As other people have stated, how much fluid are you leaving in the recovery tank, are you filling it all the way, or just to the indicated level?

 

OK, good deal on the block test, but did you do the pressure test as suggested?

Is your coolant reservoir just emptying dry, with the level not staying somewhere within the MIN and MAX marks with engine cold?

Are there signs of coolant drippage on heat shield below the coolant expansion tank?

Is there coolant (under radiator) pooling on top of plastic skid plate and maybe not dripping through - but blowing off when car is in motion?

How much coolant are you losing/adding incrementally - and over what period of time?

Does your engine oil look good, i.e., no "milkshake" consistency?

Is the second (lower) bleed screw in the steel coolant pipe (which runs just below and alongside the thermostat) tightened?

Running out of questions to ask...

 

Honestly man, just reseal the cooling system at this point.

• There's a seal for the water pump to the block (great time to do supercharger oil)
• a seal to the thermostat
• and the infamous heater core plastic pieces.

I've had to replace the plastic pieces before (same symptoms, loss of coolant every 100 miles or so) and it's a total f*$#ing pain unless you have remote hose clamp pliers. I did it with long needlenose and it took HOURS to just disconnect the hose clamps. I have a set of spare plastic tee connectors at my place, except for the one with the little nipple. I can ship em out to you for $10+shipping, let me know.

Also worth checking is if your radiator holds pressure. Block off one side, fashion some fitting with tape and plastic bits, and pump that puppy full of water. Make sure it's not something dumb like a hairline crack in the radiator. (JB weld will totally fix that, by the way).

 

Again, just want to thank everyone for the feedback and suggestions. To answer some of the questions: I add just enough coolant so that it is a hair below the max line. The coolant level drops very indiscriminately. Sometimes I go a day of normal driving (approximately 100 miles round trip in mostly highway traffic, with some local city traffic) and the coolant doesn't budge, other times by the time of return from work and check the level, it's just around the height of the plastic "S" shaped curves at the bottom of the tank.
There is no coolant leaking from the tank, I'm not seeing coolant dripping below the radiator, and i don't see any leaking from the two relief valves. Oil looks clean, not milky.
Unless I'm getting some strange loss at the radiator, I can't see what else it can be.
I'm going to try a coolant pressure test, as was suggested, but I can't see how I'm losing that much coolant consistently even if there is a pressure issue.

Again, thanks and I'll tell you one thing, there is no better auto community than the Mini community!

 

Of course man, that's what we're here for! By the way, this were the little plastic tees I mentioned.



If you get under the car with a good flashlight, look around for white coolant deposits on any coolant/heater hose lines.

For future reference, you can always go to realoem.com when you're looking for parts inside any system on BMWs. That's where this screenshot is from. Good luck and keep us updated!

 

Maybe that's part of the problem; checking the level with the engine still hot from appx. 50-miles of driving home from work? It's a pressurized system where the level will fluctuate some depending on the temperature of the engine. If you're adding coolant based on a reading taken with the engine hot, and the level not "settled," then you don't really have an idea how much to add, or whether you're possibly adding too much... If you're over-adding, the system will find the path of least resistance to purge itself of the coolant.

Create a uniform environment to check your coolant level, the best time being first thing in the morning, cold, before starting the engine. Take note of its level, make adjustments, then check again on subsequent mornings and see where you stand.

In the meantime, try the pressure test to see if the system holds 18psi, and to check whether there's coolant spitting out somewhere. I imagine if it's quiet, you might even hear pressure escaping if it is...

 

I am warming up to the idea you have a small leak that occur when the coolant pressure is high enough, which happens when it get sufficiently hot. You recently had the cylinder head replaced and poor workmanship can left something nasty behind.

Let's start with one basic simple thing. Check to see if any of the factory spring clamps for the hoses had been replaced with wormscrew clamp. Most generic wormscrew clamps create uneven clamping pressure on the hose. This leave a small sector prone to leak under pressure and temperature change. If you have tired old hoses that aggravate the likelihood.

I have seen bad mechanics putting on screw clamp too large on small diameter hoses, like the one for the plastic T mention above. The hose got distorted to crap. You should pay attention for the trace of coolant residue when you look.

 

I only check the coolant level after driving back home to see if it's gone down. I just shine a flashlight into the tank and I can see that's it's very low. I then check it again in the morning after the car has cooled off, and it's still very low. That's when I add coolant.
I will check all the clamps to see if there's any fluid leaking from those locations.
Again, the car doesn't overheat, the fan will come on when necessary, yet the coolant level is "disappearing". And I will perform a pressure test as well.
Keep the ideas coming, much appreciated!!

 

I don't want to alarm you, but if you are having to put coolant into it often, you may have a head gasket, or head issue.
If you do a pressure check and you have a pencil camera , remove the spark plugs and see if any cylinder is leaking into the cylinders. It is better to know now, than later!

 

I'm not saying that any of the above suggestions don't have merit but I can tell you from personal experience that, if I fill the recovery tank any higher than at the top of the S-shaped partitions in the recovery tank, the coolant will disappear. To the OP, assuming that your system passes the pressure test, please only check your coolant first thing in the a.m. and see if it, like mine, does not go any lower than the S-partitions.

 

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So happy to hear your found and fixed the problem! Now drive it like you stole it!
Congrats!

 

Been there; done that...
https://goo.gl/8Lm4K2

Glad you're all set.