Temperature Gauge Reading & Coolant Bleeding

[GearHead]

Hi folks

I have a question on Temperature Gauge Reading and Coolant Bleeding

Since the beginning of the year, the Temperature Gauge on my 03 X5 3.0 started to behave strange. Fortunately it did not read HOT but as the engine warms up, I see the needle on the gauge gradually creep up. It sometimes stays dead center but in most cases, it drops down gradually and fluctuates.

While searching around for answer, the possible causes I have found are
1) possible trapped air and needing possible coolant bleeding
2) possible stuck Thermostat

Before going down the path of replacing the Thermostat, I tried bleeding

Base upon the various instructions, this was what I did
a) Remove the reservoir cap from the expansion bottle,
b) Remove the bleeder screw (next to the expansion bottle
c) Turn the key to position II (did not start engine)

Before I could do (d), coolant was gushing out from the area where the bleeder screw was. It was so bad that it appeared like a fountain with coolant gushing out

BTW, (d) was to be turning the interior temperature gauge to high temperature to open up the valve to the heater core

I am very stumped.

I will like to validate whether trapped air within the coolant before going to the dealer to replace the thermostat

Any ideas?

Many thanks

Jeffrey