Ways To Flush Dirty Coolant

How To Flush Out Dirty Coolant Completely

Summer is the ideal time for racking up some kilometres on your ride as you get to bask in the essence of open, clear roads and the ability to travel long distances to places less ventured. But this freedom is bound by the condition of the vehicle and the most common route to failure in summer driving is an overheated engine. 

However, this can be easily avoided by maintaining the coolant in your car and regular inspection of the coolant can tell a lot about the condition of your vehicle’s engine. 

How To Determine If My Coolant Needs Replacing?

The coolant is made of distilled water and antifreeze at a 1:1 ratio or 50/50. The antifreeze’s function is to lower the freezing point and raise the boiling point of the water. The freezing point needs to be lowered so that the water doesn’t freeze in cold temperatures which can cause cracking and damages the engine. And by increasing the boiling point, the antifreeze stops bubbles from forming which can cause engine overheating due to the presence of air in the coolant lines. 

The coolant also has additives like rust inhibitors that prevent rusting. However, these additives degrade over time and increase the acidity of the coolant causing corrosion in the engine block, deteriorates the engine gaskets and dissolves or eats away the metal surfaces. This is why it is important to replace the coolant at regular intervals to prevent acidic conditions that damage the cooling system and engine components. 

Before you start off, wait till the engine is cool to touch. Working on the cooling system when the engine is hot is very dangerous. 

There are four ways to determine the condition of your radiator coolant:

  1. Visual Inspection: Taking a look inside your coolant reservoir or radiator cap can help you determine any abnormalities. The colour of the coolant says a lot about its condition. Your engine can take either blue, green or orange coolant and if you see a drastic departure from these bright colours then the coolant may need immediate attention. The coolant flows through the entire engine block and has rust inhibitors that prevent scaling. If the coolant is too old then you may see suspended particles and floating bits which can be an indication of corroded engine parts. Also, look for grime or sludge build up on the radiator cap or inside the reservoir as that could mean the gaskets are busted. 
  2. Hydrometer TestCoolant hydrometer testers show the quality of the coolant based on the specific gravity of the mix. To test it, suck the coolant to the indicated level in the hydrometer tube. Shake out any air bubbles and let the needle or float in the tube set. Read the corresponding value the needle shows and that can tell you the condition of the coolant mix. The minimum coolant temperature is about -370C or -340F and the maximum is 1290C or 2650F. Check where your coolant lies in this scale which tells a lot about the coolant condition.
  3. Multimeter Test: A bad coolant causes electrolysis which basically means the coolant is eating away the metal in the engine block. This is obviously bad for the vehicle. To test it, grab a multimeter and set it at 20 DC volts, then place the positive probe inside the radiator cap or in the reservoir, touching the coolant and the other end to ground. With the engine, off, take a reading. Less than 0.3 is fine but any value above that is bad. Start the engine and check again. if the reading reached above 0.3 then the coolant is causing electrolysis which can slowly degrade and destroy the entire engine.
  4. pH Test: The pH scale of the coolant can tell the acidity or basicity of the mixture. The ideal pH of the coolant is between 8.5 and 10. Using a digital pH scale or a pH strip find the exact pH of the mix. If it is lower than 8.5 then you have a very acidic mixture that can destroy the engine and requires complete flush of the coolant. 

You may do any one of the above tests after a visual inspection to ascertain the condition of the coolant and to be very certain you can try out all the tests. Failure in the visual inspection alone can be enough reason to carry out a coolant flush. Always follow the replacement guidelines stated in your owner’s manual for the coolant servicing and stick to the manufacturer’s recommended coolant colour or type. 

How To Flush The Coolant?

After your engine has cooled down completely, you can begin to start working on your cooling system.

The tools you will need while flushing out your cooling system are:

Here is the procedure to flush out the old coolant and add fresh coolant:

  1. The first step is to use a chemical coolant flush that clears out any contaminants like dirt, debris and metal bits resting in the car’s radiator or engine block that can damage the components and drastically reduce their life. 
  2. To do this first drain all the old coolant from the radiator drain plug or petcock. The petcock is a small valve that has a nut which when loosened, can allow the flow of coolant from under the radiator. Make sure to tighten the bolt after the coolant has drained. 
  3. If your radiator lacks a petcock then you can drain the fluid via the lower radiator hose. Undo the clamp on the radiator hose using a plier and let the coolant drain into a catch pan or fluid drain. Always open the radiator cap or coolant reservoir cap to speed up the draining process.
  4. You also need to drain all the fluid from the coolant reservoir. To do this locate the hose or pipe that connects the reservoir to the radiator. Remove the pipe from the radiator and hold it such that the coolant reservoir is higher than the pipes opening. Using a pump, suck the air out to allow the coolant to drain out into the catch pan. 
  5. To allow the cool mixture to flow throughout the engine while flushing, you can remove the thermostat in your car. It is usually located at the end of the upper radiator hose connected to the engine. To do this simply undo the bolts holding the hose and the thermostat is exposed. Remove it and redo the bolt to the hose. 
  6. Once the old coolant is out, fill the reservoir with distilled water and coolant flush. You can bleed the air out of the system via the coolant air bleeder located at the top or front of the radiator. 
  7. You then need to run your engine for a few minutes so that the coolant flush can get to every nook and cranny of the engine and clean out all the dirt. Also, make sure to open the heater in your cabin at max setting to let the coolant flush circulate in the heater core and clean that out as well. 
  8. A chemical coolant flush is especially recommended if the testings from the previous section revealed that the coolant has a buildup of grime or has a sludgy residue. The coolant flush cleans out all the dirt and debris accumulated in various sections caused by oxidation and scaling of the metal components in the system. 
  9. Once you run the vehicle for 10 minutes, you can begin to drain out the fluid and examine the colour. 
  10. The process for draining is the same as before. Once drained, look at the condition of the mixture. If the colour is still dark and rusty then you will have to carry out another flush. Basically, you need to flush the system until you see almost clear distilled water when you drain it. This may take up to 5 flushes.
  11. After carrying out all the flushes, make sure to replace the thermostat with either a new one or the old one you previously removed. Change the thermostat gasket as well. 
  12. Now you can begin to add new coolant to the system. You can either opt for a pre-mixed coolant or get concentrated anti-freeze and make the 50/50 mixture yourself. 
  13. Tighten all the fittings like the drain plug or lower radiator hose and attach the coolant reservoir pipe to the radiator. Using a funnel add the coolant via the radiator cap opening or through the reservoir. Run the engine for a while and top up the reservoir to the indicated level. 

Depending on your vehicle and it’s coolant condition you may need to flush the system a few times with distilled water to get all the dirty coolant out along with the particles and contaminants. Note that during the first flush use distilled water and a bottle of coolant flush. For the rest of the flushes, use only distilled water. Do not use mineral or tap water as those contain minerals that can harm the engine. Distilled water is pure chemical water suitable as coolant. 

To ease the job of repeated draining and refilling your cooling system you can opt to use a cooling system refill toolkit. This setup creates a vacuum in the cooling system that eliminates airlock in the radiator and coolant lines which can cause overheating and the HVAC system to fail. 


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John Framigllia
Technical Writer
Our technical writer is known for simplifying automotive parts and services. Intuitive with various vehicles and manufacturers, he knows how to simplify complicated problems.