Nothing.
This was the 5th Mercedes I owned, and my first S-Class, and it's one of the greatest cars... ever! The sheer size of this long wheel based version was overpowering to other road users, the car drove as new.
The car was 100% reliable, though it was scary to own as it feels very expensive, and you know any repair needed will set you back.. the main reason I didn't keep it for too long!
The car ran on petrol and LPG, using the very latest LPG system, and there were never any faults with it. The gearbox was the more reliable 4-speed and silky smooth.
In review.. very reliable, the 3.2 delivers great performance, I did 135mph in it (in Germany obviously) and it was smooth and quiet. Returned low 20's to the gallon, but the LPG made up for it. Every Merc enthusiast owe's it to themself to own one at some point.
Fully agree. a fine car great power. considering the weight. hitting the scales at 2000kgs!!! things do wear in them, its normal, mainly dues to the cars excessive weight!! but if you check around on the net etc you will pick up parts for them. in fact they are now becoming a classic (w140 series) many argue that its the best car merc built for years. as for fuel figures they are OK. I can get 32mpg or so while keeping at 90kph. or sometimes 200miles to 25lts. then these cars are not for boy racers. they move with grace. I tested mine in germany as well. it's a 94 s320 with 120000miles on the clock 1 owner from new full history etc. did 700 miles almost non stop. in about 10hrs. no bother with burning oil or overheating etc. but if you are going to buy one. do consider that they burn juice if pushed and can be heavy on tyres. but engines can go for ever. do watch out for minor oil leaks. I mean minor. as some diesel cars burn more oil than these do. electrics are generally fine. but cn be costly to fix if mechanic ain;t sure what they are doing. a lot better than the merc s class from 1998-2005/6. will defo be a classic car. make sure you buy the right spec, leather and auto, and clean alloys and perfect body. do watch for a little rust around the wheel arches. mainly due to salt on roads. I hope someday merc make cars like this again.