2002 Cadillac SLS

2002 Cadillac SLS V8 All Wheel Drive Automatic 85000 miles

I have had a sudden drop in my MPG average. It has gone from 22 mpg to 12.6 mpg and is running hard. Any idea what this might be or how to fix it?
July 27, 2009.

Is there a check engine light on, with that significant of drop you should have some indicator. By running hard, what do you mean?

Jul 27, 2009.
Yes my service engine light is on, the guy at the hop said it was bc my airfilter senor was unplugged but when he plugged it back in my car made this weird noise and souned like it was going to die. He had a hard time putting the cover on the air intake valve and im wondering if that is the problem.

Jul 28, 2009.
It is common for the vehicle to change running at the point the mass airflow sensor is reconnected, because it goes from default value, back to calculating. By air intake valve, do you mean the odd shaped cover that makes up the top of the air filter housing. That could cause a problem since it is after the mass airflow sensor. If you have a leak after, that would be an unmetered air leak, and would change fuel calculations with little symptoms noticeable.

Jul 29, 2009.