Changing radiator coolant

Civic 2001-2005

  1. Nighthawk
    Difficulty Level:
    Quite a simple one this really, but it needs a bit of patience and is the same principle for pretty much any marque car, not just Hondas although there will be obvious design differences/locations.

    Radiator coolant needs replacing every 6 years or 120,000kms according to Honda. Not only does the coolant help keep the car engine cool in Summer, it also lubricates the water pump which is a rather expensive repair. So, I have changed the coolant on the missus EP2 purely because I don't have a record of when it was last done. This is part of its major service I am doing but I felt that this could do with being its own DIY.

    A few things - you will eventually be dealing with hot liquid - if you are not comfortable dealing with hot liquid, STEP AWAY FROM CAR :Whistle::Laughing:

    On a serious note, be careful.

    I took off the radiator coolant tank to give it a clean, you do NOT have to do this, but I wanted to as well as flushing the system out. As far as I know, Honda do not state you have to do this, but old habits die hard. You can easily do this DIY by skipping Steps 3 and 4 if you wish to do so.

    - 10mm spanner (for removing the reserviour tank and removing the battery if doing the res tank)
    - Radiator coolant - (the cooling system on these cars holds 4.1L according to Honda)
    - Drainage container
    - Plain water (hose pipe is ideal)

    STEP 1:

    This is a preparation stage.

    Firstly - Turn your temperature dial inside the car to max hot with the engine running, after a few seconds, turn the engine off. I did this the last I drove the car as I knew I was changing the coolant today. This ensures that the valve to the heater matrix is open as you want to drain the fluid from that. This is an important step don't SKIP IT.



    For those that do not know, you have a heater matrix inside of your car. The engine coolant runs through this and provides you hot air within the cabin. This needs to be flushed out with new coolant as it could rust and subsequently leak into the car cabin. It is controlled by a valve which is opened and closed depending on the position of the car temperature dial that you adjust.

    STEP 2:
    Remove the radiator drain plug. This is a plastic plug which unscrews anti clockwise. You could remove the lower tray off the car, but I didnt see a need to so left it there.

    This will dribble out initially, so make sure your drainage container is underneath it. Now remove the radiator cap and coolant will start to gush out.


    Ok, pet hate here, who puts a radiator cap on the wrong way? Seriously?? I don't know how I never spotted that before hand.

    Anyway, let the coolant flow out until it naturally stops.

    STEP 3:
    This is a personal preference here which I would advise, but flush the radiator out. (No pictures here as I forgot). Stick a hose in the top of the radiator and let clean water flow through it. This will flush out any rust elements (hopefully) and any old coolant. Tighten up the bleed screw at the bottom of the radiator, disconnect the top return hose and flush more water through so it flushes out the cooling channels. Once everything runs clean, reconnect the return hose and reopen the bleed screw and drain the water out again. Once this is all finished, retighten it and its time to move onto the next stage.

    STEP 4:
    Again, personal preference but I would advise it. Remove the coolant expansion tank and give it a clean. Mine was filthy on the outside and could not read the level on it.

    The coolant tank works simply by, if pressure drops within the radiator due to a leak etc, the pressure pulls coolant from the tank into the radiator to keep it topped up. As a result, coolant can stay in the coolant tank for quite a long period of time if its not pulled up. This is held in place by a 10mm bolt

    And held in place by a support bracket which just clips in. I couldn't get a picture due to its awkward position but its easy to work out once you get into it. Remove the 10mm bolt, remove the overflow pipe which just pulls off, and just pull the entire unit straight up. I disconnected and removed the battery to make access easier for this.


    With the coolant bottle removed, empty it out and flush it with clean water and give it a good clean.

    Put it back in the car, clip the lower section back in, you can see the clip in the above picture and retighten the 10mm bolt.

    STEP 5:
    Make sure the bleed screw it tight, now its time to start to fill the radiator. Top up the coolant expansion tank to its max mark. Open the radiator cap and start pouring coolant in slowly. As you have opened the system up, it now has air and you need to bleed this air out to ensure effective cooling/heating. This is quite time consuming as you cannot really rush it, but as you pour, the laws of science will force air up to the surface causing air bubbles. After you have emptied around 3 litres or so, air will naturally stop popping to the surface and will now have become trapped by the fluid in the highest points of the engine/coolant channels etc so you will have to force it out.

    STEP 6:
    Start the car engine and let her idle. Ensure the temperature dial is on max still and turn the fan on slightly so it is pulling hot air into the cabin. Keep the radiator cap off, there is no real risk at the moment as there will be no pressure building up as the system is not sealed (with the cap off), so water will not spurt at you. As she runs, she will force the coolant around the system forcing more air out of the system. The coolant will start to rise within the radiator, this is normal and is the air pushing the coolant out from behind. This will eventually force the air out.


    As the air pops out, science dictates that the fluid level within the radiator will drop. As there is no pressure, the coolant tank won't pull coolant in, so you need to top it up. Top it back up, not all the way, just enough to cover the radiator core completely, and let science continue its magic. Once the car starts warming up, raise the revs ever so slightly to help speed the process up. Keep an eye on the temp gauge to ensure she is not overheating. The coolant will now start getting hot, so BE CAREFUL. Again, no pressure behind it, but the bubbles could cause it to spit at you and burn. The increased revs, and I mean just slightly increased, will help bleed things out faster, rev it for 20 secs or so, then let it drop back down to normal, and top up again. Keep doing this until she is up to temperature. You will be able to feel the hot air coming into the cabin, and you will be amazed at how hot it will actually be blowing out if you havent changed your coolant before.


    Keep this process up until there are no more obvious bubbles (can take around 15 minutes or so of idling and gently rev increase). Check the levels, replace the radiator cap (the right way around please unlike the muppet who topped up the fluid in the radiator on my car in previous ownership), and take her for a drive. Drive normally. Come home, let her cool down, and once cool, check all levels, and top up again as necessary.

    Job done

    Enjoy the new hot air and your water pump will love you even more for the clean lubricant.

    On a side note, I used genuine Honda fluid. It actually cost less to buy OEM than any other after market brand, so just stick OEM. It was cheaper and its the right stuff to use.

    Ichiban and Nels like this.

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  1. legend-ary
    fantastic write up.
  2. Ichiban
  3. SpeedyGee
    Excellently explained @nighthawk!
  4. Nels
    Nicely explained. Thank you for sharing.