Coolant Expansion Tanks

Is your coolant boiling over, or has the original plastic tank cracked after countless heat cycles? An expansion tank gives the coolant room to expand as the engine warms up. This category is for you if you want to replace a tired original tank or need a universal solution for a project.

Expansion tank or overflow tank?

An expansion tank is typically part of the pressurised cooling system: coolant circulates through it, air collects and is separated out, and the pressure cap often sits directly on the tank. An overflow tank is usually unpressurised and simply collects fluid that is pushed out – you will find that type under coolant overflow tanks. Which solution your car uses depends on how the cooling system is designed. See also the full category of overflow tanks and catch cans.

How to choose an expansion tank

• Vehicle-specific or universal: Vehicle-specific tanks, e.g. from Mishimoto for the BMW 3 Series (1992-99) or Jeep JL/JT, are made for that exact model. Universal tanks, e.g. from Radium Engineering, require you to plan the placement, brackets and hose routing yourself.
• Connections and cap: Check the number of ports, the hose diameters and which pressure cap the tank is designed for – the opening pressure must match your system.
• Placement and volume: The tank should typically be one of the highest points in the system so that air can collect. The space in the engine bay often determines the size more than the volume you would like.
• Material and mounting: Aluminium can be an alternative to plastic, which can become brittle with age. For mounting there is, for example, a base from Torque Solution. The brands in this category also include Moroso.

Installation and maintenance

Always install on a cold engine and bleed the system afterwards, as air pockets can cause localised overheating. Check the hose ports for leaks after the first few drives, and never open the pressure cap while the engine is hot.

Frequently asked questions

What is the difference between an expansion tank and an overflow tank?

An expansion tank is typically pressurised and an active part of the cooling circuit, often with the pressure cap mounted directly on the tank. An overflow tank is usually unpressurised and only collects fluid that is pushed out when the engine is hot.

How do I bleed the cooling system after installation?

Fill coolant with the engine cold, let the engine run with the cabin heater on full, and keep an eye on the level as the thermostat opens. The procedure depends on the car, so follow the manufacturer's instructions and top up once the engine is cold again.

Why is my expansion tank losing coolant?

Typical causes are cracks in an aged plastic tank, a worn pressure cap or leaking hose ports. Coolant loss can also be caused by faults elsewhere in the engine, e.g. at the head gasket, so find the cause before you replace the tank.