The oil cooler sits in the V valley between the two cylinder banks, and its seal hardens from the heat. From then on it leaks along two paths. Outwards the oil collects in the valley, but no dripping appears under the engine, so all that happens is that the oil level falls and the smell of burnt oil comes from the engine bay. Inwards the core of the water-to-oil cooler starts passing, and the oil mixes with the coolant.

The failure of the seal is reported above 70 000 miles, that is above roughly 113 000 kilometres, and the maker supplies a cooler revised with a PTFE-lined seal in place of the A2730100315, under service bulletin 07.01-P-1089.

This is regularly put down to the valve cover gasket, although replacing the valve cover does not solve it, and the leak carries on. The cooler can only be reached after the intake manifold has been removed, so the labour makes up the larger part of the bill.

Three signs set it apart. The oil stands in the V valley, and there is no trace under the engine. In the coolant tank an oily, brownish layer or a mayonnaise-like mixture is visible. If, however, the front of the engine is wettest low down, then this is not what is leaking but the seal of the front cover, from where the oil runs onto the belt and the exhaust, and comes with smoke and a burnt smell.