Damp bores

Look for spark plugs that are suspiciously steam-cleaned, suggesting that coolant is leaking into a cylinder. A misfire when the engine’s first started is another telltale.

Gas leaks

Sniff the coolant to see if it smells of fuel or exhaust fumes. Gas pockets can collect in the head, leading to hot-spots. An infrared thermometer will identify if the head – or part of it – is excessively hot.

Compression

A compression test is handy for identifying head gasket leaks between cylinders. Look for adjacent cylinders with low pressures that don’t improve when oil is squirted in.