There are four engines to choose from with the E63 6 Series and they comprise 4.4- and 4.8-litre V8s, a 3.0-litre six-cylinder petrol and a twin-turbo 3.0-litre turbodiesel. Dealing with the smallest petrol first in the 630i, it's a very reliable unit and, with correct servicing, is happy to go past 200,000 miles. Early versions suffered with rattly top ends when cold, but this should go away when the motor is up to temperature.
With the 635d's twin-turbo diesel, the only points to look for are collapsed pipework for the turbochargers. This will be noticeable by the car feeling down on power and performance. Replacement pipes are not expensive, though labour costs car ramp up the final bill. If these pipes fail, it can also cause the Exhaust Gas Recirculation thermostat to stop working, it won't let the diesel particulate filter regeneration get up to temperature and this will clog up with soot. That will be an expensive fix and be wary of any diesel that has only been used for short hop trips. BMW remedied a problem with broken turbo actuators by using a stronger mounting bracket, so look through the service history of any car carefully to see this has been carried out.
Now we come to the V8s and the cars most PHers are interested in. The 32-valve N62 motor is known to leak a little oil from the cam covers and oil pipes. Fixing the cam covers is simple, but sorting the pipework can be more involved due to access. The engine needs 8.5-litres of quality oil at every service, though these can stretch out to 18,000 miles depending on use and the car will let you know its service indicator.
More oil can escape from the plastic gearbox sump that warps over time with heat soak. It's £500 to fix and you can expect to carry out this work every 50,000 miles. The gearbox is sealed for life according to BMW, but reckon on changing the transmission oil every 60,000 miles. Also budget for replacing the electrical connector seal at the same time to stop any future leaks. Otherwise, the automatic and manual gearboxes are strong. The rare SMG (sequential manual gearbox) option can be more troublesome and wear through clutches quickly, especially if the car is used in town a lot.
Perhaps more worrying than any oil leak is a coolant leak with the N62 V8. The coolant pipes to the water pumps leak water from the back of the engine. You can expect a substantial bill of £3,000 or more if it's the O-ring seal on the valley pan as the engine has to be removed to do the job properly. Replacement parts are not a major expense, but labour costs can turn this into a very expensive job. Aga Tools offers a replacement pipe that cures this, though you'll still be looking at a large bill. The 650i doesn't seem to suffer from the same problem.
Under-bonnet heat can lead the V8 engine's diode packs to fails. The parts are cheap to replace, but access makes this a tricky job.
A lot of owners fit an aftermarket exhaust to free up more engine noise, particularly with the V8 models.
"During ownership of the car, it always brought a smile to my face when I took it out to drive, it sounded awesome when I put my foot to the floor (too often) and servicing costs weren't as horrible as I was expecting."