I just just went down that road with our 1990 740. The car still ran good, but the exhaust was really heavy of rich gas fumes. I pulled my hair out trying to figure out what was causing it. (I was thinking bad Cat, poor injector spray pattern, you name it.... My imagination was running wild.) The wierd thing was that it still got good MPG on the highway. The spark plugs still appeared a nice tan color. I then noticed that if I drove it for a long period of time, it seemed to be not so bad. It was the short trips to the store, or, driving through the fast food places - now that was embarassing.

So I decided to check out the engine temp sensor. I did the old "heat water on the stove with cooking thermometer in the water and measuring the sensor resistance at various temeratures" test. I charted my temp vs resistance readings on graph paper and connected the dots with a hand sketched curve. I then did the same with the data in the 700/900 FAQ, and sure enough, there was a significant difference in the two curves. What was interesting was that when I took the sensor up to about 190-195 degrees, it started to bend over toward spec. That may have been why the car seemed to run cleaner when it got really good and warmed up.

So the other day I went ahead and sprung for a new sensor. After driving it for a couple days now, it appears to be cured. (Still too early celebrate, but it's encouraging.)

One other thing I noticed while working on the car (for Regina owners) - I found the rubber ends of the MAP sensor vacuum tube to be starting to rot. If the line leaks bad enough, it probably could cause the sensor to see low-vacuum or even no-vacuum, which would also cause an over-rich condition.