Well, I just picked up my '03 MCS from the shop. $700 later and the SES light is still on.

Great mechanic, but they were still scratching their heads when I left. Trial and error gets expensive. Throwing a post cat code, replaced the O2 sensor. Then found leaks and patched, still throwing some code that sounds like the cat.

The mechanic mentioned that he wasn't certain, but it seemed the crankcase might be building up excessive pressure, and the PCV valve could be the issue. (His gut was that it was the cat, though)

I tested when I got home, and with the filler cap off and the engine running, I was a little surprised at how much air shoots out of the oil filler hole. You can definitely feel it fighting a little when you screw the cap on.

Is this normal? My recollection while growing up was that the classic Chevy 350 produced nothing when the cap was off.

 

Do you have an Oil Catch Can (OCC) installed?

 

I saw that online while researching this problem. No, I do not.

Btw, the mechanic also mentioned that my car was "sluggish" for an 'S.'

I was perplexed by this. Unless it has been slow degradation, it appears to me to drive the same as when I bought it ~3 years ago.

 

Did you guys check out the innards of the CAT? Sometimes, as the cat gets worn and the steel liners inside get worn, they can dislodge and block the outlet. That would cause sluggish acceleration and more crankcase pressure.

 

Have you had a compression test or a cylinder leak down test?
Worn cylinder bores or worn valve guides are often the cause of
blowby and high pressure in the crankcase. I would also change the PCV
if there is one on an MCS and check that the crankcase ventilation system is not clogged or pinched. You could also try a "cat delete" test with
a spacer/adaptor to move the behind the cat O2 sensor out of the exhaust flow to prevent check engine light (or SES in MINI world). One trouble with the "cat delete" test is that the engine typically runs so much freer that many owners don't want to put the cat back on!