Mitsubishi 4G9 (4G92 / 4G93 / 4G94)

Mitsubishi's 1.6, 1.8 and 2.0 four — in the Lancer, Mirage, Galant and Carisma. Its smaller sibling is the 4G1 and its larger the 4G6.

The thing to establish first is whether it is a GDI, because Mitsubishi's early direct-injection versions of this engine have a reputation nothing else in the family shares. GDI was among the first petrol direct-injection systems sold in volume, and it ran a stratified lean-burn mode that deposited carbon heavily on the intake valves and in the combustion chamber. On fuel with higher sulphur content than the system was designed for — which is most of the world outside Japan — the deposits arrive far faster.

The result is a rough idle, misfires and a loss of power that a conventional diagnosis does not explain, and cleaning rather than replacing is the answer. Many were converted to run on LPG or simply retired early for this reason.

The non-GDI versions have none of it and are ordinary, durable, port-injected engines. The badge on the car is what tells you, and getting it wrong sends the diagnosis somewhere expensive and useless.