How To Flush and Bleed Coolant

The job of refilling and bleeding coolant is not that bad – it’s just the process of removing air from your car’s newly serviced cooling system. But there are some important tips and tricks to arm yourself with before you attempt this after any job that requires cracking the seal on a cooling system. Before you spend hours trying to get rid of pesky air bubbles, let me guide you out of the dark, sweet-smelling forest with my shitbox wisdom.

Safety

Protective eyewear and gloves are recommended any time you mess with automotive fluids, including coolant. Take extra care when working with coolant/anti-freeze if you have pets — animals are attracted to it and of course, it’s poisonous. Best to keep them away altogether but it’s not the worst idea to look up your local emergency vet’s number as part of your workspace prep.

Coolant Flushing Procedure And Tips

The flushing or replacing of coolant is the simple part of this gig, even if it’s quite messy.

Draining Old Coolant

Draining the old coolant is easy enough; jack the car up and secure it safely, and locate the radiator drain (sometimes known as a petcock), usually on the lower part of the radiator. It is usually plastic and held in with a thumbscrew, a flathead, or a Phillips-style screw. Most cars will have it and make the coolant drain much less messy. 

For the poor souls who don’t have a radiator drain, it will get a lot messier from here but I can still help. If you want a cleaner but longer process, start with the lowest and smallest hose you can find, like a coolant overflow line from the upper or lower radiator hose. In the case of my 2010 VW GTI, there is no clean way to drain coolant without doing a segmented approach. I could remove the hose from my auxiliary coolant pump to drain most of the coolant from the engine, then remove the coolant temperature sensor on the lower radiator hose to further drain it in a controlled way. Then fully remove the lower radiator hose to complete the drain.

If all else fails, get a large drain pan, remove the lower rad hose and pray. All the coolant will come out for sure.

Flushing The System

Before filling up the cooling system permanently with fresh antifreeze, this is the perfect time to do a flush. There are a number of chemicals and methods you can use for this, but I think some distilled water and a big bucket will do the job just fine, depending on the age and condition of your car. Hose water could even be used as an extremely temporary solution by just running it through the open cooling system, but if you want a decent flush I’d recommend getting a few gallons of distilled water which is usually available at stores like CVS and Walgreens.

Close the cooling system back up as normal and fill it with distilled water. Let the car run it for a few minutes, even up to operating temperature to open the thermostat and get the water through the whole system. Once that’s done, let it cool and then drain. If there was color in the water, then it needed some flushing to get the maximum amount of fresh coolant into the system. You can repeat this process as many times as you like, even chasing a totally clear drain. I say doing it once works just fine. Now to finish the job for real.

Re-Sealing The System

Once all the coolant is drained and definitely not splattered on the garage floor, we can reinstall the removed hoses permanently and swivel our focus towards filling and bleeding the cooling system. From here on, the job becomes something of a trade between time invested and quality. The more time you spend massaging the air bubbles out of the cooling system, the more likely you won’t be overheating and stuck somewhere because of a pesky air pocket. Let’s start by filling the coolant up to specified capacity.

Re-Filling Coolant

For cars with external reservoirs or expansion tanks, there will be a fill level. Fill the coolant to that fill line but don’t expect it to stay there for the first few pours. The coolant level will continually go down until the coolant fills all of the air pockets within the opened part of the system. Fill until it stops taking coolant. If you want to future-proof yourself from any coolant bubble shenanigans, you can also take the time to raise the part of the car where you fill the coolant to make the entry point higher, thereby helping any air bubbles get there. Air wants to rise to the highest point naturally.

For cars with a simpler radiator cap cooling system, I highly recommend buying a no-spill funnel for about $40 from your local auto parts store. It will make everything much easier by allowing for a lot of extra coolant capacity for bleeding and it raises the highest point of the cooling system much higher than the rest of the car. 

Most important of all, make sure to use the correct coolant. If it calls for pink coolant, do not use green. Especially do not mix different kinds of coolant unless it is explicitly designed for it. There are exceptions: blue coolant can replace green coolant in some 2000s Japanese cars. For the most part, stick to the service manual or what it says on the coolant itself.

Run Up To Operating Temp

The job is not yet done because the difficult part of bleeding is getting up to operating temperature (OT) and opening the coolant thermostat and running the heater so the system is operating as intended. Check for leaks or loose clamps then start your car, set the heater to max heat but minimal fan speed, and rev it to around 2,500 rpm until the car gets to OT. If your car doesn’t have a coolant temperature gauge, an OBD tool will come in handy here. If you can’t get your hands on it, generally you’ll be revving your engine until the coolant level significantly drops. That’s when you’ll know that the thermostat opened and allowed coolant to fill all passages within the engine.

On my GTI, I have three coolant temperature sensors I can monitor. I know my thermostat opens when my main coolant temperature drops and my radiator temperature starts rising. In any case, take a moment to refill the coolant once the thermostat has opened then continue revving the car at a steady rpm. 

The folks with a no-spill will be able to monitor from the driver’s seat if there are any bubbles still coming out from the cooling system. Depending on the car, some vehicles will have a smartly designed mostly self-bleeding system or have a series of annoying bleed screws to open like on early BMWs. Whichever is the way, make sure to follow the factory-recommended bleed sequence. Most cars aren’t that complicated and only take some patience. Wait for bubbles to subside.

Some cars have two coolant fill points, most notable of those being older turbocharged Subarus. In this case, you bleed from the highest point which is the coolant cap by the turbocharger. You should use the lower one on the radiator to help fill the system before you bleed.

Modern cars with electric coolant pumps also have their own bleed procedures that can be incredibly easy. For example, my 2007 BMW 335i simply required me to fill the coolant to the proper level, turn on the ignition, set my heater to max temp and minimum fan and hold the throttle pedal down for 10 seconds. The water pump would self-bleed and the job was done. For my 2010 VW GTI demonstrator, the bleed is done once the thermostat is open and the coolant is at its proper level. Modern VWs are designed to continually self-bleed after some driving so it will take a few cycles of adding minor amounts of coolant to finish the job.

Shakedown

Remove the no-spill (if applicable) and close the cooling system up for a test drive. Make sure to put the car through its paces and monitor coolant temperature. If all went well, you shouldn’t see any overheating. If the car is overheating, its likely that there is still an air bubble in the system and you’ll have to re-bleed it. 

With that, you should be bleed up and ready to go for a rip. Make sure to dispose of the old coolant properly and check for any leaks after your test drive. Happy non-overheating, car friends.