The engine is a Motronic, 3.5 liter (3430cc), in-line 6 cylinder. The compression ratio of the engine is either 10.0:1 or 8.0:1. The compression ratio depends on whether the car is a "green" or USA spec, or the regular, non-catalytic converter equipped engine. The euro engine is the more desirable engine, and the difference can only be ascertained by decoding the VIN. These engines are bullet-proof practically. A US-spec engine reponds VERY well to a chip, either Jim C or Dinan for example. Bentley has great Shop Manuals both for the car itself, and a Bosch Fuel Injection Manual which covers these engines. The close-ratio transmission (identified by the shift pattern with first gear being the "dog-leg") should shift well, and should not be overly worn. These are not as robust at the standard over-drive Getrag transmission, but should last well into six figure mileage.
Originally, the E28 brakes were adequate for use in the heavy E28s. Nowadays, the performance is lack-luster. Bigger calipers are a bolt-on fix (E31, E34, and E32 cars have calipers that fit either in front or rear depending on model). Maintenance is key to caliper and brake component longevity as well, so make sure there have been routine fluid bleedings. The only other problems that you need to be aware is that of the brake bomb. The 3.5 Motoronic equipped cars require a hydraulic pressure accumulator, rather than the normal vacuum pressure. The pressure accumulator can go bad (aka the "bomb") and may need replacement.
The Suspension & Steering
The E28s have a relatively weak front suspension. Front suspension bushings wear away quickly and cause shimmying and other handling maladies. Rotors warp easily, and tie rods wear away as well. Steering boxes aren't the most precise things either, but you can always adjust them, if out of adjustment. Upgraded 750 upper control arm bushings are a good idea, as are E34 aluminum lower control arms to reduce unsprung weight. The rear suspension wears at the subframe bushings and the pitman arms. These will cause oversteer or generally loose handling. The good news is that there are many modifications available for the E28s in the suspension department. Dinan, Alpina and Hartge all have good suspension systems available and are very well tuned. H&R, Racing Dynamics, Eibach, and Bavarian Autosport all have springs, and you can fit Boges, Bilsteins, or Konis. Of course some combinations work better than others. And you always have the M-tech springs and Bilstein option.
The Interior & Electrics
Make sure all the electricals work. The E28 electrical system is very robust, yet complex. You want to make sure all the systems work and you don't have problems with the OBC, the check control unit, or other accessories. Also, the sanding off of the corrosion applies to ALL FUSES and the fuse box, as well as contacts at headlights and sockets in general. The interior wears well, and should not have any mildew/standing water/rips/tears/etc. Everything is very robust, but seals do wear, so door/window/windshield seals are all suspect after 10 years. Bad seals cause water leaks which can lead to rust.
Written by: C. Graff