The BMW 3 Series is one of the most popular BMW models ever made. BMW brought the first generation to the U.S. in 1977 as the replacement for the 2002. Since then, the 3 Series has gone through four additional generations. To differentiate them, many enthusiasts refer to the various iterations of the 3 Series with the internal designation that the factory gave them. Models made from 1984 to 1991 are E30s and so on. By and large, the S Series is a very well-built, reliable car, though there are some things to be aware of.
E21 (1977 to 1983)
The 320i, which was the model sold in the U.S., is probably the least popular 3 Series and can cause its owners many problems. The 2.0-litre and later 1.8-litre engines are robust but can often suffer from blown head gaskets. The thermal reactors for the emissions systems can be problematic. On the fuel injection system, beware of dried and cracked fuel lines that can leak fuel. 320s can suffer from a front end shake at around 50 to 55mph. The shimmy is caused by worn swaybar bushings. To fix the problem, you need to inspect the control arms, upper strut bearing, wheel bearings, control arm bushings and steering rack bushings, and replace any worn parts.
E30 (1984 to 1991)
The E30 was introduced to the U.S. in 1984. It was with this model that the 3 Series began to attain the popularity that it enjoys to this day. The E30 also happens to be one of the most durable of the 3 Series and one of the most fun to drive. The M20 six-cylinder version features a cam-belt instead of a more durable timing chain. Failure to replace this can result in a catastrophic engine failure. The cooling systems can oxidise over time, which can cause the head gaskets and the water jackets for the throttle bodies to leak. The front control arm bushings wear and cause poor suspension behaviour. E30 M3 bushings provide a dramatic improvement. Failed rear suspension carrier bushings result in thumping noises from the rear. Installing new bushings fixes the problem, but it is a difficult job.
E36 (1992 to 1999)
The third generation of the 3 Series also happens to be the most problematic. General build quality and the durability of the car's materials are not that great. Dashes and door panels come apart. Electronics issues are common, so expect multiple failures here. The engines themselves can suffer from a range of issues from VANOS (valve timing system) failing and noisy lifter. Gearboxes have been known to have problems with first and second gear as well as a separate issue with fifth gear. The rear control arm bushings wear out and need replacement with post-1996 parts. Ball joints also wear out.
E46 (1999 to 2006)
When it was first introduced, the E46 had its share of problems, but unlike the E36, these were generally ironed out by 2004. The M52 and M54 engines are pretty robust. The ECUs have been know to have problems and need reprogramming. Cooling systems are a weak point and need new water pumps at 60,000 miles and radiators at 100,000 to prevent issues. Other issues include excessive belt-tensioner noise, worn outer ball joints, problematic sunroofs and radio display failure.
E90/E92 (2006 to present)
The E90 (sedan) and E92 (coupe) are both two new for long-term issues to rear their heads. For now, the most common issue is failure of the high-pressure fuel pump for the direct injection system on the 335. Some automatic gearboxes have suffered from jerky and hesitant shifting.
Any 3 Series is potentially a great car with the proper care and maintenance. A pre-purchase inspection is highly recommended before purchasing any used BMW, however.