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Recently I had difficulty starting my 70,000 mile 2005 4.2 XK8. When it did start there were billows of white 'smoke' coming from both exhausts and the rev counter was all over the place.
I have had no problems with this car during my 10 years of ownership. I have just done a gear box and engine oil and filter change. I suspected head gasket trouble but was confused by the colour of the 'smoke'. The car has not done much mileage lately and I was advised that the problem could be due to E10 fuel lying in the tank for a couple of months and the white 'smoke' is actually water vapour, but I'm doubtful. I've been running the car up for a few days and have noticed that it is becoming increasingly difficult to start but when it comes up to temperature the white 'smoke' largely disappears and the car runs perfectly normally in every way, until the next day when everything repeats itself.
Any ideas?

These cars have never liked being taken off the road for long periods and the E10 situation may well make things worse. Start with the easy stuff, so first take the car for enough long runs to empty the tank of its old fuel. Then, refill with fresh petrol, preferably with E5 and see what the car is like afterwards. Running the car around the block a few times won't be good enough to eliminate the possibility of fuel storage issues. Mike.

Agree with Mike, see how it is after a few good runs. If it’s head gasket it’s just going to get worse, is the coolant level dropping? With head gaskets you would expect the vapour/ smoke to improve as the engine warms as the gasket will expand slightly.

I thought I’d let you know that you were absolutely correct about the cause of the ‘white smoke’ problem with the Jaguar. It was E10 fuel left in the tank for too long. Putting E5 on top of it was simply wasting money as it just meant the problem lasted much longer. You wouldn’t believe the variety of problems I suffered until I eventually drained it all off and filled it with E5 at which point the problem immediately ceased to exist. The car is now running better than ever, on Super unleaded. By the way all of my false fault codes and messaged disappeared after a hard test – Thanks for all of the info.

Brilliant! It's always good to hear about a happy ending to an initial tale of woe. Mike.

Brilliant! It's always good to hear about a happy ending to an initial tale of woe. Mike.

I'm late to the party but I thought I'd add that I recommend we all use Esso super unleaded. Except for a few areas (see link here: https://www.esso.co.uk/en-gb/fuels/petrol) it is ethanol free. By law they have to label their pumps E5 but it is mainly E0. My 2002 XK8 certainly runs noticeably better on this that E10.