I have a 91 740 wagon (non turbo) and I've recently had the check engine light come on sporadically. There are also been a change of smell in the cab when this has occurred. I checked the code and got a 221 reading. The chart on the FAQ identifies "Fuel trim (lambda control) too lean at part load." I'll appreciate it if a better mechanic can help me interpret what all that means. I recently had all of the belts replaced. Is it possible that my mechanics left an air leak in the course of that operation? There's nothing that appears obvious to me.

Thanks in advance for your responses.