I own a 1990 745 turbo with 286,000kms on it. Ever since summer, the car's run rough when cold. Idles very low. (Idle is normal when the engine is warm) And I can smell an extremely strong rotten egg smell of the cataletic converter at stoplights even when I've been driving conservatively.
This would indicate to me that there is unburnt fuel coming out the exhaust.
I think the low idle when the engine's cold and the cat working overtime are related as they happened at the same time. I might add, the car's getting AWFUL gas milage.. seems to have plenty of power, though.

I thought I may of melted a valve last summer when I pushed the engine extremely hard in very hot weather. (maintained 15psi boost for several minutes during extremely hot weather), But I just did a compression check and all 4 cylinders are all reading 162-170psi. That all seems normal.. I'm relieved, but that doesn't solve the problem. The plugs looked perfect when I pulled them out to do the compression test.

One of the first things I did was borrow my dad's fuel mixture readout that connects to the oxygen sensor lead to check the mixture. The mixture is right where it should be. I think maybe the oxygen sensor is bad. But I can't afford to replace it at the moment.

Any ideas? I'm desperate for help soon as I have to take the car through emissions testing soon in order to get insurance.

Thanks!