Overheating

[Rockford]

Hi,
Sorry to sign up with a problem. I've been a mostly happy Insignia owner for seven years and gained loads of useful advice from reading these forums. Thank you.

Now, my Insigna Techline 2.0 CDti (169 bhp) (65k miles) has started overheating after 5 or so miles of driving - it does not overheat when parked idling. There are no leaks.

First (independent) garage replaced the thermostat, saying it was blocked (RAC recovery chap thought it was that too).  That did not fix it. They then checked the radiator for blockages - all ok. Then they said it was probably the water pump.  Since I had had the pump replaced preemptively a few months ago at the main dealer, I took it back there.

Dealer's mechanic said pump was ok but the coolant header tank/bottle thread was damaged and hence no pressure in system.  They put a new coolant tank and cap in -- fortunately that was under guarantee having been replaced late last year at the dealer (I know!). But the problem is still not fixed. I only just made it home from the dealer with the needle in the red zone!  It was definitely overheating and not just a sensor/computer issue because the coolant was boiling and steaming (never a good sign).

I will have to go back to the main dealer, but I'd greatly appreciate any wisdom from hereabouts. Cheers all!