My 97 850 is getting terrible milage and there is an unusual odor in the exhaust. I bought the car about 5000 miles ago and it had a check engine light problem about 500 miles after purchase. The airpump had the normal problem of being full of water. Changed the air pump, egr valve and airpump relay. The check engine light was reset and there are no other codes at this time. Also I couldn't verify if the timing belt had been changed, so that was replaced too.

I suspect the catalytic converter might becoming clogged from the previous air pump failure. Is there any way to clean out the converter?? Also I'm not sure if the O2 sensors might be the problem. Either way with gas prices soaring ($48 for refill of 16 gallons after 260 miles - mostly highway) I need to resolve the issue promptly. I don't want to blindly replace the converter unless I really have to.

Has anyone else experienced this problem??

 

I have the same year and had the same symptons this summer. The issue is emissions related. For me, it turned out to be two things. First, there is an emissions pump located near the engine. It is primarily used just after the car is started to assist with clearing the system before the car has a chance to get to operating temp. This pump is of a poor design and often gets water logged and ruined over time. Plus the can just wear out.

The car ran fine without this pump, and threw no codes (no check engine light). Over time, the Oxygen (02) sensors gave out. THAT is what leads to the black smoke, sweet, acrid smell, and poor mileage.

The 02 sensor replacement is pricey. I went with Bosch replacements. My guess if that a dealer will charge you ~$700 to do this repair.

Good luck.