Radiator Filling Expansion Tank

[Grey V8 Pete]

Have a look at the first few paragraphs of my earlier post. If you Tee off the top hose for a small bore pipe to the expansion tank then it will automatically purge air in the system as that will always float to the top. Air has a habit of coming out of the coolant during heat up so this system will automatically get rid of it and replace it with coolant during cool down.
Did you fit a new thermostat? If that has failed “shut” you could get your current symptoms. As Lucky@LeMans says it could be a head gasket due to the amount of pressure you are getting.
Did you bleed all the air out of the heater circuit? Make sure the heater valve is open (set it at fully hot position if the valve is part of the heater control). I have bleed valves in both heater lines. Available at CBS in diameters to suit popular hoses. Link below.
https://www.carbuilder.com/uk/black-...ner-with-bleed
Gauge will read quite low if there is no coolant in contact with it, which indicates that your cylinder head is empty! Peter.

[cbjroms]

Thanks for the help guys, really appreciate it.

@Lucky@LeMans - running the engine (up to nomral on the temp gauge) with the radiator or expansin cap off, I do not get any bubbles which would be expected it is head gasket? The coolant is a little brown but not partcularly sludgy. Hoses were defintely hard after road test and the radiator was cold so it must have been full of air rather than hot water.

@Grey V8 Pete - the heater circuit is bypassed (heater in connected to heater out). Your 'Gauge will read quite low if there is no coolant in contact with it, which indicates that your cylinder head is empty!' is interesting. Running the engine on the drive for 20 minutes gets the gauge into the high position at which point the electric fan cuts - so under these conditions head is full of water. But on road testing the gauge barely rises and this is when the problems are occuring. Perhpas I will try road testing without the thermostat in place.

[Lucky@LeMans]

Running the engine at tick over on the drive the engine might well behave as normal. If the head gasket is only just at the cusp of failure it might only show itself as faulty when you drive the car under load. When driving the head gasket takes more load as does the cooling system in general.