Anything that could have gone wrong, mostly engine trouble.
Most comfortable car we ever had, but as for performance and reliability I certainly could not recommend the Citroen Synergie.
A 2.2 TD? Surely the only TD options available from Citroen at that time were the 1.9 and 2.1 (the 2.1 was never used in the PSA MPV range)
You are right about Citroen not doing a 2.2 version in the Synergie, I however do own a 2.1 TD SX version with full leather which I bought new in Belgium in 1999, best vehicle I have ever owned, Have driven it all round Europe, and the length and breadth of the UK on a number of occasions, It now has just short of 200K miles on the clock and it still returns around 40 mpg on a run,
Had a new clutch fitted at 130k miles, and then only as a precaution, so apart from normal maintenance ie, cam belts, Brakes, Tyres etc, nothing else has had to be done and certainly no problem with the cylinder head, that problem you are referring to only occurs if the vehicle has been driven hard on a number of occasions or there has been a number of different drivers in the vehicle over its lifespan.
I have a friend who uses a synergie SX TD (Roland Garosse) version as a Taxi, again no real problems with it and I believe it is just approaching 435K miles.
Fantastic vehicle, highly recommended, and as with anything, treat it right and it will treat you right.
Note: A Comments RSS Feed 
is available. New comments appear in the Members Area before the main site