My car has been in for repairs for 16 weeks - should I keep it?

I bought a Volkswagen Passat Estate from a big car supermarket last year for £18,000. It was 10 months old, had done 5000 miles and was in immaculate condition. Had it week weeks when the emissions light came on - got it seen to under warranty. Light kept returning and car started to emit black exhaust smoke. Volkswagen have now had the car back 10 times and I've had a replacement hire car for the last 11 weeks. Car has been with Volkswagen for a total of 16 weeks. Still no idea of the cause. Even if they fix it, should I keep it?
Assuming this is a 2015 and therefore the current B8 model of Passat, then this is very unusual and begs the question why it was for sale at a car supermarket and not a Volkswagen dealer. It could've been rejected by the first owner for a fundamental fault and as soon as a Volkswagen dealer hooks it up to his computer its history of problems will come up. So ask the Volkswagen dealer who has been trying to fix it. If he admits it has a history of faults then you could attempt to reject it to the car supermarket for having been fundamentally faulty on purchase using the ruling in Clegg v Olle Andersson, House of Lords, 2003. See: www.honestjohn.co.uk/faq/consumer-rights/
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