2001 B5 1.8. Noticed that there is some sort of bubbling occurring at the top hose in the coolant expansion tank. Just had the coolant flange replaced (thinking of going aluminum next time if it fails again). Thought that might fixed it but it didn't. Any idea as to what the reason is?

 

Could be trapped air escaping from the flange swap.

 

keep an eye on temps and drive it around a while, ideally up and down some steep hills.

likely its just trapped air escaping and you need to burp it a bit.

 

The bubbling was occurring before the swap. Did a timing belt water pump change and thought that might be it. Gonna try to bleed through the hard pipe and heater core. That small hose is the vent hose?

I was thinking that. When I drive uphill I notice the temp moves up between middle and the first tick. Never higher. Some people told me that it's normal for the B5. The gauge wasn't exactly accurate just as long as the temp stays at 90C on the VCDS which it does. I'm not losing coolant except right before I changed the flange. I did notice one time I thought the level was low but after a few days off driving, level went back up to Max. I'm gonna bleed it today and see if it helps.

 

Someone in another thread said that it's the vent hose (the small hose on the top of the expansion tank) and that it's normal for the most part. I've never seen any car that bubbles like that. My 90 Integra didn't do that. My dad's 03 B6 never did that (and the temp gauge never moved as much going uphill or hard driving). I don't think it's normal and I don't want something to happen if I decide to do a long drive. The longest I've driven was 40 minutes. Car ran fine, temp gauge moved slightly above half but not before the first line.

 

It was still bubbling after the drive?

I recently had to drain/refill my coolant and it bubbled for quite a while during the filling/burping and for a bit after a quick drive around, but once I got it mostly burped, I took it for a drive through traffic (and up and down some hills to try and get any bubbles out) and it finally stopped.

 

I didn't drive the car for almost 2 years. Finally had the time and money to get the timing belt, water pump, thermostat, all done (head needed to be remachined since I had bent valves from missing timing belt teeth). After that, I noticed the temp gauge would fluctuate when I drove uphill or had my AC on. Never past the first line though​. Ran the VCDS, engine temp read 90C which is operating temp. Gauge went up one day i headed to work. Found out the coolant flange went out from a cheap aftermarket one of the previous owners changed. Got one from Audi this time and thought that might've fixed the bubbling in the tank but it didn't. No coolant loss yet since I just changed it but still bubbling from the small hose up top.

 

Bled the system but I'm still seeing bubbling in the expansion tank..

 

The expansion tank is sealed and pressurized, so the only way it could be bubbling is if it's transferring air in place of coolant.....unless you have some sort of head gasket type issue (even though I've never heard of that symptom).

Are you losing coolant? Did you do a pressure test of your cooling system? Those will tell you more than anyone on here can.

G/L

 

A shop suspected a cracked head or head gasket wasn't sealed properly. Going to do a compression test as well as a block test to see if there's any CO2 getting into the system