ive got a 2002 318i 2litre n42 engine bmw.2 weeks ago i had to have a new timing chain tensioner,chain guide and oil and filter change.for the first week the car ticked over a bit lumpy at about 600-700 revs and was a bit tappy,but drove fine.know the car is still tappy,but once it has got warm,it revs itself from 600-700 revs upto 2000 revs with about a 4 second pause in between.once it hits 2000 revs it drops straight back down.this also happens when driving the car.