How to Change Radiator Coolant: A Step-by-Step UK Guide

Terry Twoo

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Summary

• Engine coolant is a critical fluid that prevents your engine from boiling over or freezing. It needs changing periodically as its protective properties degrade.
• Performing a full coolant "flush" is better than a simple "drain and fill" because it removes old fluid and contaminants from the entire system.
• Always work on a completely cold engine, use the correct coolant type specified in your owner's manual, and dispose of old coolant responsibly.

Of all the fluids in your car, the brightly coloured one in the reservoir is probably the most misunderstood. It’s not just “antifreeze.” It’s engine coolant, a liquid with the critical job of keeping your engine from both boiling over in the summer and freezing solid in the winter. Think of it as your engine's personal thermostat.

Over time, this coolant breaks down. Its anti-corrosive properties fade, and it picks up tiny bits of rust and gunk from inside your engine. When that happens, it stops doing its job effectively. The result? You could be looking at an overheating engine, a damaged radiator, or a failing water pump—all of which are far more expensive and frustrating than learning how to change radiator coolant yourself.

Don’t worry, this isn’t a job reserved for the pros. With a bit of patience and the right approach, you can flush and replace your engine coolant at home, ensuring your engine stays happy for years to come.

First Things First: Are We Flushing or Just Draining?

You'll hear "coolant flush" and "drain and fill" thrown around, and they aren't quite the same thing.

• A drain and fill is simple: you open the plug, let the old coolant out, and pour new coolant in. It's quick, but it leaves a good amount of old, tired fluid and sediment behind in the engine block and heater core.
• A full flush, which is what we're covering here, involves pushing all the old fluid out, cleaning the system with a flushing agent or distilled water, and then refilling. It’s the best way to ensure the entire system is clean and protected.

Before You Touch a Single Tool: Safety and Supplies

I can't stress this enough: never, ever work on a hot cooling system. That radiator cap is holding back boiling, pressurised liquid. Opening it on a hot engine is a guaranteed way to get a nasty steam burn. Let the car sit for at least a few hours, or better yet, do this first thing in the morning.

What You'll Need

• The Right Coolant: This is the first hurdle. Check your owner's manual. We'll dive into the different types below.
• Distilled Water: Several gallons. Do not use tap water! It contains minerals that can cause scale buildup in your radiator.
• Radiator Flush Solution (Optional but Recommended): A bottle of this will help break down stubborn deposits.
• A Large Drain Pan: Make sure it can hold at least 10 litres.
• Funnel: To avoid making a mess.
• Basic Hand Tools: Pliers, a screwdriver, and a socket set should cover most vehicles.
• Safety Goggles and Gloves: Coolant is toxic. You don't want it in your eyes or on your skin.
• Rags or Paper Towels: Spills will happen.
• Sealable Containers: For safely transporting the old coolant for disposal.

The Great Coolant Confusion: Which Type Do I Buy?

Walking into an auto parts store can feel like you need a chemistry degree to buy coolant. The colours and acronyms are baffling. Here’s a simple breakdown, but remember the golden rule: your owner’s manual is your bible.

Can you just use a "universal" coolant? While many modern universal coolants are very good, they're a jack-of-all-trades. If your car is still under warranty or you want the absolute best protection, sticking with the manufacturer-specified type (like a pink HOAT for your Toyota) is always the safest bet.

What happens if you mix the wrong types? Bad things. Mixing IAT (green) with OAT (orange) can cause a chemical reaction that turns your coolant into a thick, gunky gel, clogging up your system and leading to major overheating. If in doubt, stick to the book or do a complete flush to start fresh.

How to Change Radiator Coolant: The Step-by-Step Guide

Alright, coffee's kicked in, the engine is stone cold, and you've got your supplies. Let's do this.

Step 1: Park and Prepare

Park on a level surface and pop the hood. Make sure the engine is completely cool to the touch. Put on your gloves and goggles. Turn your car's interior heater to the maximum heat setting—this opens up the valve to the heater core, ensuring it gets flushed too.

Step 2: Drain the Old Coolant

Slide your drain pan under the radiator. At the bottom corner of the radiator, you'll find a small drain valve, often with a plastic wingnut handle, called a petcock.

1. Remove the Radiator Cap: Slowly press down and twist the radiator cap off. You might hear a slight hiss as any remaining pressure escapes. This allows air to enter the system so it can drain properly.
2. Open the Petcock: Turn the petcock counter-clockwise to open it. A stream of old coolant will start flowing into your pan. Let it drain completely until it’s just a slow drip.
3. Close the Petcock: Tighten the petcock back up. Don't overtighten it—snug is good enough.

