Unfortunately I could not see a crack in the old tank and upon refilling I found out what the real culprit was. When I started the engine with the expansion cap off I noticed that there were a lot of bubbles in the coolant, it was almost frothy. Furthermore, it didn't seem to matter how many times I ran the engine and bled the system there was always more air coming out.
I went for a drive and came back to find that there was coolant all down the outside of the expansion tank, just as there had been with the old tank.
I found that if I revved the car past 3000rpm at idle coolant would exit the tank via the expansion cap. The expansion cap of course is also a one-way valve so that coolant can exit if the pressure in the cooling system gets too high.
Having seen almost identical symptoms on a VW Polo, I decided the most logical conclusion was the head gasket. Everyone thinks about oil in the water or vice versa when it comes to head gasket failure. However, it is also possible for a gasket to blow between the cylinder and the cooling jacket. This lets exhaust gases into the cooling system, which subsequently pressurises it and forces coolant out of the expansion cap. It is also possible for it to go the other way and for coolant to be burnt in the cylinders, but this was not my problem.
Bizarrely, I wasn't experiencing any overheating and it seemed to leak more when the engine was started and stopped so I actually made it to my brothers in the South of France. This was a slightly nerve racking experience as the coolant light was permanently on. I simply stopped every couple of hours, let some air out, put some coolant in and then carried on.
I covered 1500 miles and used ten litres of coolant on that journey.
I didn't have an in-car heater for most of it either, so it wasn't the most comfortable of journeys!
I have always fancied a 6 cylinder engine and with a newborn around I didn't want to be faffing around with a head gasket change, so I decided to use this as an excuse. I sold my trusty 318i touring for a e46 330i touring.
So it was a fond-farewell to our trusty 318i SE ...
And I warm welcome to our new 330i M Sport: