Join Date: Apr 2007
Location: Orpington, Kent
There are three ways to test a head gasket, methinks.
1. Compression test on cylinders. If there's leakage, it's normally the gasket. However, it's not infallible as the test can appear OK when it's not. Doesn't sound as though your guy did this anyway.
2. Chemical test of the water using a kit, to detect exhaust fumes in the coolant. Never had it done, but I'd imagine it's reliable.
3. Mechanic sniffs and looks at the coolant. If he's experienced, he'll be able to tell - but obviously you're trusting him to know what he's talking about.
The other way of spotting it is if the coolant gets pressurised when it's hot. Drive the car, then let it cool down and take the header tank cap off. If air escapes with a hiss, and the coolant level drops as soon as you've removed the cap, then exhaust gases are leaking into the coolant. Unfortunately, if this doesn't happen, it might still be faulty...