How Do You Do A Coolant Flush? – Complete Guide

Have you ever wanted to flush your cooling system, but didn’t know how to do it? I do it every year, and I can show you how.
Locate the radiator, access the drain spout and then loosen it. Drain out the coolant in a bucket. Fill your cooling system with distilled water. Start your car and check the system pressure. Turn off your car and drain out the distilled water. Dispose the old coolant. Repeat this process until you get clear water while draining the cooling system. Finally, after you get the wanted results, fill the cooling system with coolant.
Do not worry, this is just a quick overview. I am going to walk you through the process.

What Is A Cooling System And How It Works

Coolant is what circulates through the cooling system to cool your engine so it doesn’t overheat. The coolant flows from the radiator, through the water pump into the engine
block, where the hot engine gets cooled. Then it gets pumped out of the engine block through the thermostat back into the radiator. As air flows over the radiator it cools the coolant and then the cycle continues. Typically the coolant runs into your engine via the lower radiator hose and then gets pumped back into the radiator via the upper radiator hose. The hot coolant also gets pumped into the heater core where a fan blows over the heater core and that provides heat for you in the cabin. For most cars coolant is a 50-50 mix of water and antifreeze. The antifreeze lowers the freezing point of water and prevents the engine from freezing and cracking in cold weather. At that 50/50 ratio, you’re protected a negative 34 degrees Fahrenheit. Antifreeze also raises the boiling point, so if the engine does get really hot, it won’t boil and create air pockets which could cause overheating. So at that 50/50 ratio, you’re protected to 265 degrees Fahrenheit and when the engine is running, the cooling systems pressurized which further increases the boiling point. Most cars have a pressure limit of 15 psi and the radiator cap has a built-in purge which alleviates the pressure in case of it built higher than that. Antifreeze also has additives that prevent rust, and prevent acid conditions. When antifreeze is bad, it turns more acidic and has irons which promotes electrolysis and that eats away the metal in the engine. That’s obviously bad and bad antifreeze can also degrade gaskets. Old coolant will also protect less from freezing and boiling. Now that you know about the cooling system and how it works let’s go test.

Do I Need To Flush My Coolant?

There are four simple tests that you could do to check your coolant.
  1. The first test is just a visual test.
  2. The second test is using a hydrometer for the specific gravity of your coolant.
  3. The third test is a multimeter test.
  4. The fourth test is a pH test.
You want to make sure that you do all of these tests when the engine is cool. You do not want a hot engine.
  1. The first test and that’s the visual test, and you just need to check out the antifreeze. So for the visual test, you need to open up the radiator cap to check the back side of the cap. If it’s all sludged up, that could be an indication that the coolant has gone bad. You want your radiator cap to be clean and not have any sludge on it. Also, check the inside of the radiator. If you also see gunk and buildup and stuff, that isn’t good and the radiator fluid just doesn’t look good in there. Now, you can’t always get a good picture from checking out the fluid in the radiator. So, go over and check out the coolant reservoir. In some cars you don’t have a radiator cap, you might just have the coolant reservoir only, so just go in here and take a look at the coolant. If you have green antifreeze, your antifreeze should look green. It should be a bright green. If you’re running red or orange coolant, it’s a lot harder to diagnose because rust is orange. But orange coolant should look bright orange with no debris in it. It shouldn’t look like the muddy color. In either green or orange coolant, you don’t want any type of muddiness. Pour your coolant back into the reservoir. Pour it slowly, and that’ll leave any big rust pieces or debris at the bottom. So, you can see what’s really in there. Check your owner’s manual to see when your coolant should be changed in your car, but typically coolant last three years or 36,000 miles, and some coolants last up to five years and 150,000 miles. But it’s always good just to do these tests once a year just to give you an idea of how your coolant is doing.
  2. For the next test, you will need a hydrometer to check the specific gravity. All you have to do is fill the hydrometer up to that with coolant, and the needle in there will float based on specific gravity to a certain reading. Then, you just read it. Get the coolant directly from the radiator. It doesn’t matter where you get the coolant from. You could get it from the reservoir if you want. This is just easier for me. So, after you fill this all the way to the top, there’s going to be bubbles causing the arrow to float. So, you just want to tap until all the bubbles come off of that float. Make sure you get the Celsius and Fahrenheit reading straight the bottoms. It should be protecting a negative 34 degrees Fahrenheit and 265 degrees Fahrenheit. If you do not meet these parameters, you definitely have to change the coolant.
  3. For the next test, you will need to use a multimeter to see any electrical current that might be in the coolant, which will identify any electrolysis. So, just set your multimeter to twenty DC volts, and either go in through your radiator cap or you could go in through your coolant reservoir. With the engine off, get a reading by sticking one of the leads into the coolant and the other lead onto a ground. Now, you want to see a reading of 0.3 or less. If you see a reading of 0.3 or higher you have too much of an electrical current running through your coolant. What voltage in your coolant means is that there’s an exchange of electrons between metals in your engine and your cooling system which will slowly eat away and corrode those metals so much so that you could actually get leaks in the gaskets and hoses because they won’t seal properly anymore. Combine your result with that hydrometer test and the visual test to see if your coolant needs changing.
  4. And for the last test, you will need to be using pH tubes. You can use a PH test for a fish tank, or you could use a pH reader, an electronic one, and that also tell you the pH. Put the test strip directly into the coolant, and then you match the color. It tells you the pH. You can buy specific pH testers for your coolant, but why go out and buy them when you have them for your fish tank. When testing your coolant you want to have a pH reading between 8.5 and 10. So that’s basic. Remember a pH reading of 7 is the middle below 7 is acidic, above 7 is basic so a pH level below 8.5 means it’s time to start thinking about changing your coolant. Any pH reading of 7 or below, definitely means that you need to change your coolant because the acidic coolant will eat away your gaskets such as your head gasket, and also eat away your rubber hoses. So all you’re going to do is just dunk the test into the radiator, pull it out and just match up your color to the color that’s on the packing.

