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Discussion Starter #1 (Edited)
My way - DIY R18 coolant “flush”.

**Somewhat time intensive and boring method**

**Snapped pics of almost everything to make it comprehensive. If too much, sorry.**

So while this might not qualify as a complete coolant flush per se, it’s a comprehensive way to get the following done.

-No radiator hoses needing be removed
-All new fluid (effectively)
and my personal favorite reason…NO TAP WATER EVER ENTERS THE SYSTEM.

I’m obsessive when it comes to my cars in some ways. I check my tire pressures weekly, I check my oil every time I get fuel, I double up on maintenance (trans fluid every 30k vs. 60k, timing belts at 50-60k vs. 100k on my timing belt cars) etc etc. Not necessary I know, but I don’t trust manufacturers, often times they pad the numbers to make it seem their car is cheap to maintain according to Consumer Reports or something (i.e. Honda CR-V valve checks every 105k miles in the U.S. vs. 30k in Europe) just as one example.

So 30k, it's new coolant time for my car.

So anyway, when people say “flush your car with tap water through the radiator”, I go nuts. It’s stupid to me because 1, you're mixing tap water with your clean coolant (full of calcium, minerals, etc that you don’t want), and secondly, a lot of people often just stick the hose in the radiator and leave it running, thinking it’s flushing the block and heater core…but it’s not. Cold water, thermostat is closed, engine is not going to allow a lot of the radiator water in but why would you want to? It's tap water! I know it’s likely going to be fine whatever you do (I’ve seen people leave coolant for 10 years without huge damage) so a little tap water isn’t so bad in comparison, but for me, I want no tap water at all.

A few years ago, I had a little epiphany and realized that to do a simple flush (actually multiple drain and fill) was easy because distilled water is actually quite cheap, for me $2 a gallon. So, nothing hugely wasteful about flushing with a few gallons of distilled water considering I spent $10 at McDonalds for my lunch today.

So, if you get the gist of where this is going, you're essentially going to do a lot of drain and fills with distilled water only, and then at the end, fill up with coolant. There's a few technicalities I address, but you get the idea now hopefully.

First you need:

1-1.5 hours ish (more if you’re easily distracted like me and start doing other things in the middle of your job like cleaning out your other car)

2 gallons of coolant (or 1 if you do what I do exactly). There’s many coolants out there that are compatible I’m sure, but for the sake of being lazy, I went straight to the Honda dealer and bought Honda Long Life Coolant because I knew it would work. Annoyingly, they only sell 50/50 blend. Saves customers the hassle of mixing, or worse, messing up with mixing the wrong amount of water, or not mixing water at all, etc etc. Consumers are stupid, so I don’t blame Honda for going that route. Lots of places are doing it too, so it’s okay.

2-3 gallons of distilled water (I used 5, but you wont need to).

Coolant drain pan (be nice to mother earth).

Drive ramps or a jack. Make sure your ramps don't hit your front bumper. Mine almost did.

Eyes and half a brain...



Step 1: Drive the car on the ramps. This is because I’ve HEARD that it helps the system bleed air easier, but I have no idea.

Step 2: MAKE SURE THE ENGINE IS RELATIVELY COLD (radiator warm is okay, just not hot to the touch). Hope that made sense.

Drain the coolant via the plastic pet**** (lol at censoring) into your coolant drain pan. I needed to use vise grips to get mine loose. Put paper towels on the vise grips to not damage the pet****. Remove the radiator cap to speed up the draining.

This is how much came out .9 gallons.

I was also curious to see if the new coolant looked any cleaner or newer.

I emptied the drain pan, wiped it down and filled it up with the new coolant.

Looks exactly the same. :)

Step 3: Tighten pet**** (relatively loose is okay, even a little drippy is fine). Fill up with STRAIGHT DISTILLED WATER. Mine took 1 gallon.

What I like to do is buy those drink dispenser distilled water containers and let it slowly trickle in. When you fill it quickly, often the filler neck gets full and you end up spilling everywhere, so I do this (takes maybe 10 minutes to fill) but I go do something else in the meantime like sweep the garage or something. You don't have to obviously.

Step 4: WITH RADIATOR CAP STILL OFF, start the car and let it idle.***

***I have heard various accounts from mechanics on why and why not, regarding if you should rev the engine to bleed air out.

Reason on why you should rev, rev the engine so that the water pump can push more coolant through the passages and force air out better.

Reason on why you should not rev. The impeller itself on the water pump can “smash” up the big air bubbles in the coolant and emulsify air inside the coolant itself if you rev it "too high" (like how CO2 mini bubbles in soda get mixed in). Then they don’t get bled out and later on combine inside the motor when it’s stationary and creates another air pocket again, so the air isn’t bled out.

I personally let the car idle because it’s easier and can watch the radiator filler neck without having to man the gas pedal. Watch the bubbles come out blip by blip.

Misc info, my car starts to rise in coolant level (expanding) around 6 bars on the temp gauge.

Sorry, should have mentioned this earlier, make sure the heater is on. I often check to make sure there is some hot air blowing. If it feels nice and hot, then there is coolant flowing through the exchanger (good). If it feels oddly cold, then there's air trapped in the system somewhere (bad).

