Mini Cooper, Coolant Bottle Cleaning

When working on the Mini yesterday one thing I noticed was that the coolant overflow bottle was full, despite the cooling system being empty. Clearly something wasn’t right there.

In a normal working cooling system the overflow bottle is used as a storage of coolant for when the coolant expands due to heat. As the system heats up it will push a certain amount of coolant into the overflow bottle (or in older systems, onto the ground), and a system with an overflow bottle allows that coolant to return back into the radiator as it cools down, ensuring the system is kept full and minimal coolant is lost. It’s a closed loop system and usually works really well, also allowing easy visual level checks and topping up as needed.

The R50 Mini (not the R53 Cooper S, which uses a pressurised expansion tank) uses a standard overflow bottle in its system

Being that the cooling system was empty and the tank was over full I had my suspicion about the expansion tank, it must have been blocked. There was also an indication that the bottom of the bottle has some level of sediment in it.

The bottle is only held into the car with a single 10mm nut, so its quick to remove. I also pulled the hose off the “radiator” cap neck.

The cooling system had tried to use the overflow, as there was some coolant in the hose, but nothing at all came out once that drained. The bottle was 110% blocked.

Looking at it, there is really no surprise

That dark section in the bottom of the tank is sediment. I’m not sure what it actually was, but it was a fine grit. Almost like sand.

The liquid that was in there was no good either. Not coolant, just dirty water.

That sort of thing didn’t happen overnight, it had somehow built up over a period of time. Obviously the previous owner just looked at the bottle, saw it was full and thought everything was OK.

I gave it a thorough clean out until there was no sediment left in the bottle. There is still some slime stuck on the sides of the bottle, but I’ll clean that when I flush the cooling system.

But even with the bottle cleaned out, there was no flow of water out of the hose…. Hmm.

Compressed air eventually did the job, and even then it took a bit to blow all the chunks into the bottle. The hose was blocked solid with sediment.

I refit the bottle back to the car, topped the system up again and now it all seems to be working as it should. I took the car for a good long drive and it’s proving to be a very nice little car to drive and great fun on the back roads. A belly full of 98 octane seems to have awoken a couple more torques, I guess the previous owner was running it on 91. Its far from fast, but it doesn’t feel as flat anymore.

Unfortunately it does appear there is actually a leak in the cooling system. Its weeping out of this plastic coupling for the heater hoses (not the bleed screw, it seems to be weeping from the underside of the top pipe). Other than limited space it doesn’t appear to be too hard to change, so ill get a replacement on the way and swap it when I flush the coolant.