The brief
The owner of this 316i had been topping up the oil more often than felt right, and there was a small patch of fresh oil on the garage floor each morning, sitting right under the line where the engine meets the gearbox. After a longer drive the cabin would pick up a faint burning smell.
Where the oil lands is the giveaway. That spot only weeps when the seal at the very back of the crankshaft, tucked behind the driveplate, has gone hard. It is a small rubber seal spinning against the crank every second the engine runs, and once age stiffens it, it stops keeping the oil in. The burning smell is that oil reaching the hot exhaust nearby and cooking off.
The diagnosis
On the lift it was clear: oil weeping from where the crankshaft passes out of the back of the block, then running down the bellhousing and dripping off the lowest point. On this engine there is only one thing in that position, the rear main seal.
The complication is access. To reach that seal the gearbox has to be separated from the engine and the driveplate unbolted, so it is a proper job rather than a quick wipe and reseal. We showed the owner how quickly the level was falling and the choice was easy, because the only other option is topping up forever and hoping you catch it in time.
The work
The gearbox oil was drained, the driveshafts and propshaft disconnected, and the gearbox lowered onto a transmission jack and drawn back off the engine. With the driveplate off, the old rear main seal was exposed and pulled.
A new BMW-spec rear main seal was fitted square to the housing, and while the area was open the gearbox-side seal was renewed too rather than leave a tired one sitting next to a fresh one. The driveplate went back on to torque, the gearbox was mated back up with new bellhousing bolts, and it was refilled with the correct oil.
The outcome
A dry bellhousing, nothing on the floor after a long road test, and the oil level holding exactly where it should.
The 316i went home with the leak stopped at its source, which means no more chasing the dipstick and no more burning smell on a run. Since this seal does not get touched again unless the gearbox is out for something else, doing it right once usually means it stays right for the life of the car.
