My 03 650 Dakar seems to overheat.

The issues are:

The engine runs for a short period, maybe 5 minutes, before the water is being pushed up into the expansion tank and then squirts out of the top.

In this time the fan does not come on neither does the overheating light. Once it gets seriously hot i turn the engine off.

Now i know what you are all thinking, check the usual things. Have done all that. I have:

1. checked coolant mix
2. checked fan with 12v supply - it works
3. changed fan temp switch
4. removed thermostat - it justs seams the take longer to do exactly the same thing.
5. flushed through whole system with a hose
6. checked the water pump is spinning - that seems fine.

I have discovered that when the engine is running, if i unplug the fan temp switch then the fan comes on. It will do this when the engine is cold. Is this normal? Will someone check this with a normally functioning bike. Maybe BMW have supplied a part that is difficent!

I have been told by a BMW mechanic that the thermostats never break? Not sure if that is true or not. Also does anyone know much about the radiator cap? Is this likely to be the problem.

I rebuilt the bike over last winter, not really any major internals though, but have toured around France on it, during that time it only overheated twice, once on really hot day in traffic and once on my way home again.

Does anyone know how to test the fan temp switch - can you just put a multimeter on it and test it cold and hot to see if it allows current to pass?

Not sure if i should get a new Rad cap and a thermostat to make sure.

Cheers

Mat Brook