Huge engine temperature fluctuations

[Boomslang]

My problem is now fixed: we took the engine to bits piece by piece and inspected/tested each of the components (stat, water pump, exhaust manifold, the head block) until we got to the headgasket where there were at least 4 blackened points of exhaust escape from each of the four cylinders.

 

My mechanics professional opinion was that the amount of exhaust gas escaping into the coolant system through the head gasket was causing airpockets in the head leading to the increase in temperature. These would eventually displace as coolant was pumped and the temp would drop back down.

 

£1500 later the temperature gauge sits steady even when pushing hard against the flow of the Avon.

 

As an aside one of the firsts things the mechanic did was test for exhaust gases coming out of the coolant filling port using a leak test fluid that changes colour. We did this test twice with two different liquids but neither of them changed colour despite enough gases coming out of the port to nearly empty the testing vial.

 

Any thoughts on why that might happen? I cant for the life of me work it out....