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2005 Mercury Montego Drop in Fuel Economy, Lack of Accerleration
I have a base model 2005 Mercury Montego with a 3.0L V6 engine and about 135k miles.
About once a month the Check Engine light will come on for codes: P0171, P0174, and both pending.
P0171 = System too Lean (Bank 1)
P0174 = System too Lean (Bank 2)
The car is idling normally, and seems to drive without problems. However, I have noticed a steep loss in fuel economy (from about 21 MPG to 15-17 MPG), and a periodic lack in acceleration. The car doesn't seem to want to accelerate like it usually does. About 75% of the time, the vehicle has a hard time making it to 70 MPH when getting on the highway, and that's with the pedal to the metal. The engine will get up to about 4000 RPMs and seem to struggle to go any higher. However, about 25% of the time the engine will shoot to 6000+ RPMs when the petal is put all the way down and I have no problem making it above 70 MPH. Weather doesn't seem to play into this behavior at all.
I am NOT mechanically inclined, but do know a little. I have a relative who is a mechanic who suggested a vacuum leak. However, when he checked he did not hear or see a leak. I also had the Fuel Filter replaced, cleaned the MAF sensor, cleaned the Throttle Body, changed the spark plugs, and a few other small things that I can't think of off the top of my head. I have also tried running fuel treatment through the gas to no prevail.
I deliver pizzas part time while I attend college and the lack of power and fuel economy is killing me right now. Does anyone have any suggestions?