Tips on Buying a Mercedes
Buying A Used Diesel MercedesW123
Body The unit bodies on W123 mercedes vehicles are inheritly very strong structures. Main points of interest would be the rear suspension mounts, where the trailing arms meet the body, as well as the front frame rails. Excessive rust on either of these components is not good and should be investigated. The sills of the body, on the sides as well as underneath, should be poked with an awl or other similar object to assure that they are not soft or rusted out, meaning that water could enter the subfloor frame should a crack occur. Front and rear jacking points on the sills should also be solid, even though the original Bilstien jacks in these cars should never be used. (Most cars cannot stand up to the stress on the sills, cracking, denting usually occurs) The spare tire well should be opened fully and poked for rust, as water tends to pool here if the trunk seals are leaky. The trunk seals themselves should be inspected as well to ensure that they are secure all around; a loose area can trap water which then rusts out the lip in which the seal is bedded. The plastic trunk liners in each side of the rear wings should be removed and the underlying floor checked for water damage or perforation. This is also a good time to take a look at the belt-line moulding buttons, (the little plastic retainers that hold on the trim that goes around the car). These buttons go through the body and should be visible from the inside of the trunk wing. The original buttons are red. If they are different colours eg. red(older style)/white(newer), then the mouldings have been taken off for whatever reason, and this should be questioned to the owner. The front and rear floor pan should be checked for rust perforation, a typical spot for rust is conviently right under the accelerator pedal. Taking up the carpets and checking the sub-floor for water damage is a good idea. These cars typically aren't the most water-tight of machines, so dampness/water if it has been freshly washed or raining isn't too much of a concern, however, major rust or a mushy floor pan is a detrimental problem. By design, LHD cars suffer from typical drainage issues with the passenger cowl vent. If water is directly sprayed into this vent, more than likely it will end up in the passenger footwell. Water also tends to pool inside the left front (LHD cars) fenderwell inside the engine compartment, right beside the washer reservoir. Windshields will also leak if they are not original, unless a very exceptional job has been made whilst replacing it. Rear windows also leak, but thankfully not as often. Sunroofs should be leak-free, if not, adjusting their height is a long and difficult process. Replacing their seals is also a tedious endeavor.
Drivetrain Mercedes-Benz diesel engines are, by nature, extremely reliable engines. The 617 series engines used in the 300D's and 240D's are nearly bomb proof, so long as one stays away from the 5100rpm red-line in sustained service. Excessive valve clacking or injector knock (not normal diesel noises) and blow-by that chuff's like a locomotive are signs of an engine that has been either abused, not serviced at the proper intervals or has very high mileage. Knocking right before the transmission shifts (which is the time the engine is free-wheeling with no load), also while free-revving the engine in Park or Neutral is a sign of injector troubles, which could be as simple as a dirty fuel tank screen, air in the return lines, or as complicated as an engine in need of new injectors.
[Kyle Blackmore's Spotting a 123 Dog]