Cooling system mystery - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 18 (permalink) Old 08-08-2015, 07:45 AM Thread Starter
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Cooling system mystery

So, as I mentioned in the "Lots to sort out" thread, the bike has generally been behaving well. One exception has cropped up, however, and that is with the cooling system.

The bike seems to run at normal (for this bike) temps. During long idles, the fans turn on at 220 and, temp immediately begins to come down and fans shut off about 208. While cruising, the temp is anywhere from 180 - 205. My problem is with coolant recovery. The overflow tank fills itself up, but does not draw the overflow back in to the radiator and ultimately pisses some coolant overboard. In fact, The system seems to stay pressurized LONG after the engine has cooled down (I'm talking after DAYS of not running). Literally a week after the big ride to Tellico, I popped the radiator cap and it blew off positive pressure! Meanwhile, the overflow tank is completely full and levels in the radiators and definitely down.

My first instinct was that the cap was either malfunctioning or perhaps the wrong one, but if the system doesn't generate a vacuum as the engine cools, then of course there's nothing to draw the coolant back in to the system, regardless of the cap. Anyone have any ideas on what might cause this? Maybe I should try a new cap anyway? Kinda stumped...
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post #2 of 18 (permalink) Old 08-08-2015, 12:52 PM
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I'd say first try a new radiator cap.
It's supposed to open when expansion or excessive pressure occurs and vent to the recovery tank, then allow the system to pull from the recovery tank when the system cools off.
Sounds like it's not doing it's job and acting like a one-way check valve.
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post #3 of 18 (permalink) Old 08-08-2015, 09:30 PM Thread Starter
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Definitely worth a try.
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post #4 of 18 (permalink) Old 08-09-2015, 08:38 PM
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One item worth mentioning is to remove the recovery bottle and clean it out.

I usually pull the bottle and fill it with hot water and a few drops of Dawn dish washing detergent. Let that sit over night and then take a soft bottle brush and scrub out the inside.

Make sure to clean the nipple where the return hose attaches to the bottle, it has been discovered these have become clogged over time. Replace that line attached there if it clogged, just for cheap insurance.

This all sounds like a PIA but it is worth doing.



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post #5 of 18 (permalink) Old 08-10-2015, 01:01 PM
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I second the cap

1986 Honda Interceptor...learned on it
1988 Yamaha Radian...wheelied on it
1990 Yamaha FZR...friend totaled it
1997 Honda Shadow...wife stole it
1998 Yamaha YZF...noob tree'd it
2002 Honda 954RR...just sold it
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...if I stop runnin' the evil little clowns will get me!
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post #6 of 18 (permalink) Old 08-17-2015, 05:51 PM Thread Starter
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New cap installed, took it for a short ride and allowed to heat up to fan temp twice so now we'll see if it pulls the excess back in after a cool down. I'll post what I find tomorrow.
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post #7 of 18 (permalink) Old 08-17-2015, 08:52 PM
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Quote:
Originally Posted by Half a V4 View Post
New cap installed, took it for a short ride and allowed to heat up to fan temp twice so now we'll see if it pulls the excess back in after a cool down. I'll post what I find tomorrow.
Good time to place our bets then?




1986 Honda Interceptor...learned on it
1988 Yamaha Radian...wheelied on it
1990 Yamaha FZR...friend totaled it
1997 Honda Shadow...wife stole it
1998 Yamaha YZF...noob tree'd it
2002 Honda 954RR...just sold it
2005 RC51...finally found it

...if I stop runnin' the evil little clowns will get me!
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post #8 of 18 (permalink) Old 08-17-2015, 09:23 PM Thread Starter
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post #9 of 18 (permalink) Old 08-18-2015, 08:01 AM Thread Starter
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OK so the coolant level was down this morning by perhaps 3/4 cup (in the radiator). Overflow tank slightly higher than yesterday but nowhere near full. There seemed to be a bit of positive pressure on the radiator when I removed the cap this morning.

I replenished the coolant and will see if it keeps filling the overflow with subsequent running. I don't understand how the system is not generating a vacuum as it cools down though. It has been off since about 6PM yesterday evening.
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post #10 of 18 (permalink) Old 08-23-2015, 06:29 PM Thread Starter
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Well I just returned for a great trip to western NC on the RC this weekend. Bike behaved well and my riding confidence on it continues to improve.

Unfortunately, the cooling system is still blowing coolant into the overflow tank and not recovering it in spite of a new cap. Yesterday, after a cool down during lunch, I looked in the radiator and it was down over 2 inches from the top. I replenished the coolant and of course, the bike did fine on the remainder of yesterday and on the ride home today. The temperature seems to vary quite a bit with riding conditions, but any time it gets to about 220, the fans kick on and it immediately begins decreasing. This is pretty much the same behavior I've observed on the VFR with a very similar cooling system.

The bike now needs a front tire and I'm considering taking it to a local shop to have a cylinder leakage test performed along with the tire. I have no problems with how it runs and I have not observed or smelled any coolant/steam from the exhaust but I'm beginning to run out of ideas here. If it turns out to need a head gasket, I may just shoot it! (JKJK!)

I suppose a thermostat could be tried as well, but since the temperature seems to behave appropriately, I kind of think changing it would be pointless. I'm open to any other ideas or suggestions.

Thanks in advance guys.
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