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A three cylinder engine lacks basic balance - the crankshaft will wobble when rotated because the off-axis masses are not in the same plane or symmetrically balanced around the axis of rotation (as in a 6 cyl engine).
The "fix" is a contra-rotating weighted balance shaft. Unfortunately, in my experience, VAG don't engineer it properly and use a chain to drive this shaft, together with the oil pump.
The problem with driving a balance shaft with a chain is that the torsional oscillations set up by this out of balance system quickly damage the chain tensioner system and ultimately, the chain itself. Failures are beginning to occur with this engine, which manifest themselves as a loss of oil pressure when the chain lets go. The engine is then wrecked. On examination, the oil pump is fine and the chain drive is in pieces.
It takes a little longer for the 3 cyl diesel to wreck its balancer chain than the 4 cyl engines did - because the Lanchester shafts fitted to a 4, rotate at twice crank speed - so generating 4x the out of balance forces. F = mr (omega) squared, where F is the out of balance force, m the effective mass, r the radius and omega the angular velocity.
Think washing machine - unbalanced load - delicate drive. Not a bright idea...
VAG were forced to modify their 4 cyl engines to a geared drive system after a fiasco with the chains - but the 3 retains a chain drive to the best of my knowledge.
In engineering, never try to buck the Laws of Physics - otherwise the design will bite you.
VAG have, in my experience an extremely poor record when it comes to correcting serious design faults or compensating customers who suffer the results of them. The 3 cyl chain drive usually lets go at about 100k miles - so well out of guarantee. They therefore won't change or fix it.
Avoid. If you want a diesel Polo, get the 1.9.
659.
Edited by 659FBE on 25/05/2011 at 15:33
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