Leaking oil cooler gaskets are a common problem that affects most BMW engines made in the last 20 years. This happens as gaskets become hard and brittle with time, which makes them unable to seal properly. In the beginning stages, the leak will only occur during engine warmup.
The main problem that comes with this issue, in addition to obvious oil loss and potential stains on the driveway, is that the oil drips on the alternator, which sits just below the oil cooler.
This means that even the smallest leak can lead to potential alternator issues on the long run.
There are two gaskets on the four-cylinder N46 engine (marked with arrows on the image below), one between the oil filter housing and engine block, and another between oil filter housing and cooler.
I would advise changing them in pairs, regardless if one is not leaking at the moment. Both gaskets are cheap and the additional work involved is minimal. The situation is similar with six-cylinder engines, as well.
Useful tip: if following official instructions, you would need to drain the cooling system and disconnect cooling hoses that go in and out of the oil cooler.
This is done to gain access to the bolts that hold the cooler in place. However, I’ve managed without this step, which saves quite some time.
- Depending on the year and model, the oil cooler may be hidden below the acoustic cover. If that is the case, remove the microfilter housing and scuttle drain to gain full access to the acoustic cover. Remove four nuts that hold the cover and the cover itself.
- Remove the oil filter cap, using an oil filter wrench. Pull out the filter and let the oil drain out the housing. As the engine is slightly tilted, some oil will remain at the bottom of the housing, so you need to wipe it off with a clean rag.
- The housing is bolted down to the engine block with three bolts, indicated with arrows on the image below. Unscrew them and slowly lift of the whole housing assembly.
- Although the coolant hoses are still connected to the housing, limiting the movement to some extent, you can still rotate and position it in such a way to get access to four torx bolts that hold the oil cooler in the place, indicated with arrows on the image below. Unscrew them, and remove the cooler from the filter housing.
- As the filter housing is now fully disconnected from the surrounding engine parts, you can take away from the engine. I would suggest working on a desk or similar work surface, as removing old gaskets requires some patience. As being brittle, they tend to break into small pieces while you try to remove them. Don’t do this over the engine, as small pieces may fall into the engine.
- With old gaskets removed, thoroughly clean the groves using brake or carb cleaner. Do the same with the mating surfaces on the oil cooler and engine block. Apply some oil to the new gasket, then correctly position it.
- Refit everything back up in reverse order. Tighten up all the bolts to the specified torque values using a torque wrench.
Important note: Most sources state that torque value for bolts that secure the housing to the engine block is 22Nm, and this is only true with M13 bolts. However, most engines use M10 bolts, and proper torque value, in that case, is 10Nm!