If the leak is actually around the gasket at the flange and engine block, it is gasket VW# 028 115 441 c on the mk3 TDI. After you remove the oil cooler you will see 3 allen bolts. Remove them and the flange will come off. The gasket is underneath. The torque for the 3 allen bolts is 18 ft lbs.
If you see oil around the top of the flange first inspect the oil
pressure sensor/sender since they can leak through the metal prong sticking out
the top. An
example of this is pictured below. Before you order an oil pressure
sender, check the color. Grey is VW#: 068 919
081 a , blue is VW# 028 919 081 d.
Some have 2 oil pressure senders, one low pressure and one high pressure.
Confirm which one you need and then order the part from your vendor. If the leak is
confirmed as the o-ring, make sure the 27mm nut holding the cooler in place is
tight. When the oil filter is screwed on, it should also help hold the
cooler in place. The other leak for the oil cooler is internal. This
will cause the coolant and oil to mix and should be immediately repaired.
If you have oil in the coolant, see 1000q:
Always test your new oil cooler. Mine was defective and immediately mixed the oil and coolant, wasting a lot of money and time. The normal industry practice is that parts vendors aren't responsible for defective parts made by someone else. I suggest applying pressurized air at 40 psi or higher and immersing it in water. Bubbles will reveal any leaks. Clean the water out of the oil passages with compressed air afterwards.
The difficulty rating is 2/5 but remember that you may also have to flush the coolant system and change the oil which increases the time involved. The spring hose clamp remote pliers are needed since the spring hose clamps may be in a position where it's impossible to loosen them with regular pliers.
27mm deep socket
oil filter wrench
new oil filter and engine oil
spring hose clamp remote pliers (pictured below)
o-ring VW# 038 117 070 A
oil cooler VW# 068 117 021 B (X suffix indicates rebuilt part)
If you are just replacing the o-ring, it is possible to remove the cooler
without draining the coolant, do not disconnect any coolant hoses.
If you are replacing the cooler you must drain the coolant and if there was a leak you need to do a flush anyways to clean out the old mixed coolant/oil. Follow the procedure listed in 1000q: coolant flushing mk3. Disconnect the 2 coolant hoses going into the cooler. The spring clamps may not be accessible so I suggest using the remote spring clamp pliers on them.
Drain the oil and remove the oil filter. You will see a 27mm nut
holding the cooler on (pictured below). Do not loosen the nut until the two coolant hoses
are disconnected. The nut is thin and tapered on the edges so I
suggest a deep socket and making sure you are holding the socket flat against
the nut. I found that a ratchet with a head that can bend at an angle works
best. Universal joints promote twisting on the socket. Universal
joints also throw off a torque reading - more useful tips like this at 1000q:
torque wrench FAQ. If the hoses
are blocking removal of the cooler, rotate the cooler 90 degrees as you pull it
down to clear the engine and hoses. If you are just replacing the o-ring,
avoid damaging the threads as you pull the cooler down. It's possible to
remove the threaded center rod to ease removal but I do not suggest this since
it can damage the threads.
The o-ring has tabs to help hold it to the cooler. See the above picture of the new cooler with the o-ring attached. Put a thin coat of oil on the o-ring and replace the cooler.
Pictured below is the oil cooler and
o-ring after removal. Replace the new o-ring and cooler and torque the thin 27mm nut to
18 ft-lb. Remember, the oil filter helps hold the cooler on so don't
kill the nut, especially since it's thin and tapered.
Installation is the reverse of removal.
What was your experience changing the leaking oil o-ring? Feel free to share at the Volkswagen TDI forums here: myturbodiesel.com forums