Temp gauge inop after catastrophe
 

So, on my buddies 2004 boxter the tensioner pulley exploded and things went very south from there, they didn't realize it going down the highway..... and the engine overheated, the surge tank blew out.... literally melted and blew out, the trunk was sealed so well somehow that it blew out the back right tail light. anyway. So today, i replace everything tensioner pulley, belt, surge tank, and tail light assm. Everything went smooth got it all done in about two hours. Cranks right up, bleed it by flipping up the little hidden switch in the surge tank, heater on high, and add and rev it a bit to get things going. Then I notice the needle is just not moving. I also removed B1 due to a write up I saw, but that was the instrument cluster for me. Anyway, bleed it, no leaks, fans are kicking on and off, just no temp gauge, the little red light is blinking by the 240(means low on coolant i read) and the coolant light in the bottom left is on. From here im a bit clueless, seems to be running fine. I just don't want to risk it actually not reading a temp and over heating. Just wanting some input and thoughts.

It sounds like you have a large air pocket in the system, but....
Are you sure the water pump is turning and pumping fluid?
Are both radiators getting hot at the inlet & outlet piping?
What caused the tensioner pulley to "explode" - I've never heard of that happening before - could the water pump have seized?
On a side note, if you can change out the coolant tank, tensioner, belt, replace coolant, etc in 2 hours you are very quick - I wish I could work that fast. I hope the overheating has not seriously damaged the engine.......

I'm going to guess there's a lot more collateral damage than just the items you replaced. I'm also amazed you could correctly replace the coolant tank in 2 hours much less the other stuff. That job is a bear. I'd start with looking at temp sensors or what sensors drive that gauge. They are probably fried if you aren't getting any info.