Sorry if stating the obvious, but how are you checking the coolant level? Make sure you check it under the radiator cap (when cool!), not just the expansion tank.

Hi Big John. No, if youare sitting in the divers sear it sounds like its coming from just infront of the passenger side where the coolant tank is.

Hi Big John. No, if im sitting in the drivers seat it sounds like it is coming from just infront of passenger seat where the coolant is.

Air lock in the heater matrix? I don't know the coolant system bleeding procedure on the Insignia but I thought Vauxhalls didn't have bleed valves these days. It also can be a sign that the coolant level is low - but that would hopefully show up on the header tank.

As elekie&a/c doctor has asked - is the heater working as expected?

Important - you mentioned that the engine seems very warm after running - carefully feel the main radiator core is it hot? If not you may have a circulation problem such as blocked radiator/ failed closed thermostat/ water pump . Why did the head gasket fail in the first place, indeed does the fault still exist?

Edited by Big John on 13/10/2022 at 08:59

Sounds like air in the system. Some cars (VW Golf seems to get special mention) are difficult to fill

My 2001 1.4 16v Octavia(same oily bits as similar Golf) was a doddle to fill after a new radiator was fitted - After filling kept squeezing large hoses and topping up then run up to temperature (fans running) with the header tank cap removed. Checked level and replaced cap - job done - perfect thereafter. Header tank is high up with pipes feeding from many places.

A previous Fiat Punto was a nightmare to bleed as the "header tank" was at low level next to the radiator feeding the cooling system from the bottom. No wonder head gaskets used to fail shortly after a coolant refresh!! It had bleed valves near the heater matrix. Myself I chose to refresh the coolant live - filling and draining at the same time. Wasteful but no air introduced in the system.