Driving to work the past two days I encountered the same problem in the exact same spot with my coolant system. It takes me about 3 miles to enter the freeway and my coolant temp is typically around 1/4 and won't reach 1/2 until I get up to speed. These past two days my coolant temp has gone past the 1/2 mark about two miles into my freeway ride until I was given the coolant warning in red with my temp going over 3/4. I immediately took my foot off the gas at which point the coolant went to the 1/2 mark and stayed there for the remainder of my 12 mile ride on both days. Both days after work it worked just fine with no problem it was only my commute in the morning where the temp was in the 50s this morning and the 30s the morning before. Another interesting part was that once my coolant temp settled at 1/2 I had instant heat where there was none before that…this may not be a real issue as I typically don't get much heat until I'm about 8-10 miles into my ride. Anyone seen this issue before or know where I should start looking? I looked at my coolant reservoir and it was a bit low and I added some coolant today but not sure if that would cause the problem. Also when I got home and took the reservoir cap off it released a bunch of air, then it sounded like it was "bubbling" into sections of the engine or lines. Not much of a car guy, I fix my own problems but don't really understand the mechanics of some systems so I'm perplexed on what could be causing this.

Sorry forgot to add that this is a 2004 audi A4 1.8L…B6

 

I had the same problem with my 2003 A4 1.8. Immediate problem was air in the system. That usually results from the coolant level getting low. The coolant system is pretty testy on the 1.8 so keep an eye on your coolant level, more than just waiting for the low coolant warning light.

Top off the coolant with the proper G12 pink coolant. Then bleed it properly. There should be a bleeder valve on the solid coolant line that goes across the fuel rail. It is a 7mm or 8mm allen head flange screw. Best to break this loose before you get the engine up to temperature. Get the engine to temperature, heat on high and slowly open that valve to bleed out air. Do this multiple times.

Second I would look for a coolant leak. First spot is the coolant flange. Passenger side firewall of the engine. The o-ring that mates with the block is a highly known leak location. You won't see a puddle or anything as the leak from there drops directly on the exhaust so at most you would see residue looking straight down from that corner of engine. Replacement flange will run you about $35, but the labor is tedious. Very doable yourself with patience and a few tools as long as you are ok with a couple dinged up knuckles by the time you are done. While you are there, may as well replace that coolant temp sensor. Other parts you could use for this job are hose clamps to put everything back together when you cut the OEM clamps that Audi uses.

Good luck. Plenty of threads on the Coolant Flange and a few good DIY writeups with pictures etc. Simple search will get you there.

 

I agree with checking or replacing the coolant temp sensor, since that may be causing the gauge to read incorrectly, and perhaps keep the blower from running. The symptoms of "air" releasing when the reservoir cap is removed, and the bubbling, is quite normal. When some parts of the engine are hot enough to boil the coolant at atmospheric pressure (but won't when the pressure cap is on) suddenly relieving pressure will result is localized boiling, which causes the effect you see. By the way MKE, there is no need to have the coolant hot, or the heater turned on in the car. There is no heater core valve, so running the heater is a waste of time. But more importantly, you don't run the engine at all while bleeding the air out. Bleed the air with the coolant cold, so you don't get scalded, and engine off. If you have the coolant pump spinning, the low pressure in the heater return line will always drop the coolant level so it can't reach the bleed hole in the hose. You will actually be pulling in air, rather than pushing it out.