cooling fan coming on more often on 85 KRT100

[pinhead]

I have a 1985 BMW KRT 100 for 10 years.  It has 85000 miles and has run fine.  I have noticed the past 2 years that the cooling fan is coming on more often when I am idling.  Originally years ago, it would only come on if the temperature was over 90 F and idling.  Now it comes on almost anytime I idle for even a short period of time even in cooler 60 F temperature. I have power cleaned the radiator external twice with no improvement.   I was wondering what is happening to make this change.  Flush and clean cooling system?  Other?

[blackie1]

failing temp sensor?

[koapono]

the obvious is often overlooked: is your radiator and coolant tank full?
if it's topped off then temp sensor or thermostat.
Hopefully, not the pump!
it's a pretty simple system.

g'luck

[frankenduck]

Another component in the system is the temperature relay which is what turns the fan on.

[The Mighty Gryphon]

Make sure your thermostat is working properly.  There aren't many who know that the thermostat not only controls the coolant flow through the radiator, but also the flow through the radiator bypass hose that helps the warm up of the engine. 

If the thermostat doesn't open completely it won't block the radiator bypass which will drastically reduce the amount of cooling available.  Working on small boats, I have seen this problem occur frequently with overheating marine engines.

[frankenduck]

If you plan to mess with the thermostat to remove or inspect then be very careful with the gasket around it. It is fragile and not available as separate part. You need to buy an entire new thermostat to replace it.



https://shop.maxbmw.com/fiche/PartsSearch.aspx?&searchtype=undefined&parts=17111460716%0A%0A

[pinhead]

Wow thanks you guys for all this information.  Great resource here.   And $76 for a thermostat!!   I used to replace them in cars and the cost about $5!  I am going to do winter service and check all this out.  I will let you know what I find out. 

[frankenduck]

And $76 for a thermostat!!   I used to replace them in cars and the cost about $5!

BMW = Bring More Wampum

I just replaced one in a car - $18.

[Past-my-Prime]

Somebody, somewhere here, posted that replacement thermostat of same dimensions is available for a Ford Fiesta or something similar? I can't find the reference.

I took mine and put it into water, heated it up and watched when it opened. I can't recall the exact opening temperature, but it was in spec.

It's not a big job.

The water pump is a medium-sized job, but if you need to do it, I have two recommendations: buy two kits. And get the gasket/bearing tool.

[frankenduck]

[rbm]

Somebody, somewhere here, posted that replacement thermostat of same dimensions is available for a Ford Fiesta or something similar? I can't find the reference.

https://www.motobrick.com/index.php?topic=4903.msg30736#msg30736

The thermostat's rubber gasket is too large for the K75 so a BMW replacement needs to be procured.  See the discussion in this thread:
https://www.k100-forum.com/t7589-thermostat-gasket-not-o-ring