Same thing happened on my '02 Dakar. Replace the radiator cap, the springs are tired. On the F650 (and some other bikes!) it works like this: When the fluid reaches a specified temperature the cooling fan engages. If the fan fails or the temp reaches another temperature level the bike will emit a coolant overheat red warning light. If you continue to ride, the springs on the radiator cap will allow the fluid to escape into the expansion tank- when the tank is full a tiny hole on the removable black cover will spew coolant. When the springs are tired/old fluid will fill the expansion tank before the fan engages or the warning light is activated due to simple expansion of the coolant as it warms. In the Sudan the bike and I cruised through sand at temps exceeding 56c with no probs. The new radiator cap cost me 12usd. Safe roads.

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Thanks for that Mr Hook, i was suspecting as much.

I measured just under 1L in earlier after i made sure there wasn't any air, tried it again and it did exactly the same. After i sat and thought through it i came to the same cycle that you described.

Knew it couldn't be anything to do with the oil. It is foaming ever so slightly but no more than i would normally expect for quite newish oil (1000m). Plus it hadn't risen either.

Thanks one and all for your time and help. Always useful to know that there are others who have gone through it before.

Mat. Brook

Let us know what is was.

If it were me I'd start by trying a different cap as well. They may seem simple, but there are actually a brilliant piece of engineering. Most simple things are.

I'm going to assume you don't know anything about maintenance on this bike for clarity. If you know all this (which is seems you do as you've flushed it) all the better. Other people may not know this though and/or you may have forgotten something.

Than I'd drop the coolant fluid and check if the water pump is turning by turning the motor off by hand with the real wheel. You have to remove the water pump cover off course. While you're there make sure the weep hole is unblocked. BMW fills it up which is the stupidest thing.

Than flush the whole system with a hose (low pressure) and flush it out the RHS of the engine as per the picture. Newer bikes don't have that nipple but a bolt instead. Flush until you get clear fluid and heaps of it. If you're anal you can collect the clear stuff and check it for grit of other crap.

Than re-fill with a nitrate free coolant mix of 50-50. Make sure some passes past the RHS burp bolt and put it back in and tighten when it does. It's at the high end of the system, so it may take some fillings. Rinse off excess coolant off the motor and electrical components when you put the bolt back in.

As you fill the system make sure you squeeze the crap out of the hoses on both sides to push the air our the system. Pour slowly to allow air to escape. Fill it all the way to the neck and put the cap on. Than fill up the burp tank to the max level.

Go through a heat cycle (run till the fan comes on or around the block) and check and fill if required when it's cold.

If there is still an issue, there are other things at play.

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Sorry this took me so long to get back to the forum.

It was indeed the rad cap...as soon as i saw the replacement it was obvious. On the underside of the cap there is a small cup with a black rubber insert in it. This was missing and contributed to the issue.

These things are always obvious after they are solved.

Thanks one and all for any help.

Mat. Brook

You reckon it's floating in your system somewhere now?

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I had the same problem in Swaziland once.

The light came on, fluids came out of the radiator. I tried to find the problem, I suspected the radiator at first, but starting the bike and revving didnt show any leaks. Removing the radiator cap didnt seem possible.
I then rode until the light came on (every couple minutes or so). I managed to get to a BMW dealership (40KM around the corner considering being in Africa) they had trouble finding the problem. They said it was the thermostat.

The bike went back to Pretoria, and there they changed the radiator because they said it sprung a leak because of a little stonechip. The problem was slved, but what riddles me, is that I rode all the way to the BMW dealership without loosing any fluids. So Im still not sure what the problem was...

Glad you found out!