Oil leaks can be annoying, none more so than from the oil filter housing. It's characterised by a drip from the oil filter generally when running. It drips onto your diff, covers everything and there's a few things that can cause it. They say that leaks tell you that there's still oil in the engine, which is good but for the sake of doing this 15 minute job it will save a lot of oil loss (in my case).

There's a lot going on around the oil filter housing - pipes in all directions and it's not the easiest place to get to. I had a trial and error approach to finding what was causing the leak. It's best to clean the area up to find where the drips comes from. Remember that a drip might mean that the leak is above that area, rather than in the area itself. The cause could be one or more of the following:

• Crankcase breather pipe
• Loose oil filter
• Oil cooler pipes
• Waxstat adapter
• Oil pressure switch
• Housing to block seal

In my case it turned out to be mainly the waxstat adapter (the oil thermostat) and a bit from the crankcase breather pipe. The filter housing to block seal is a more involved job, so best to eliminate the others first. These can be cured as part of the same process below. If you can see a definitive leak, go for that first and see what happens. The best first attempt is to see if the oil filter is tight enough. If it is, doing the rest as part of an oil change is easiest.

Potential parts for the 300Tdi

• Waxstat O-ring - ERC5913
• Crankcase breather hose - LLH500170 or ERR3084
• Oil cooler pipe O-ring - ESR1594L
• Oil pressure switch - PRC6387

Crankcase breather pipe

I replaced the crankcase pipe first as it was split and weeping oil from the bottom. The potential highest cause of a leak (barring the rocker cover gasket). Undo the jubilee clips and put the new pipe on. It will take a bit of wiggling. Update: I have since replaced the pipe twice more in 5 years due to the same problem. Using genuine hoses, it keeps cracking in the same place. I have resigned myself to this leak!

Check the oil filter seal

Remove the oil filter and check that there aren't two seals there (one left behind from a previous oil change). If there isn't, check the mating face is smooth.

Waxstat and oil cooler pipe O-rings

Undo the oil cooler pipe with a 24mm spanner and remove it. It will start to leak oil so plug it. There should also be a small O-ring that either came away with the pipe or is still in the housing. Retrieve it.

Undo the two 8mm nuts on the waxstat adapter. The spring inside will push the adapter out so be careful to not let it fly out. Pull it all out as one and keep it together. The main O-ring goes around the sleeve, which as you can see on this one had degraded over time to become a triangular shape in cross section.

Fit the new seal and lubricate with new engine oil. Put it back in carefully and tighten the bolts to 9NM. Fit the oil cooler pipe with a new O-ring also and "nip up" the nut, which means "tighten until it won't go any more without forcing it". The other oil cooler pipe follows the same procedure but goes directly into the filter housing.

Oil pressure switch

If your pressure switch is leaking, which mine wasn't (the part with the wire sticking out), then this is simply removed and a new washer replaced.

Result

It seems my leak was due to waxstat adapter seal being degraded, so now there's no more leak from the oil filter housing. However, the process is quite iterative, so i'd also replaced the crankcase breather hose in the process, which was looking tired - so two attempts for me to cure this leak. The driveway now loves me...

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