Hi guys,

My indie mechanic recently replaced the water pump and thermostat in my '90 300se, and I've conincidentally started to notice a huge drop in fuel economy. Whereas I used to get around 20MPG, I'm now getting around 12MPG! I've noticed that the engine runs a little cooler than before (80C), and I'm not sure which thermostat he used.

The first thing I thought to check was the duty cycle. I found that the car was running a little too rich, so I adjusted it down to around 50%, fluctuating. I'm going to check the coolant temp sensor and air intake sensor later today, but was wondering if it's possible to get valid duty cycle readings if the coolant temp sensor is faulty? Also, what else can cause such a huge drop in fuel economy?

Thanks for any input you might have!