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Coolant system flush and renewal

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Hello All, 


Just did a thorough coolant flush and renew on my "new" 5th gen 1998 and set out the steps below. Hope it helps someone out. 


High mileage bike with what looks like poor maintenance. Thank God it's a Honda or it would be scrap by now. 


Anyway, I began by pulling the side fairings (I'm doing other work - you don't have to remove all the fairings like I did) and putting the bike on its side stand to drain the old fluid.


Note the extreme lean angle achieved by finding a nice dip for the side stand. You can also just muscle the bike over to the left whilst lifting the front wheel. Easier than you might think. I found having the bike on an uphill slope drained the most coolant. 


To to drain the system remove the "odd" 5th bolt and washer on the thermostat (right hand side, near your foot peg). Place a container underneath and slowly unscrew the coolant filler cap. Unscrewing the cap lets air enter the system and it flushes really fast. WARNING: this stuff shoots about 4 feet! 


There is also a coolant drain bolt under the front left exhaust downpipe which is awkward to reach but also dribbles a bit of fluid if you really want to get every last drop out. I did, but it may not be necessary on a well maintained machine. 


Finally, detach the hose from the overflow bottle on the left and let that drain. It just pulls off easily. 


In all, I only got about 1.75 litres (nearly 1/2 gallon) of fluid out. The bike takes twice that to fill so clearly a maintenance fail by PO. Coolant looked red and dirty. 


Now the technical bit. I replaced all bolts/hoses and refilled the system slowly with equal mix of distilled water and white vinegar. Vinegar helps remove hard water deposits and scale from aluminium and is used to clean coffee machines.


The engine was set to idle and vinegar mix circulated for a while to thoroughly clean the system internally. 


I ensured the thermostat kicked in and juice was fully circulated by waiting until the cooling fan kicked in. Another way to check is by massaging the cooling system hoses and feeling when they warm up. Or you can look down the filler neck and see the fluid flow. 


After it cools you drain the system again as described. Strong vinegar smell and coolant was full of crud that the acid had cleared out. Really happy with results. 


Now I had to neutralise the acid from the vinegar (don't want that corroding the aluminium) so I mixed up some Soda Crystals in distilled water and filled her up again. Soda is a base and will neutralise the vinegar's acid. Luckily soda is a buffered base so you can't add too much - it never overdoes it. I poured it through an old sock to filter out any undesolved soda crystals. 


Ran the soda water to full temp as before, let cool and drain. Weaker vinegar smell came out and the coolant looked like it had collected a bit more crud so I was very pleased. 


Next step is to rinse the soda residues out so filled the system up with 100% distilled water and ran it for some time. Let cool and drain as before. This time I poured a few jugs of distilled water through the system with the drain bolts out, just to flush straight through. 


Fitted new bonded washers to the drain bolts because someone had been a bit heavy-handed tightening them and I could feel the threads were damaged. The bonded washers seal easier than copper washers (don't need as much tightening) which gives the threads a break. 


Steel bolts into alloy is always a recipe for disaster so I do the same with my sump plugs. Really saves the threads. 


Ran a die tap over the bolts to straighten the threads and applied a light smear of copper ease. Good as new. 


Remember to burp you cooling  system by blipping the throttle hard a couple times and massaging the hoses. Cranking the bike over sideways also helps get air bubbles out. Pouring the coolant in slowly (no glugging) is also helpful. 


Now the the system is perfectly cleaned and I am confident in it once more. 


Happy days! 













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Sorry, the last pic showing rusty forks shouldn't be there but I don't know how to delete it. Please just ignore that one. 

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Wow!  That's some filthy water man-amazing how much crud came out.  Thanks much for the info-mine needs the same treatment.

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The second pic of drained coolant shows the soda mix after it had settlled overnight. Crystals are at the bottom making it look white. Think I added too much...

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I appreciate the amount of effort you're putting into cleaning out the inside of the bike's cooling system.  But the proverbial "last mile" in heat rejection occurs where the aluminum of the radiators meets the free-flowing air around it.  When I recently purchased my "new to me" '99 VFR800 I found that the radiators were covered in a layer of road goo and grime (especially on the inside, I guess the front tire throws up a lot of stuff) and there was a lot of tiny rocks and road debris stuck in between the individual fins of the radiators.  To get them properly clean I ended up having to take the radiators off of the bike.

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That's some great insight, GregDenver. 


I was planning on giving the bike a proper scrub while it's apart for maintenance. Don't think it has seen the wet side of a rag for over a decade! 


Might take your advice and dismount the radiators for a proper clean, as you suggest. 



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