Flushing a heater core


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The fixes here can be applied to all models of Land Rovers : credit: © Alisdair Cusick
Unhappy with your heater output? Alisdair Cusick explains what may be wrong and how to make your heater efficient again

Need to know

Time: 30 mins
Difficulty: 1 out of 5
Models: All Land Rovers.
Tools needed: Sockets and screwdrivers, hose, water supply, suitable catch container, funnel.
Parts used: Antifreeze.
Work safely: 
• Use the right tool for the right job.
• Always allow an engine and the coolant to fully cool and depressurise before removing a radiator cap or disconnecting any part of the coolant system.
• Always dispose of used coolant properly – never down a drain.
• Keep hands clear of belts and fans on a running engine.
• Get an expert to do the job, if in any doubt.


Servicing is an investment in our Land Rovers to help keep them in top form. In doing that work, we all have those little extra jobs that go a little bit further, to gain additional benefit. One of the simplest is to flush the heater core whilst doing a coolant change.

A warm driving environment is something we expect in a vehicle, though it should be remembered that in the first Land Rover a heater had to be ordered as an option. Those days are long gone, thankfully, and ample cabin heat is now a prerequisite when driving.

A heater works by using a small heat exchanger or matrix mounted behind the dashboard which connects to the engine’s cooling system. At working temperature, engine coolant is pumped through the heater matrix where a fan blows air across the matrix and into the car, taking heat with it. There is extra pipework both to direct the air to different areas, and to mix ambient air in to permit temperature control.

Whenever the coolant is drained, it is worth flushing out the heater matrix to remove any internal debris, thus allowing maximum flow through the matrix and optimum heat transfer to the interior. It’s a quick job and definitely worth doing. Here I explain the job on my Series I because the heater matrix is easily removed for demonstration, but the principle is the same on any vehicle.


Heater problems

Heater issues can vary depending on the complexity of the system. A classic Defender or Series heater is simpler compared with, say, a Range Rover or Discovery which may use air conditioning, blend motors and electronic controls. Such systems can mean electrical issues raise their head, but in terms of a lack of heat, the main causes are the same, regardless. All you need to do is check the symptom.

Firstly, lack of heat. This could be down to low coolant level, in which case check for leaks, repair them, and refill. If a heater blows hot air on the move but cold air at idle, then there is likely to be an air lock in the heater, in which the heater matrix traps air – it’s a particular problem for 1994-onward Range Rover Classics (soft dash). You may also notice gurgling noises from the heater as you accelerate, which is a sure sign of air in the matrix. Cure that by following the correct filling procedure. But some people swear by raising the front of the car while filling the system, to help bleed air out on Range Rover Classic and Discovery 1 and 2 (all engines) models. A total lack of heat could be a failed thermostat in the engine’s cooling system. On more complex systems, all electrical issues begin with checking fuses. Blower motors, heater resistors and blend motors can, and do, fail.

If the coolant level is correct and there are no air locks, yet heater performance is still lacklustre, then the heater matrix may be blocked. Being off the main flow, with smaller internal spaces than the engine pipework, they can collect debris which, over time restricts the flow through the matrix. A lower flow means less heat is transferred, so less hot air flows into the car. The way to check this is to feel the two hoses that feed the heater matrix. If one is hot and the other cold, you either have an air lock, or a blocked matrix.

What is it? The fins of the heater matrix are visible at the side. Unlike this Series I, the heater is usually difficult to reach, hidden behind the dashboard.

How does it work? It is fed by two pipes that tap into the engine’s cooling system; one feeds coolant in to the matrix, the other returns it back into the system.

How to locate: On later Land Rovers with busier engine bays, look for two smaller bore rubber hoses that typically pass into the bulkhead, as on this V8 Range Rover Classic.

Identify flow: The coolant feed inlet pipe then needs to be identified. It will be the hottest of the two, and this can be carefully checked as the engine warms.

Connections: Decide the best way to connect a flushing hose to the matrix connections, either using those bulkhead connections, or to pipework leading to them.

Cold engine only: Having confirmed the engine is cold and depressurised to avoid the risk of scalding, remove the radiator cap. Never remove a radiator cap on a hot engine.

Let it out: Drain the coolant, usually by removing a lowest coolant hose joint, allowing coolant to escape. Some engines and radiators may have a small drain tap, like this.

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They all do that, sir… Sediment build-up in the block can mean the tap needs a prod (or suction) to clear. That same sediment is, of course, what blocks the heater matrix.

Get in there: The bulkhead connectors are undone to access matrix supply. Always use a socket on hose clips. A screwdriver may slip, potentially causing you to stab your hand.

Removable? This Series I matrix is easily removed by unbolting the fixings on the bulkhead and disconnecting the wiring. Impractical to remove for a routine flush on many vehicles.

Against the flow: We flush the opposite way to the usual coolant flow, to push the crud back out. In this case, this is the connector we’ll use to flush the matrix.

No fancy kit: No need for fancy connectors, just hold a hose against the pipe, and open the tap. Clean water pushes through the matrix, forcing out the residual coolant first.

Crud out: The coolant is followed by brown water, which is all the crud being carried out. Notice the sediment collecting in the bottom of the bowl. A good sign.

Sparkly clean: The water emerging will then clear as the worst sediment is removed. I then flush the opposite way, too, and repeat this. Eventually, the water always runs clear.

Important jobs: Refit any hoses removed to drain the system. Here, I refit the matrix and close the drain tap. Check hoses and clips, renewing any that are defective.

Chemistry: The correct concentration of neat antifreeze for the coolant capacity is measured, then added to the car. This allows for any residual water in the cooling system.

Top up: Then top up with de-ionised water to reduce the chance of mineral build-up, as can occur with some tap water. Replace the radiator cap, and start the car.

Bleed, check, test: Bleed the system as specified in your workshop manual, set the heater to hot and the fan on. Check around the system for leakage before starting the engine.

Hot, hot, hot: As the engine reaches operating temperature, you’ll feel extra heat from the heater, a noticeable improvement. Switch engine off and make a final check for leaks.


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