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I changed the thermostat this morning as the temp gauge kept creeping up then going down again, bought a new one from the dealer, £60 or so. Made sure everything was fillted back on correctly and filled the system with coolant. Started the car and let it idle till the needle was in the middle then took her out for a burn and almost straight away the needle starts to shoot up, after a few miles it's past the first marker and stayed between that and the red until I took it easy again.
So what could it be?
Duff thermostat?
Head gasket?
Worse?

Did you bleed the system after installation?

No, actually not sure how...I read in the manual that I should run it till hot and then ease the radiator cap off but not sure if I read it correctly or what.

can you advise, Reck?

Thanks

Don't start worrying about head gaskets until all the cheaper stuff is eliminated!

316's are a PITA to bleed cause they don't have a bleed screw as far as I'm aware like other cars.

-Raise front of the car slightly (on a jack or a kerb).
-Take off radiator cap.
-Put heater and blower on full setting.
-Start car and let it warm up to normal temperature- this'll take ages since the heaters on.
-Once warmed up start squeezing all the cooling pipes you can see to encourage any air bubbles that may be stuck in there out. Especially do the ones around the rad/thermostat. Watch your fingers on the fan though! A pair of thick gloves isn't a bad idea.

You might see the water level drop during this time as air pockets in the system are replaced with water, so keep her topped up. Once you've done this for a few minutes and you think the air is out, you can put the rad cap back on and go for a drive. If it's still overheating, try it again. It can take a couple of attempts to get it all out in my experience.

Pretty straightforward once you know how, but if you're unsure of any of this I don't mind having a go for you tomorrow as we'll be at the workshop (again with Jim's permission of course).

Check your fan belts not slipping! If the water isn't getting pumped around it won't cool when you give it the berries.

Cheers for all the advice, I'll get on it

And then you will end up buying a new radiator. What's the one in the car look like??

The radiator's in pretty good shape, actually, I've had it out before. I reckon I need to bleed the system (fingers crossed amyway :~) and that'll do it