My car never passed 9 bars on the temp gauge, but it's a really weighted gauge because it went from not boiling to somewhat bubbling (10-20 degree difference maybe?) and it stayed at 9 bars the whole time.

Let the car idle for around 15-20 minutes to really get the coolant mixed up and circulated through.

Step 5: BE CAREFUL, or let the car cool down. Drain the coolant out of the radiator again and fill up your pan. See how much lighter the coolant is? Obviously, you're getting the old coolant out still with this "flush".

Comparison to pure coolant.

Step 6: Fill up the radiator (after tightening the pet**** of course) with distilled water again.

Step 7: Repeat the “heat up and run” process. You might be apprehensive about shock cooling (causing cracks because of introducing cold water into a hot system). I’ll be upfront, I’ve never done this to this car (R18 auto) before, but I’ve never encountered problems with my other vehicles (even aluminum block ones) before, so take it for what it’s worth. I also use relatively warm water (it’s 90 something degrees today). If it was freezing outside, I would actually take the time to heat up my water that I was draining and filling with on the stove, not joking. Just would make me feel better. After doing this to the car, there was no issue with driving around or overheating, etc so I assume it's fine.

***I repeat the radiator drain and fill process 2 or 3 times because of the following logic.

Original coolant
50/50 mix.
1. Drain 1/2 and fill with water = 75/25 water roughly
2. Drain and fill again = 87.5/12.5
3. Drain and fill again = 93.75/6.25

I know I likely did that bit of math wrong mostly because I don’t know the block capacity and such, but you get the idea. With three drain and fills, you get 93 percent new fluid. Good enough for me. **Actually no, it wasn't good enough. I did it 5 times, lol but three is plenty fine.

Here's the 3rd time I filled it...getting lighter.

2nd time


Anyway, I also like to fill and drain it a few times because the idea of flowing a lot of new fluid through the system gives the car a better chance to get rid of sediment that would otherwise not be removed in the first or even second fluid change.

Step 8: NOW, take off the coolant over flow tank and rinse, flush, whatever, it out. Fill 3/4 full with 50/50 coolant. I overfill it because often times there is still air in the system, so I find that over time, the overflow tank goes down a little. Also when removing it, I promise you can get it on and off without touching the battery, but it's a tight fit.

Now, top off with coolant. Note my lazy method again, I filled the distilled container with new coolant obviously.

Step 9: Here’s the point where it may cost you a little extra than you might have wanted to pay at first. Do another start and run procedure, and then DRAIN again. You’ll be draining new coolant out…noooo…

***I personally don’t do Step 9 because I like to run a 75/25 ish mixture of water and coolant. Coldest it ever gets here is 60 degrees F but often almost 90-100, so no chance of freezing, very hot, and water actually cools better than antifreeze anyway. 25% coolant I feel is enough for the corrosion resistant properties, but makes the coolant more “efficient” at removing heat, which is what it’s doing 100% of the time where I live, rather than protecting from frozen coolant, so I like more water.

I’ve actually done tests with my old Mitusbishi Eclipse and found that it would run roughly 10 degrees cooler on 75/25 water/coolant with no ill effects on reliability. I had the car for roughly 160k miles when I sold it, no issues.

Step 10: Fill up the radiator again with 50/50, run another air bleed cycle and you’ll be done. This should get you close enough to the 50/50 originally. Wouldn't be any different than in you had left residual tap water from putting the hose in the radiator to flush it out.

“Step 11”: No need to bounce the bumper up and down to burp the system, that really doesn’t do much. As long as your car idles but doesn’t overheat, you’re okay to do a little drive around. Drive around your neighborhood a little and watch the temp gauge. If you don't overheat, great, likely no air or little air left in the system. However, ehe next few times you check the car WHEN IT’S COLD, check the overflow tank level and the radiator fluid level. It will probably have dropped because some more air came out in the meantime. Top off as needed.

So there you go, a somewhat time intensive but comprehensive "flush" that's really easy to do and not that expensive. If you really want to save draining the $18 of new coolant, okay, just drain and fill once or twice with distilled water as the "flusher fluid". Just don't use tap water, that was my whole point of this.

Either or, this is the way I’ll always do it so I just wanted to share.

Happy motoring, hope this helped anyone.
-RH Polarbear

Super Moderator
2,160 Posts
RHPolarbear's method is nice b/c you safely "drain" most of the radiator fluid out. I'll be doing this tomorrow as per my records, I haven't drained and filled my radiator in a few years so it's time.

Not mentioned is the coolant capacity. I've seen it's 5.8 quarts (1.45 gallons) so buying 2 gallons of the OEM Honda coolant is in order.

Sorry to revive an old thread, but it is good way to "flush" the radiator.

Thanks RHPolarbear for this (I'm pretty sure he sold his R18 a while back). Good luck everyone!

29 Posts
So you took the radiator cap off at the beginning, ran all those drain, refill, runs, then put the cap back on at the very end? You left the cap off the entire time until you were finished?
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