My MPG had dropped pretty low in recent weeks, down to about 16mpg and I figured it was my O2 sensor going bad even though check engine light didn't come on. I replaced the O2 sensor and mileage appears to be back to normal but I won't know for sure until the whole tank is used.
I checked other sensors (temp sensor, coolant temp sensor) and they checked out ok. Lambda is next, and I'll adjust it if it needs to be, then EHA. I figure I should be getting at least 20/21 mpg in city/hwy mixed driving.
I'm just now getting the hang of using this handy multimeter -- great tool to keep your fuel system optimized
'91 300E, 215K miles