I had oil leaking from the turbo oil drain pipe, and it was running into the turbine side of the housing through the manifold, and now there's a lot of carbon build up in there (about 2mm thick with black soot).
Will it be okay if I just bolt it back on? With the extremely high temperature and the constant flow of exhaust gases through there, it would probably clean itself up after a while wouldn't it?

 

if you end up sending a chunk of carbon that is too big through the engine.........................goodbye apex seals!

 

the compressor side is isolated from the turbine side so I don't think that could happen could it?

 

Your description is confusing. If oil was leaking from the turbo's oil drain line, it would just drip all over the ground/engine. But if you had a restriction in the drain line, pressure would build inside the CHRA, which would force oil past the front seal into the compressor housing. You'd then get lots of oil in the intake, and lots of smoke out the exhaust and potential carbon buildup. The front seal can also just fail with time. Similarly, if the rear piston ring (seal) on the turbo's shaft has failed, you'd end up with lots of smoke and potential carbon buildup.

I wouldn't bolt the turbo back on without removing the turbine housing to clean in out. Loosen all 6 bolts, then use 2 as "jacks" by turning them against the oil flanges. Note orientation with a marker or scribe. If the drain line had a restriction, replace/fix it. If you have internal seal failure, rebuild or replace the turbo.

 

why not wire brush it off and blow it with compressed air? i also don't see how the oil made it into the turbine housing from an external leak. pictures would definitely help.

 

When I removed the oil drain pipe I saw that there was only half a gasket on it where it connects to the turbo. This is where all the oil was leaking from. 2L of oil drained out of the car completely in 10 minutes.
If anything there would have been less oil pressure on the turbo because of this. The turbo is fully rebuilt too, so it has brand new oil seals.

 

Ok, but that's doesn't explain how oil would have gotten inside the turbine housing. Was it smoking badly?

 

well from the explanation it should be perfectly fine, but pics would still help your case if you are worried that it may be an issue slapping it back together. a little carbon won't damage the turbine wheel.

 

There's only two explanations I can think of:
The first is that the turbo underwent a dodgy rebuilt and the oil seals aren't secured properly.

The second is that because I didn't replace the turbo to manifold gasket with a new one there was some small gaps that oil leaked into then it got forced into the turbine assembly by exhaust gases.

I don't know how to determine which it is though.

 

Something I should have mentioned is that the inside surface of the part of the turbo that connects to the manifold is completely covered in carbon as well.
I can't see how oil leaking from one of the turbo's oil seals could find its way back to the exhaust entry part of the turbo. What do you guys think?

 

^ That would be the turbine housing. The leaking drain line could get oil all over it and anything else in the area. It would burn off and smoke like a chimney.