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VANOS is variable valve timing - on that car it changes the exhaust valve timing - double vanos does both.
Double VANOS has a higher output rating and better TQ numbers (unsure off the top of my head) You can tell if the VANOS is ok when the car is dead cold- when it starts to fail due to internal leakage the car wants to run real rough and hunt cold- you have to start it, and basically drive off easy. If that is starting to fail be prepared for big $$$. Not a common failure, but it happens time to time. No difference in reliability really between the two.
ONE LAST THING- make sure you get a flashlight and have a good look inside the oil filler hole and around the inside of the valve cover. If it appears sludgy and everything is coated- walk away.
BMW was doing 15K oil changes based on the oil life meter, and usually the oil goes bad after 7-8K- the result was a lot of cars were sludging up and motors were dying. If the PO followed the service indicator light for oil changes that car is in trouble.
rjp
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