Bike overheats at high speed only

Zanie · Dec 24, 2014

I am placing this query in the general technical section; throwing the net wide to see whether any other bike has had these odd symptoms.

Symptoms

Here is an example case of the symptoms: I would be driving along a national highway at 120 km/h (yes, I do follow speed limits). After roughly 20 min, the overheating light comes on. I stop on the side of the road and wait roughly 3 min. If I try to restart the engine before the light goes off, it doesn’t sound happy and doesn’t want to start. I hit the road again; dropping my speed to 100 km/h. 10 min later the overheating light is back on. I stop and wait. Head out again when light is off. Stay at 100 km/h. Light back on in about 5 min.

I have played around with speeds. The bike will not overheat if I keep it at 80 km/h. Anything over 85 km/h is asking for trouble. The problem doesn’t manifest on very cold days, i.e. 10°C. It has never overheated in traffic or at low speed. I am a newbie at motorbikes and off-roading. I did one 10 km stretch of sandy-ish road mainly in second gear and paddling in sections. The bike didn’t complain.

It feels really unsafe, never-mind extremely boring, driving 80 km/h on a highway. Driving only off-road will solve the issue (I definitely keep under 80 km/h there), but my nerves are shot after my last off. I was hoping to build up some confidence on good ol’ blacktop.

Bike

2001 BMW 650 GS. 37,000 km. Bought it last year. Full service history. Had its 30,000 km service Oct 2013. Replaced oil and oil filter Jun 2014.

Stuff done so far

Ok, so here is the fun part. As a non-technical person, I will try to describe what has been done to the bike already. As I said, I am a newbie. I know how to maintain/fix my bicycle, but not my motorbike, though I would love to learn.

I took my bike to Kingtek. They specialise in BMW bikes and I’ve only had good service from them. They managed to replicate the symptoms on the work-bench; running the bike at 120-130 km/h and waiting for the light to come on. They hadn’t found the problem before they closed for the Christmas holidays. I fetched my bike. I will take it back to them next year after they have done some research on more leads to follow/try out. They have run out of all the usual options.

Stuff checked so far:

Dash bulbs replaced and linked electrics ok
No weeping of any fluids anywhere
Coolant bled/replaced
Thermostat ok. Was swapped out temporarily for another one. Problem persisted.
Temperature sensor ok
Cooling fan works
Radiator core was 30% blocked. Core cleaned/repaired/resprayed. Didn’t sort out the issue.
Oil level ok
Water pump plastic gears (impellers?) ok. No visual check (no time before Christmas holidays to take more bits apart), but no “notchy” feel.

That’s all that I can recall for now.

Has anyone come across a similar problem? If anyone has any ideas/suggestions as to the potential cause of this mystery malaise, I am all ears.

FYI

I found this interesting snippet while surfing the web:

While the cooling system on the F650 classic models is fairly good, the cooling system on the Fuel Injected models is marginal (at best). Even small changes to the Fuel Injected models can result in greater stress on the cooling system. In March 2003, BMW NA released two service bulletins (17 001 03-012 and 31 001 03-013) for the GS/Dakar/CS models about voiding your warranty by installing radiator guards and fork boots/gaiters.

Source: https://faq.f650.com/FAQs/Overheating_FAQ.htm#Overheating and Gaitors

I don’t have fork boots, but I find it interesting that it could compromise the bike’s cooling system.