Yes - that is probably a very common issue
According to the posts on these digests, and my own experience a number of years ago with a mysterious minor gradual drop in coolant level I strongly suspect that there are many cars out there with very minor leaks around hose fittings.
The leak is just enough to let vapour out when it is hot, and then allow air to be pulled back in for a short time till it cools again.
This will gradually replace coolant with air, and also means that your system never reaches full pressure so you may also be getting boiling at the hot spots inside the engine (more air into the system and also poor cooling at those points).
ANY sign of overheating, or ANY sign of lowering of coolant level should always be investigated - a full pressure test (hot and cold) is advisable. You need to look for a very gradual loss of pressure.
Re: Yes - that is probably a very common issue
I've fought the issue for some time. It's my opinion that there is a very small crack in the head gasket---so small that it's not detectable by block testing kits. It takes about 8-12 months. Coolant is burned off, and some air develops. I elevate the nose, open the bleed screws while car is running, turn the heater thermostats to max. Then remove the bleed screw at the thermostat and add coolant through that bleed opening until full. All will be well for another 8-12 months.
Jim - Why high pressure system in BMWs?
Why does BMW use a high pressure system? Is this for better pollution control through higher engine operating temperatures? Better gas mileage? If so, is the trade off worth it? Seems like the my brother's Infiniti G35 does not have a high pressure system.
Re: Jim - Why high pressure system in BMWs?
Well there may be many reasons - but the items you mention are certainly major factors in the latest cars.
They want the engine to operate at higher temperatures for efficiency and emissions - and that means a higher pressure system.
Now BMW uses a "closed loop" system - which means that there is no connection with an external "overflow / reserve" tank where you fill the coolant - and the overflow tank is not at pressure.
Instead the BMW system has an "expansion / reserve" tank that contains a specific amount of air - trapped at a point above the coolant level and held at pressure with the system. The pressure release cap is on this tank, as opposed to on the top of the rad on the other system.
This is not new - this was on my 82 BMW although the expansion tank was more remote. Basically this method of operation means that the coolant is never exposed to the outside atmosphere, and therefore is less likely to pull in air if the reservoir tank goes empty.
The higher pressures also mean that there is less chance of localised boiling at any "hot spots" inside the engine - therefore no air introduced to the system by boiling action.
I am sure that a BMW engine guy would be able to talk to this for days - but I don't have that level of detail or time :-)