not have top tanks like older top fill radiators, so a separate recovery tank mounted somewhere under the hood serves as both a coolant reservoir and expansion/recovery tank.
As engine coolant heats up, it expands. In older top fill radiators, an inch or two of dead air space in the top of the radiator provided the necessary room for expansion. If the coolant got too hot, it would force its way past the spring-loaded radiator cap to relieve pressure, and any coolant that escaped would flow through a discharge tube into a recovery tank.
With modern coolant recovery systems, the radiator cap is no longer on top of the radiator, and the radiator has no upper tank to allow for expansion. So the radiator is connected to an external reservoir that serves as both an expansion tank and coolant recovery tank.
The coolant recovery tank is typically a white molded plastic tank. It might be mounted near the radiator, on an inner fender or against the firewall. The plastic tank is translucent so you can see the fluid level inside. The tank has markings on the side that tell you when the coolant level is low.
The coolant level inside the tank should be maintained so that it does not exceed the highest marking when the engine is at normal operating temperature. The highest level is often marked HOT FULL.
A lower COLD or ADD mark may be on the tank indicating the lowest level the coolant should be at when the engine is cold. If the coolant level inside the tank is lower than the COLD or ADD mark when the engine is cold, coolant needs to be added to the reservoir to bring the level up above the mark.
WARNING: NEVER open a coolant reservoir cap or radiator cap when the engine is hot. The air and coolant inside the tank may be under pressure (especially if the engine is overheating) and may spray out of the opening with great force and burn you! Wait at least 20 minutes or more after shutting the engine off to open the cap.
A hose or pair of hoses connect the coolant recovery tank to the radiator and allow coolant to circulate back and forth between the radiator and tank as the coolant temperature and pressure changes.
On most newer vehicles, the coolant recovery tank is not pressurized.
But on many older applications (1990s), it may be pressurized. You can tell if the reservoir is pressurized or not by the type of cap on the tank. If it has a plastic screw-on cap, the recovery tank is not pressurized. If it has a metal spring latch style cap (like a conventional radiator cap), the tank is pressurized.
On pressurized coolant recovery systems, the radiator usually does not have its own separate filler cap, and the recovery tank is mounted higher than the radiator so the system can be refilled via the tank when coolant is needed. The pressure cap on the recovery tank will have a specific pressure rating, so if it is not holding pressure and needs to be replaced make sure the new cap has the SAME rating as the original.
Pressurized recovery tanks are used on a variety of older domestic and foreign vehicles, including many 1993 and newer GM passenger cars with 4.3L or 5.7L engines. On GM applications with a "reverse-flow" cooling system, coolant flows through the heads first and then down through the block before it returns to the radiator. This helps the heads run cooler and improves emissions and resistance to engine-damaging detonation (spark knock) and pre-ignition.
Coolant recovery systems require no maintenance except for checking the coolant level periodically (say once a month or when you change oil).
If the coolant level is low, it may indicate a leak.
* Visually inspect the radiator, hoses, water pump and coolant recovery system for any signs of coolant leakage.
* Check the operation of the cooling fan. An often undiagnosed cause of overheating is an electric cooling fan that fails to come on due to a faulty temperature sensor, relay, motor or wiring problem. If the engine has a mechanical fan with a clutch, check the clutch for signs of fluid leakage or excess slippage. The silicone fluid inside a fan clutch loses viscosity over time which increases the slippage in the clutch and reduces cooling.
* Remove the filler cap or radiator cap and inspect the gasket inside. If the gasket is cracked, deformed, damaged or missing, the cap is leaking and must be replaced.
* Look up the pressure rating for the filler cap or radiator cap for your vehicle (which can be found in a radiator cap parts catalog). Compare the rating with the pressure rating marked on the old cap. If someone replaced the cap with the wrong one, it may not be sealing properly and is leaking pressure (and coolant).
* A repair shop, radiator shop or car dealer can use a special pressure tester to test the cap and cooling system to check for leaks. A leak-free system should hold pressure for several minutes. If the pressure drops during the test, it indicates a leak somewhere. The worst case would be an internal coolant leak caused by a bad had gasket or a cracked cylinder head or engine block.
* Inspect the coolant reservoir cap and opening for nicks, damage or cracks that might allow coolant to escape from the system.
* Inspect the recovery tank for cracks, split seams or leaks. If damaged, the tank must be replaced. Replacement tanks can be purchased at most auto parts stores or car dealers. Don’t waste your money on a used coolant recovery tank from a salvage yard because plastic can become brittle with age and the used tank may be no better than the one you are replacing.
* If the recovery tank contains sediment, the tank must be cleaned or replaced. Sediment may clog or jam the pressure relief/siphon valve in the radiator cap, and can contribute to accelerated wear in the water pump if it is siphoned back into the cooling system.
* If the recovery tank is dry (and the radiator is full), the recovery tank may be leaking, or the hose or tube that connects the tank to the radiator may be loose, pinched, plugged or leaking.
If the coolant level is low and coolant needs to be added to the recovery tank, add premixed antifreeze or a 50/50 mixture of full strength antifreeze and distilled water (never ordinary tap water that contains minerals and salts).
Also, use the SAME type of coolant that is already in the system, or a "universal" coolant that is fully compatible with the coolant in the system.
Changing the coolant on some late model vehicles can be a challenge because air may become trapped inside the engine, heater or hoses. To vent the trapped air, one or more "air bleed valves" may be located at various points in the cooling system. Opening the valve(s) when the system is being refilled allows air to escape so coolant can completely fill all the voids.
If the trapped air is not removed, it may block the flow of coolant through the heater core resulting in no heat output from the heater. Trapped air may also cause the engine to overheat when the vehicle is driven.
If there are no air-bleed valves, you may have to temporarily loosen a heater hose to vent air. Another trick is to use a jack to raise the front of the vehicle so the radiator is higher than the heater core. This should allow most of the air to escape as coolant is added.