1990 240- Running extreamly rich. Plugs are totally black after a week. Had check engine light and changed the plugs, the light went away. The Whoopie was short lived. Three days later the light came back, Geeer. As the car is new to me I don't know where anything is, where is the o2 sensor?????????? I don't have a book yet.

Da Grump

There is a boss on the down pipe between the exhaust manifold and the catalitic converter. It has a bolt screwed into the boss. Is this where the O2 sensor is supposed to be??????

Da Grump

That's the test port. I wish an oxygen sensor would fit. I forgot which year you said your 240 is, but my instinct tells you to look under the car from the driver side at about the same place, just before the cat.

Yes, a '90. That's it.
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Art Benstein near Baltimore

Fuel pressure regulator
Oxygen sensor
Engine Coolant Temp Sensor
ECU or AMM (rarely)

You could look for a Bentley -- often on Ebay. But this site has a great FAQ resource, better than the manuals for many things. Don't be fooled by the title; most of the stuff applies to 200 series too:
700-900 FAQS

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Art Benstein near Baltimore

But you do have OBD to help out. Read the codes to diagnose.

between the cat and the downpipe. it would be my guess that it is the 02 sensor.
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Chuck B '89 245 DL, '85 244 DL