scorron's comments and questions actually started me thinking on this one, this post is to confirm his comments about gas mileage reduction due to head pipe leaks between the engine and the O2 sensor.

Since I put this car back together last year, the gas mileage has continued to drop from an average of 330 miles per (13 gallons) down to 245 miles per (13 gallons). What has changed? Well, the crack in the head pipe has been getting worse and worse. On Monday, I went to start up the car and it was reluctant but fired and died a few seconds later, smell of gas was noticeable but not overbearing. Got it started drove to work but it didn't run too well at first. Drove home and pulled the plugs, they were coated in black soot and were a bit wet and "gassy". Swapped plugs and the car ran great.

When there is a crack between the engine and the O2 sensor, O2 gets into the exhaust and is picked up by the O2 sensor, this causes the ECU to "think" the engine is running lean and it adds more fuel, ergo, engine runs rich.

Today on the way home, it finally threw the SES light, any guesses on the code? 2-2-1... the "dreaded" Fuel system compensating for extremely rich or extremely lean air/fuel mixture at cruise. It also threw a 2-3-1, same thing minus the extreme comment (which doesn't set the SES light).

In other words, car is going to the exhaust shop next Thurs. for a complete new exhaust system including cat and O2 sensor... heck, some of these pieces are 15 years old! Yes, oil change will follow as well (smells a "bit" gassy as well).

Hope that someone finds this informative and helpful!

jorrell
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92 245 245K miles, IPD'd to the hilt, 06 XC70, 00 Eclipse custom Turbo setup...currently in pieces