Step 3: Flush the System

This is where we get all the old gunk out.

1. Add Cleaner: Pour the radiator flush solution into the radiator, following the directions on the bottle.
2. Top Up with Distilled Water: Fill the rest of the radiator with distilled water.
3. Run the Engine: Put the radiator cap back on. Start the car and let it run for 10-15 minutes with the heater still on full blast. This circulates the cleaner through the entire system, including the engine block and heater core. Keep an eye on the temperature gauge to make sure it doesn’t go into the red.
4. Cool and Drain Again: Turn the engine off and let it cool down completely. This is a good time for another coffee. Once it's cool, drain the cleaning solution just like you did in Step 2. You’ll probably notice the liquid is much dirtier this time—that's a good sign!

Step 4: Rinse Until It's Clear

You don't want any of that cleaning chemical left in the system.

1. Close the petcock again.
2. Fill the entire system with just distilled water.
3. Run the engine for another 10 minutes, then let it cool and drain.
4. Repeat this process until the water coming out is perfectly clear. It might take two or three rinses, but it's worth it.

Step 5: Refill with New Coolant

The home stretch!

1. Close the Petcock for the Last Time: Make sure it's snug.
2. Mix Your Coolant (if needed): If you bought concentrated antifreeze, mix it 50/50 with distilled water in a clean bucket before pouring it in.
3. Fill 'er Up: Using your funnel, slowly pour the new coolant into the radiator. Pouring slowly helps prevent air bubbles from getting trapped. Fill it up to the base of the filler neck. Also, fill your coolant overflow reservoir to the "COLD" fill line.

Step 6: "Burp" the System (The Most Important Part!)

Air trapped in your cooling system is the enemy. It can create "hot spots" in the engine that lead to overheating, even with fresh coolant. We need to get that air out. This is called bleeding or "burping" the system.

1. Leave the Radiator Cap Off: Start the engine and let it idle.
2. Watch and Wait: As the engine warms up, the thermostat will open, and you'll see the coolant level in the radiator suddenly drop as it gets pulled into the engine. You'll also see bubbles coming to the surface. This is the air escaping.
3. Top Up as Needed: As the level drops and bubbles escape, keep slowly adding more coolant to keep it full.
4. Keep it Going: Let the engine run until it reaches its normal operating temperature (the cooling fans should kick on) and you no longer see any major air bubbles.
5. Cap It Off: Once the level is stable and the bubbles have stopped, carefully put the radiator cap back on and tighten it.

Step 7: Final Checks

Turn off the engine. Check for any leaks around the petcock. After the car has cooled down completely, check the coolant level in the overflow reservoir one last time and top it up to the "COLD" line if needed.

Over the next few days, keep an eye on your temperature gauge and the coolant level. It's normal for it to drop a tiny bit as the last few air pockets work their way out.

What About the Old Coolant?

You now have a pan full of toxic liquid. Do not pour it down the drain or on the ground. It's incredibly harmful to the environment and deadly to animals.

Pour the old coolant into your sealable containers. Most local councils have hazardous waste disposal sites, and many auto parts stores or local garages will accept used coolant for recycling, sometimes for a small fee. Give them a call first to check.

Common Questions You Might Have

Can't I just top it up instead of doing a full flush?
You can, but it's a temporary fix. If your coolant is old and contaminated, just adding more new coolant is like adding a cup of clean water to a muddy puddle—it doesn't solve the underlying problem. For more on essential checks, see our essential car maintenance checklist.

What are the signs my coolant needs changing?
Besides following the service interval, look out for coolant that's rusty, cloudy, or has bits floating in it. Other signs include your engine temperature creeping up higher than usual or a sweet, syrupy smell from the engine bay, which could indicate a leak. If you see your dashboard warning lights for temperature come on, it's a clear sign to check your system.

My temperature gauge is going crazy after I changed the coolant! What did I do wrong?
You almost certainly have air trapped in the system. Let the engine cool down completely and repeat the bleeding procedure (Step 6). It can sometimes take a couple of tries to get all the air out. This is the most common hiccup for first-timers. If you're concerned your engine is overheating, stop driving immediately.

And that's it! You've successfully flushed your cooling system. It might seem like a lot of steps, but it's a straightforward process that will give you peace of mind and keep your engine running smoothly for miles to come.

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