So after completed the 4 test, you can come to a conclusion about whether you need to change your coolant or not. If you need to change your coolant, here is how:

Before You Drain The Antifreeze From Your Car

Let’s get this straight. Some call it coolant, some call it antifreeze. But its the same thing. Now that we got that out of the way, allow me to show you how to flush your cooling system and get all that dirty old contaminated coolant out of the radiator, engine block, and heater core and just get a really good flush and replace it with a brand new coolant. Before starting any work, make sure your engine is cold. The system is pressurized when it is hot and you don’t want to burn yourself. Now, on my 2007 Hyundai Santa Fe, I have a radiator cap and I also have a coolant reservoir tank. Some cars might not have a radiator cap, so whatever I say I do through the radiator cap, you could do through your coolant reservoir tank.

A couple of days before you’re going to flush your coolant, you’re gonna want to use a chemical coolant flush. You’re supposed to empty your coolant, add water and then add chemical coolant flush into your coolant reservoir tank. The chemical flush is really going to help flush out any contaminants, loosen up any debris, and it’s just gonna make your flush much better. I use the whole bottle and then I drive the car for 100-200 miles.

What Will You Need

So now we’re ready to do a coolant flush. In order to do a flush, here’s everything you’re going to need:
  • Basic hand tools
  • A couple of drain pans to catch the old dirty coolant
  • Bunch of distilled water (make sure you’re using distilled water and not filtered water)
  • Two chemical flushes
  • Coolant (I’m using a 50/50 mix so I don’t have to add water)

Before you buy your coolant, you want to check your owner’s manual and see what kind of coolant you need to use, and also the capacity so you know how much coolant your cooling system will use. Now that you know the capacity and what kind of coolant that you’re going to use, and you know all the tools that you will need, let’s begin.

How Do I Drain The Antifreeze From My Car?

  1. Grab one of your containers and slide it underneath the car where the radiator petcock is. The petcock is just a valve that lets the coolant out. Usually, it’s in the corners of the radiator.
  2. Unscrew the radiator petcock and drain the radiator. TIP: If you open the radiator cap, your coolant will drain faster.
  3. If you have a coolant reservoir tank, find the hose that goes to the radiator and disconnect it. It’s not gonna start to drain automatically, so you will have to find something(turkey baster) to suck on it. Do not use your mouth. Place the hose into the container to drain.
  4. Take a cup and fill it with the coolant. I like to take a sample, kinda like a before and after thing, just to see the progress.
  5. After your coolant reservoir tank is empty, wash it with a hose and warm water. Make sure you get all the debris out of the tank and then let it drain. Repeat this process as many times as necessary.
  6. Recycle your antifreeze. Antifreeze is poisonous to animals. If your dog or cat gets into this and drinks it, which taste sweet, they will die. So have a bunch of containers on hand, fill them up with the antifreeze and call your town recycle center or your local auto parts stores. They’ll take it for free.
  7. Find your thermostat. Usually, to find the thermostat, you need to follow the upper radiator hose into the engine block.
  8. Unscrew the bolts on your thermostat. The reason you need to remove the thermostat is to have bigger flow even when the engine is cool. If your thermostat is hard to get to you could just omit this step. I usually replace the thermostat every time I flush the cooling system.

