Overheating to the extent that coolant trickles from the reservoir overflow
 

Hi folks,

Thought I'd post a quickie question before I have to leave my CBR1000FM for 2 weeks on a work trip. I've seen for a while that the temperature gauge has been getting pretty high (7/8ths) but that's not new and in ambient heat of 30degs I've figured it pretty "normal". However, today even on the motorway at a steady 70mph in 22degs ambient it was close to redlining on the temperature gauge all the time and when I stopped there was coolant trickling out of the overflow tube coming from the expansion tank on the RHS of the bike. The expansion tank was completely full to the top (hence the overflowing). When I stopped the engine it cooled down normally and the coolant flowed back out of the plastic expansion tank into the main system as the level in the expansion tank dropped to normal and I heard it gurgling. I'm assuming it's either a the thermostat that needs replacing or maybe the water pump, but without having the time to do any further diagnosis before having to leave I was wondering which it is most likely to be given the symptoms I've described. I'd like to get a "headsup" on it as I'll have little time when I return to the bike to do anything before I have to leave again. If it's 99% certain it's one and not the other, I may go ahead and order a replacement before being able to test either. I'm thinking that as the coolant flowed back into the main system on cooling it's more likely to be the thermostat than the water pump..... By the way, I drained the cooling system recently and replaced with fresh iPone radiator coolant. Any views???

I'd start by testing or replacing the cap. It has to be able to hold system pressure or it will push out the coolant letting it get low or lower the boiling point of the coolant.

74demon thanks for the response. I've just read in the Haynes manual that the coolant only returns into the main system once sufficiently cool so a defective cap could explain completely the symptoms. I'd assumed that any coolant finding its way into the expansion tank would automatically return to the main system even when hot. I was wrong! Do you know of a way to check whether the cap is working properly? Haynes says get it tested by Honda using a special equipment, but if that's the case it's probably cheaper just to buy a new one anyway.

Here's a LINK to the cap info.
This LINK for the thermostat.

Had the same problem on mine. Did the cap and thermostat. Easy fix

I'm told you can test a thermostat using a pot of hot water on a stove. As for the cap, I could see why that would need special equipment. Never bothered cause the parts are so cheap

Any auto repair shop should be able to test it for you in seconds. They are pretty cheap, so replacement is just as easy as getting it tested. They are not motorcycle specific, so take the old one to your local auto parts store and match one up.

Thanks for all the pointers, links etc. The only thing I disagree with is the "cheapness". Over here in Europe a radiator cap (albeit Honda) is €38 that's around $44 for a bloody radiator cap..... I'll be checking out the links from Timbuctwo to see if I can find something more realistically priced. I'm "ready" for finding I need a new thermostat too, but it'll be the easiest fix that I do first.

Don't go to Honda....

Ordered a Stant radiator cap from US for $9 +$3 delivery ready for fitting when I get back home. When's the USA going to join the European Union? Lol. I'll look at the thermostat if I need to, but let you know in any case whether the simple radcap fix really turns out to be a fix. Thanks in the meantime.

On one of my first rides in hot weather with my CBR the temp went to the red line. I could hear the fan going, but it did not drop the temp. Got home safe, but it was nerve wracking with the gage in the red. I started with the basics by checking if the system was full, and then draining it. I was shocked at what came out. The coolant / water mixture was a pale chalky color...never saw anything like that before. It also had a very odd odor. So I flushed the system with 50/50 distilled water and vinegar...3 times...until the mixture came out clear. RInsed it out completely with distilled water, and refilled with proper mix of distilled water and coolant. That completely fixed my overheating problem. I would not have thought that this would make such a dramatic impact on cooling, but it did. Now my gage rarely gets up to 3/4 on extremely hot days in stopped traffic. It's usually at the half way mark all the time. Try that.