Help Needed
 

94 S 350
At 600 rpm car idles fine. At 1000 rpm shakes and stumbles feels like a miss. At 1200 engine smooths out no stumble. car got 26-28 mpg now falling 20-22 also alot of blue smoke when cold gone after warm up. ? What mite this be? Where do I start?

Give us more info. What has been done to the car service wise? What is your mileage on the car? Are you keeoing up with routine service? Is the car new to you? etc...

Bet it's someting simple like an O2 sensor.

And S350 - that's a diesel remember