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I have a 1983 320is. Shortly after I purchased the car it began to smoke quite a lot and often. The color of the smoke was blue-ish. My first thought was that it was burning oil and that it would either be valve seals or piston rings. Since it was smoking all the time, I was afraid that is was going to be the piston rings. To confirm my suspicions I took it to a reputable BMW shop in socal. The mechanics at the shop said most likely it was worn piston rings.

In addition, the car failed to pass smog. I took it to a muffler shop and the Catalytic converter was in terrible condition. It was nearly 50% blocked with a huge dent on the bottom. I had a new MagnaFlow CAT installed. On my way to the muffler shop I noticed something very strange, the car was no longer smoking. I drove the car all weekend and for the most part the car was smoke free.

I then decided to check the compression and all readings were between 150lbs – 155lbs across the board.

My question is, is it possible that the cause of all this smoking was a bad Catalytic Converter?

Not if it was blue smoke. Perhaps a fresh cat is catching it, but that won't be for long until the new cat is saturated with oil smog and dies as well I reckon. Valve stem seals may be worn allowing oil to seep along the valve guides.