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Discussion Starter · #1 ·
I spent 3 days with friends up at Raquette Lake in the Adirondacks. The bike (2001 S4) performed flawlessly the entire time...until I got about 20 miles from my house. It was a really hot day and it was the end of an approx. 250 mile trip. The coolant light came on. It was about 2 pm and I was at the exit nearest one of my longtime watering holes. So, I rolled into the Hartford Road Cafe in Manchester, CT. Shut it down and went in and had a beer and sandwich. My plan was to let the bike cool down then ride it as far as possible until the light came on again.....repeat until I got it home. So, after lunch, I set out once again. The light never came on. Got home without incident. The next day I checked the coolant level with the bike on the side stand. (Not sure if that's the way you are supposed to check coolant. Upright?) When I took the cap off the coolant was up to the top. I'm guessing the issue involved a sensor but I have no idea and it doesn't seem to be happening anymore. Any advice or theories?
 

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Average Weird Guy.
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When was the last time you replaced your radiator cap Bruce?

I had a similar problem on my RC51. Anything below say mid 80's Fahrenheit ambient temperature outside and the bike would operate perfectly normal. However on days when it got really hot outside the coolant temp warning light would light up. Same as you I'd let the bike cool down and check the coolant level and the radiators were always full. Even flushed and refilled the system,same problem.

The radiator cap has a large spring inside it,that spring is there to withstand a specific amount of pressure in the cooling system before the cap internally opens to allow the hot coolant to vent to the overflow tank.
As the years go by that spring force weakens and it eventually allows the cooling system to vent too much too soon.

I tested my radiator cap and sure enough it was opening well before the rated pressure level listed right on the cap. Installed a brand new OEM radiator cap and it cured the issue.
 

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Discussion Starter · #3 ·
Thanks. I just got this bike last Fall. It had only 8K mi. and it now has about 11K mi. I think I'll just replace the cap since it's probably an OE unit. Is this a dealer only item?
 

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Average Weird Guy.
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Is this a dealer only item?
OEM is simply the safest/easiest option,that way you know you've got the correct part.

However if the radiator cap is the exact same size and shape,is the correct type cap for the cooling system that you have (because there are two types of cooling systems,sealed and unsealed,and the radiator caps are constructed distinctly different for each of these two types of cooling system.),and the cap has the exact same pressure rating listed on it as the OEM cap than an aftermarket cap can safely be used.

A thousand pardons for the bold type there Bruce but it's imperative that you adhere to the rules when replacing critical system components like this with an aftermarket part.
 

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Discussion Starter · #5 ·
OEM is simply the safest/easiest option,that way you know you've got the correct part.

However if the radiator cap is the exact same size and shape,is the correct type cap for the cooling system that you have (because there are two types of cooling systems,sealed and unsealed,and the radiator caps are constructed distinctly different for each of these two types of cooling system.),and the cap has the exact same pressure rating listed on it as the OEM cap than an aftermarket cap can safely be used.

A thousand pardons for the bold type there Bruce but it's imperative that you adhere to the rules when replacing critical system components like this with an aftermarket part.
This bike and coolant system is new to me so I really appreciate your advice. Thanks.
 

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Hi Bruce,

If and when you "dig in", I did the same with my old ST2. Replaced the cap with OEM. But being the fiddling engineer that I am, I looked carefully at the cap that came off. It has 2 check valves integrated into it. One to hold pressure (to pressurize the system) and the other to allow coolant from the tank to re-enter the radiator while cooling off.

These valves tend to get gooey over time, and just needed a gentle rinse and messaging. Your DUC loves messages, but it can't talk to tell you that...

If I could find my used one, I would mail it to you, but I'm still busy unpacking after selling my house!
 

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Discussion Starter · #7 ·
I checked that radiator cap and found something interesting. The rubber gasket was jammed up into the cap. And, kind of crunched to one side although unbroken. I pulled it out and re-seated it around the filler. I will clean up that cap and find another gasket. Thanks for all the advice.
 

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I spent 3 days with friends up at Raquette Lake in the Adirondacks. The bike (2001 S4) performed flawlessly the entire time...until I got about 20 miles from my house. It was a really hot day and it was the end of an approx. 250 mile trip. The coolant light came on. It was about 2 pm and I was at the exit nearest one of my longtime watering holes. So, I rolled into the Hartford Road Cafe in Manchester, CT. Shut it down and went in and had a beer and sandwich. My plan was to let the bike cool down then ride it as far as possible until the light came on again.....repeat until I got it home. So, after lunch, I set out once again. The light never came on. Got home without incident. The next day I checked the coolant level with the bike on the side stand. (Not sure if that's the way you are supposed to check coolant. Upright?) When I took the cap off the coolant was up to the top. I'm guessing the issue involved a sensor but I have no idea and it doesn't seem to be happening anymore. Any advice or theories?
So when you say you took the cap off do you mean the radiator or the overflow bottle. Typically you top up at the overflow bottle up to a level marked on the side about half way up. Once the bike is cool radiator should be full and bottle half full, standing upright.
 

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Discussion Starter · #9 ·
So when you say you took the cap off do you mean the radiator or the overflow bottle. Typically you top up at the overflow bottle up to a level marked on the side about half way up. Once the bike is cool radiator should be full and bottle half full, standing upright.
Thanks. The original owner of this bike has CF panels everywhere. So, I can't really see that bottle. Will try to get at it today. It was the radiator cap that I took off.
 
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