Hopefully i can help..
What actual tests did they perform?
Does the engine acutally overheat still / and excessive vapour/smoke from the exhaust?
Have you noted any coolant on the ground under the car since or prior?
Was the cylinder head checked?
The mechanic did the 'gas' test twice and on each occasion the testing probe turned to the appropriate colour indicating that there was a 'blown' head gasket. He then changed the gasket but the system has continued to lose water, but not at as high a rate. According to my wife whose evidence is not totally reliable, the temperature did not rise to excessive levels at any time. Coolant was observed on the ground on a couple of occasions. I do not think the head was checked when the mechanic changed the gasket but I am not absolutely sure about this, he is generally honest and reliable. In any case could he judge a warped head simply by observation? I don't think there was exhaust smoke or overheating at any time
Hi..there thanks for your reply..
No sadly you cant diagnose a warped head intially with it on the engine - however it depends on the extent of how the heads warped, that has to be checked if a engine has overheated alot..sometimes you can check with the head on a bench clean off all the old gasket traces and use the edge of long steel rule to check how level the head is..if there are any gaps seen or light comes through between the head and the edge steel rule..even shone torch along the opposite side..either way any gaps/light coming through would be a sign of the heads warped..like i say it depends on the extent of the warp or the head itself could even have very small hairline crack..in which case the head needs to be tested at a specialist to confirm that..and if the heads at fault then another one has to sourced..