On refilling my Boxer with cooling fluid, I noticed I had an air lock preventing the rad from filling. The engine had just heated to about 90oC when I saw it, and I had the heater matrix working seemingly Ok.
What I found was that there are three hoses running into the rad; two from the engine block, and another running from the side of the water expansion bottle. This one acts as a breather for the rad and allows the air to escape into the expansion bottle when the system is filled. In my case, this breather was blocked with sediment on both the rad end and the bottle end.
When removing the hose from the bottle end, you may notice a small piece of brass inserted into the connector. This is a strengthener to prevent the part from snapping when removing hoses...
So, when refilling the system, ensure you observe bubbles rising into the expansion bottle from this hose.
The procedure in full is:
1. Drain (from the bottom rad hose) and wash system.
2. Reconnect and tighten all hose clips.
3. Add cooling fluid slowly to the system from the water bottle. (Thermostat will be closed at this stage, rad full of air more than likely.)
4. Start engine in idle, run heater matrix on hot.
5. Closely observe the following: temperature reported on the dash, leaks, fluid level, heat from vents.
6. Once the thermostat opens, the rad should expel air from the top hose into the expansion bottle.
7. Recheck levels, temperature and leaks.
Don't let the temperature rise above 90oC. This could mean that you have an air lock and a local hotspot could cause boiling of the fluid and a massive discharge when the pressure is released (burns... the rest).
Please add comments if you think any of this is sh*t...