Joji-
First off, the coolant comes rushing back up when you remove the cap b/c you are releasing the pressure in the system. When the pressure is released, you are also decreasing the boiling point of the fluid in the system. Not a good idea to do.... especially quickly. Anyhow, sounds like you definitely have air trapped in the system. You might also have an issue w/ t-stat or water pump.
You can try either the short cut bleed method or the long one. Either way, w/o proper coolant or coolant flow, you're running the risk of creating hot spots in your motor - definitely not good!
Short cut method of bleeding the system of air:
Not 100% guaranteed effective but...
Open the bleeder valve (and leave open). Drive around w/ some random accelerations, upto 5K RPM, for about 15min. Close valve and test - make sure both radiator fans are working when at operating temp.
Prescribed method of bleeding the system of air: (from Bentley)
- For tiptronic, remove fuse B1 in fuse panel to disable ATF cooler shut-off valve)
- remove filler neck trim cover
- open up bleeder valve locking clip
- fill w/ coolant to bottom edge of filler neck
- run engine at idle. Top off w/ coolant until no more can be added when engine idle is raised sharply
- replace coolant cap and tighten. Warm up engine for about 10 min at approx 2500 rpm until coolant thermostat opens. To be sure engine is fully warmed up, check to see that radiator cooling fans in front wheel wells are on.
- allow engine to run addl 5 minutes at 2500 rpm. Every 30 seconds briefly rev engine to 5000 rpm
- open reservoir cap, relieving coolant pressure w/ extreme care. Top off coolant, close cap and repeat process of intermittently revving to 5000K rpm for 5 minutes
- allow engine to idle until radiator cooling fans cycle on and off once. Shut off engine and open reservoir cap, relieving coolant pressure w/ care
- top off coolant until level reaches max
- flip down bleeder valve locking clip
-replace filler neck trim cover and cap.
- If tiptronic, replace fuse B1
The cooling system is pressurized to 19psi and has a total capacity of 18 liters (~ 4.5 gal) so don't be afraid of adding just distilled water (entire system ratio s/b 50/50 water:approved coolant).
Good luck!
