What Car? Q&A - What's the yellow goo under my oil cap?

Q: I have recently noticed that under the oil cap of my Audi A3 there is a yellow custard-like substance. Do you know what this might be and why it has appeared? How costly an issue could this be?
James Blandy

A: The substance is almost certainly an emulsion caused by water finding its way into the oil system. If you don’t use your car regularly, or only do very short journeys, there is a chance this is simply caused by condensation.

Alternatively, and more dramatically, your Audi has suffered a blown head gasket. This has probably allowed coolant from the cooling system to find its way into the engine’s oil system.

We’d advise getting the car looked at as soon as possible.

As for prices, we called a local Audi dealer and were quoted £950 for the repair, although you should be able to find independent garages willing to carry out the work for less.