93 K75S - Expansion Tank Coolant Mess

[GasStation]

The expansion tank on my 93 K75S must be of volcanic heritage as it spews coolant out onto the surrounding righthand ABS modulator and bag. The coolant level in the expansion tank is halfway between the full and low marks. I suspect the coolant it is being regurgitated out of the expansion tank rubber cover. Why? I really don't know. 

I have read that the radiator cap has two valves, one of which relieves coolant pressure from the radiator to the expansion tank at a certain value. Could the culprit be a defective radiator cap/valve letting too much pressure build up in the radiator then overpressurizing my expansion tank causing coolant to spill and smear the surrounding area? Or is it simply the rubber cover with pin-sized hole in the nipple protrusion that needs replacement?

[Inge K.]

Could be that your cooling system don`t function properly.
Check fan function, and coolant level...at filler neck.

Inge K.

[GasStation]

Could be that your cooling system don`t function properly.
Check fan function, and coolant level...at filler neck.

Inge K.

The fan turns on as advertised and when required, the coolant level was recently changed and replenished during oil/water pump seal replacement IAW BMW specs.

The coolant stains are clearly downwind and seem to emanate from the expansion tank rubber cover. I just don't know if it is because of too much pressure in the expansion tank downstream of the radiator, faulty rubber cover or just the peculiarity of this K75S, ie. keep the coolant level very low in the tank. The previous owner told me that he had noted the overflow and attributed the consequence as a 93 K75S oddity.

Does anyone else have this experience on his/her K75S/C/RT?
 
Here is what it looks like:

[DRxBMW]

Could be that your cooling system don`t function properly.
Check fan function, and coolant level...at filler neck.

Inge K.

Does anyone else have this experience on his/her K75S/C/RT?
 

YES, reservoir level at the minimum mark should cure the issue.

Happened on my 94 K 75 and Gene's 93 K75.

We chased new rubber caps, rad caps, sanding the reservoir plastic lip for a smoother seal to NO avail.

The bike will run fine at the minimum level.

[GasStation]

Could be that your cooling system don`t function properly.
Check fan function, and coolant level...at filler neck.

Inge K.

Does anyone else have this experience on his/her K75S/C/RT?
 

YES, reservoir level at the minimum mark should cure the issue.

Happened on my 94 K 75 and Gene's 93 K75.

We chased new rubber caps, rad caps, sanding the reservoir plastic lip for a smoother seal to NO avail.

The bike will run fine at the minimum level.

I will lower my expansion tank coolant level in search of the elusive "sweet spot".

I do not mind replacing parts like covers and radiator caps. However, if a K75 odditiy, then what's the point?

Thanks again for sharing your experience.

[TimTyler]

I was once told that:

Coolant at the MIN line was fine on a cold engine. If the engine heats up to a certain point, excess engine coolant will be routed to the overflow tank, and when the engine cools, the coolant will be drawn from the overflow tank back into the engine.