It was a weird car day

I'm waiting to turn into work and I have a sudden plum of steam coming from my passenger front of my Scirocco.

I pull in and a few quick checks find the plastic thermostat housing decided to explode. I ordered up a new one and, hey why not, a thermostat while I'm down there. Lunch break gets the car fixed and mobile again.

Exorcised a few persistent air bubbles and it seemed to be running ok. Then I'm driving along, engine is nice and warm, then BAM it starts running colder.

Pull back into work (it was a very long day) let the car sit, goes cold. I decide to check the coolant level. Pop the cap off and coolant flows out as if under pressure. It over flows a bit, top it back off to the full mark, try and tripple check for more bubbles. Looks good.

Driving home it will not get to running temp, not cold as with a stuck t-stat, but not warm. It only starts running warm (a bit more than half way, but thats normal on these)

What the hell is going on here?

Dumb question, but is there a heater control valve? It sounds like there is still air in the system.

You may have bought a thermostat rated at a lower than stock temperature. Sometimes parts stores will carry a few different thermostats for each car: stock, a bit colder than stock, and maybe a bit warmer as well.

I wish I had your problem

Getting all of the air out of a system can be tricky. There is usually a procedure and it has to be followed. Are there air release valves in the system?

Sometimes jacking the front up as high as you can helps move air out of the system

Squeezing the hoses can also force bubbles out of their pockets

The reverse flow systems with the T-stat on the bottom are notoriously difficult to bleed.

No air release valves.

Book procedure says fill, run up to temp, allow to cool down, check and top off.

I've been pumping the hell out of the hoses, still seems like I have air in the upper hose. And the odd angle the hoses don't really allow to pull hose, top off rad and hose and reconnect.

Runs warm while sitting still, any decent airflow over the radiator and the temp drops. I think I've got a jenky T-stat out of the box.