Smoking Cosworth YB Engine

[Olivera]

I need some help diagnosing why my engine is smoking badly. This occurs when coasting on the overrun for a good few seconds then getting back on the throttle. It is exactly at the point of getting back on the throttle that it puffs a big cloud of smoke. It doesn't smoke when hard on the throttle or even when starting the car in the morning. I should also mention that new piston rings and valve stem seals were fitted when this engine was refurbed last year. So I'm trying to figure out which of the following is causing this:

1) Worn valve stem seals - a new set were fitted last year so shouldn't be this?
2) Worn valve guides - possibility, but there is no smoke when starting the car?
3) Worn piston rings - a new set were fitted last year so shouldn't be this?
4) Worn turbocharger oil seals - hopefully this, as its simpler to fix
5) Something else?

Can anyone help?

[stevieturbo]

Without seeing, hard to say. But turbo is a strong possibility

[gog]

First check that the crankcase breathing is clear. If blocked oil, feeding the turbo, may not be able to return to the sump adequately.

[Justin S]

oil control rings on the pistons can still give issues and may not show on a leakdown test either. If you have a blocked breather, then it may show oil leaks around the engine, especially under load causing excessive crankcase pressure. The turbo under vacuum could be causing it . Easy check, just take the inlet hose to the plenum off and check for oil . It would have to travel through the intercooler, so the intercooler may now be contaminated with oil also. Cosworth oil seals for the stems are pretty good, so unlikely.

[Olivera]

I've removed the turbo. Quite a lot of play in the shaft, and there is a hell of a lot of soot on the turbine side. Going to send the turbo off for a refurb anyway as it can definately do with one. Will then check the breather system and turbo oil return feed to sump. If that doesn't fix the problem it must be worn valve guides? Can anyone explain how a compression and/or leakdown test could help diagnosing this?

[stevieturbo]

A leakdown test wont offer any diagnosis for valve guides

[ian_uk1975]

Sounds like turbo bearings and oil seals... there should not be excessive play in the shaft; it should have a 'just detectable' amound of play when wiggling the end of it between finger and thumb. My old Mitsubishi Starion Turbo had so much play in the shaft, you could literally waggle it about all over the shop! As it sounds like your turbo bearings are on the way out, I'd expect you to have noticed slower spool-up as well, as the shaft would be binding somewhat against the housing?

[GavinPearson]

It might just be a coked up turbo drain tube. I would check this before thinking about buying any new parts.