Hello, I have a normal idle (1000) until engine reach high temp in city stop and go traffic. Before fan start idle go up between 1500 - 2000. Any advice where to look? When fan out everything look usual. Bike is 1985 K 100. Tnx

Check the heat from the fan directing very hot air over the back of the radiator and throttle assemblies is not causing the throttle cable to not return properly. Check the clearance between throttle assembly and the clips on the intake rubbers. I had that happen on my K100LT.

That's an easy check before you look for other issues.

nino wrote: Is it possible that working fan draw to much current . . .

If that is happening, be prepared for an electrical fire.

Run a simple test.....pull the fan relay and see if you get an immediate drop in tick over.

The same temperature sensor that is controlling the fan is also sending temperature information to the ECU.  When the temperature reaches 103°C, the fan relay engages and the fan turns on.  The ECU is also sensing the same temperature.  It shouldn't want to raise the idle speed but maybe some other component of the engine is feeding incorrect information to the ECU such that it is causing the injectors to run lean.  It could be the mass airflow sensor, or the Throttle position switch.

Do what Olaf suggests to eliminate the fan circuit.  Then if the idle increases as temperature increases, your problem would be mechanical and you should check the other components I mentioned.

Is this a fast idle after the fan starts, or the fan starts after a fast idle? Because it would be normal for the fan to start after a fast idle in city traffic. 

If so, an air leak could cause a fast idle when the engine gets warm, so that would be another place to look.

Could it be aftermarket no name temperature sensor to send rich signal when temperature aproach 100 C? Anybody with experience on non original sensor? Thanks all

nino wrote:Could it be aftermarket no name temperature sensor to send rich signal when temperature aproach 100 C? Anybody with experience on non original sensor? Thanks all

Ah, that is likely to be your problem. Did the issue only happen after changing the sensor?

Olaf, I dont remember if it happen after sensor change two years ago.
But I noticed that battery is weak, 12,01 Volts. One cell was empty. It doesnt receive current on idle, after idle run its impossible to start again. Could it be the cause of erratic idle? (yes Iam stupid)

nino wrote:Could it be aftermarket no name temperature sensor to send rich signal when temperature aproach 100 C? Anybody with experience on non original sensor? Thanks all

Remove IT bin IT fit GENUINE BMW UNIT.Dont fit POOR quality parts, False economy my friend  

nino wrote: One cell was empty. It doesnt receive current on idle, after idle run its impossible to start again. Could it be the cause of erratic idle? 

After you install a new battery—which you should certainly have on your immediate schedule because what you have now is junk—you'll find out.

Finally it was dirty starter motor with almost dead brushes. It tortured battery (charging) which is 4 years old but still usable. Anyhow I ll change it tommorow. 
Thanks all

Thanks for the update.