Assemble All The Parts Back Together

  1. Tighten the bolts back and connect the radiator hose into the engine block.
  2. Tighten the radiator petcock.
  3. Connect your hose back to the coolant reservoir tank.
  4. Fill up the cooling system with just plain distilled water.
  5. Run the engine and get everything circulated.
  6. Mix the water with all the contaminants and do another chemical flush.
  7. Fill the radiator to the top and get the radiator cap. Put that on and make sure it’s completely closed.
  8. Fill the coolant reservoir tank. Don’t worry about adding a little bit extra, because if you’re low on coolant this is a good backup. It’ll suck out the coolant from here and head it to your radiator. Also, want to have some room for expansion.
  9. Start the engine. Run it for 10 minutes or go for a ride.
  10. Pay attention to that coolant gauge. Make sure it doesn’t spike. If it does, that means you have some air in the system and then all you have to do is squeeze the upper radiator hose as the engine is running. Or, if you have a bleeder valve on your water pump just open that up, let the air out and close it.
  11. Turn the heat on and put the speed fan to max.

Drain The Water And Add Coolant

  1. Now, drain that water out of the cooling system. Untight the petcock. Be careful because the water might be hot. Remember to make it drain faster, you’re gonna open up the radiator cap. Make sure this is cool to the touch.
  2. Grab a sample from the water that is draining.
  3. Depending on how bad your cooling system is, it might take one flush, it might take two flushes or even four.
  4. Drained the cooling system completely.
  5. Tighten all the components (radiator petcock, reservoir hose) and repeat the process.
  6. Run your car for five to ten minutes. Drive around if you want.
  7. Drain it again. Just keep doing those steps until that water runs clear.

Okay, so you completely drained the system and the water that came out was clean. Now it’s time to fill the radiator with a coolant.

Fill Up The Radiator With Coolant

  1. Fill it up with brand new antifreeze. This is a pretty important part. You just did all this work to flush out your cooling system. You want to make sure you do this right. Check the owner’s manual to see which kind of antifreeze you need. My car uses the orange coolant, as stated in the owner’s manual. You should check yours.
  2. Most vehicles need a 50/50 antifreeze and water mix. I’m using a pre-mixed antifreeze. it’s just a lot more convenient. Sure, it is a little bit more expensive compared to buying the full concentrate and then diluting it with distilled water, but it’s just a lot easier. Also, I’m using long life. This stuff lasts 150,000 miles or five years.
  3. Since the cooling system is completely drained, before we fill it with new coolant, we want to change out our thermostats. YOU CAN SKIP THIS STEP. Take out the old thermostat and make sure you remove the whole gasket. You can use a plastic scraper so you don’t damage the metal surface. Clean the surface with a towel and some brake cleaner spray. Install the new gasket. Install the thermostat and tighten the bolts.
  4. Fill the radiator and the reservoir up with brand new antifreeze.
  5. Before you start the engine, just go down to your serpentine belt and make sure that you don’t have any liquid on the belt so your belt doesn’t go flying off.
  6. Start your engine and let it run for 5-10 minutes.
  7. Keep an eye on your temperature gauge. Make sure you don’t overheat.
  8. Fill up the coolant reservoir(if you have). On the coolant reservoir, there is a fill hot-line and a fill cool-line. Fill it up just a little bit above that cold fill line.
  9. Go for a ride and pay attention to your temperature gauge. Make sure the arrow stays at the middle as you go for our drive. You don’t want to get too hot and you don’t want to stay cold.
  10. Make sure that you have the heat on and you are actually getting hot air. If you don’t feel the heat, you have air trapped in the system.
  11. Let the car cool down and check your coolant level and make sure it’s good.

How Much Does It Cost For A Coolant Flush?

A coolant flush will cost you anywhere from $70 to $120. This is for both parts and labor(if you decide to visit a mechanic). A coolant will cost around $40-$50, and around $12 for the chemical flush. I have provided the full step-by-step guide on how to perform a coolant flush, so you don’t need to spend $50-$60 for labor.

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Hi there. I am a certified Automotive Service Excellence (ASE) mechanic since 2018 and a car detailer for 10 years.

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