AFAIK a cylinder pressure test and a coolant one are two different things.Okay if I get it , a pressure test can be done by a meter on the cylinders by removing the spark plugs and a halfords hand held tester Or according to strings ,bubbles in the coolant resevoir when cold as the gas escapes from the pots. I had a Nissan which would build pressure and release through the leak witha sound like air brakes.
The sniff test done by a hot wire detector in the water bottle which would detect the solvents in the petrol.
I am going to get a garage to do a test but keen to understand what they do
A compression test (plug removal) would not always show a possible HGF, more so in the more common failure of the gasket betwen the oil and coolant ways.AFAIK a cylinder pressure test and a coolant one are two different things.