Possible Vacuum Leak?
If my '89 325i is going down a hill, and I'm not pushing the throttle, eventually the check engine light will pop on. It also happens when it's idling after it has been running for more than five minutes. The fault code is 1222, which is lambda control 1. This however, doesn't tell me too much, just that it's running either too rich or too lean for longer than 10 seconds. What is the best way to trouble shoot and find the problem?
To my understanding this could be a defective engine temp sensor, bad injector valve, incorrect fuel pressure, secondary air leak, defective air flow meter, bad compression, or bad intake valves. I would hope something as simple as replacing the O2 sensor would do the trick, but I'm thinking it's a vacuum leak somewhere and I don't know how to find it.