The E46 cooling system uses many plastic parts, and they age from heat and pressure cycling. Two parts fail often.

The fastening of the swirl flaps in the intake works loose from the vibration of the engine, and a flap can come away. From there it gets through the intake into the cylinder, and takes the piston and the valves with it, so the cost of the repair ranges from the price of a flap to an engine rebuild. The older units with metal flaps are the most dangerous. The symptoms are a rough idle, a loss of power and occasional juddering, with the code P2015 in the fault memory. On the M47TU2 this is the most common case. Prevention comes in two forms. Either a reinforced flap is fitted, aluminium or assembled with thread-locking, or the flaps are taken out and blanking plates go in their place. The second is the more radical one and takes the risk to zero; many workshops recommend it, and the airflow does not suffer from it either.

The swirl flaps in the intake manifold are held by screws, and engine vibration can work them loose beyond 120,000 kilometres. A loose screw can come off and travel down the intake into a cylinder.

The timing chain stretches from 180,000 km, and the chain tensioner no longer holds it properly. The tensioner is hydraulic, and with cold oil at low oil pressure it does not react fast enough: on a cold start the chain goes slack for a moment, and that is when it can skip. If it skips, the valves and the pistons meet. Missed oil changes speed the process up, because dirty oil wears the chain and the sprockets. The symptom has two stages, and they do not mean the same thing. If it rattles for one or two seconds on a cold morning start and then goes quiet, the chain has already stretched but there is still time to act. If the rattling is audible with a warm engine as well, the chain is heavily stretched and the tensioner is working at its limit, and then it is not worth driving on. The chain runs at the rear of the engine, right at the bulkhead, so it is hard to reach, and on many diesels the whole engine has to come out for the replacement. The material itself is not expensive: the chain is 150 to 300 euro and the revised tensioner 80 to 150 euro. The labour makes up most of the cost.

A known structural fault of the E46 is the rear subframe mounting. The sheet metal of the rear axle carrier cannot take the twisting load of the subframe and the differential: the floorpan cracks around the mounting points, and in extreme cases the mount tears out of the body.

The turbo bearing is lubricated from the engine oil, and the most common cause of failure here is a lack of lubrication. Deposits form in the oil feed line, and because of that the bearing does not get enough oil. A missed oil change and poor quality oil speed the process up on their own as well. The symptoms are a whistling or howling noise on acceleration, a loss of power or limp mode, and black smoke from the exhaust. Replacing the turbo on its own is not enough: if the oil feed line is not cleaned or replaced, the new turbo goes the same way. Prevention is three things. An oil change at least every 15,000 km, better every 10,000, with oil approved to BMW Longlife-04. A check of the oil feed line. And not revving the engine right after a cold start, because the turbo needs oil pressure first. Above 180,000 km a preventive check pays for itself.