Sudden interim smoke

I did a full de-cat on my sup 1 week ago. The car did not smoke before, but started smoking after. If this is co-incidentle, I do not know. If I start it up, it does not smoke, which is normally a sign of valve stem seals ( could be wrong). If the engine is under load, and on boost it does not smoke (pistons and rings still fine).If i keep it on constant revs of about 1000 to 2000 rpm the smoke start pouring out the back until you rev it higher. Does not make sense at this stage. Would appreciate any input or experience with similar diagnoses.

When you do a decat you reduce the exhaust back pressure.

Inside the turbo the only thing stopping the lubricating oil coming out of the exhaust side is a piston ring between the turbine end of the shaft and the turbo housing.

The high oil pressure inside the turbo housing is counter acted by the high back pressure caused by the cats in the exhaust. This helps the piston ring keep the oil in, so normally the oil does not leak out.

The lower back pressure can, with a worn piston ring, give an easy path for the oil to exit into the turbine... causing clouds of smoke! This is because the oil pressure is still as high as before in the turbo, but there is no pressure on the exhaust side holding it back.

If your ring was dodgy before the decat, it may have only been the backpressure stopping it leaking?

On boost the oil pressure is the same inside the turbo as off boost, but the exhaust back pressure inside the turbine housing is high (compared to off boost), so it won't leak.

May be worth looking at the turbine wheels to see if they're wet with oil.

That makes sense and would explain the co-insidense. Would you recommend a compression test, It just looks awful seeing a sup with smoke pouring out the back. Maybe put a cat back until I rebiuld the engine.

It won't need a compression test mate. I'm sure the engine's fine.

Its the turbo(s). Maybe just #1 turbo infact as this sees the most use.

Are they J spec or UK spec turbos??? Standard??

It won't need a compression test mate. I'm sure the engine's fine.

 

Its the turbo(s). Maybe just #1 turbo infact as this sees the most use.

 

Are they J spec or UK spec turbos??? Standard??

They are J spec turbos

I apologize, was reading too fast and asumed the damaged ring to be piston related. I am getting oil residue by the bov valve, which I was told before could be the turbos heading for retirement. When I considered the turbos, I dismissed it because it smoke before getting on the boost, but I see what you are saying.