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02 Regal LS w/ 270k miles. Started to see the mileage decline and noticed that STFuel trim would bounce around -3.1% to +3.1%. LTFuel Trim would run 10.8 - 16.1% and sometimes as much as 20% when in 90 deg.F weather. Car was a bit sluggish at times, but no codes were set. Mileage dropped from 26.5 down to 21.6 mpg. Ruled out a partially clogged cat as the engine vacuum at idle would be 20" of Hg (mercury) and while driving depending on the accelerator position and speed would be any where between 15" of Hg and 25" of Hg. I also removed the EGR valve to ensure it was operating correctly. It was. With the EGR valve off I took a pressure reading at the exhaust port with engine running~.5 pounds. So no clogged cat issue.

IAC motor replaced - it would click, click, click with Key On Engine off. Removing it from TB it came out in 2pcs.
Fuel Pressure Regulator replaced as they are known to leak. LT Fuel trim would decline to acceptable values when vacuum line was plugged. Fuel pressure with vacuum line unplugged 54psig. Fuel pressure with vacuum line attached to fuel pressure regulator 48psig @ idle.

O-rings in connector to back fuel rail (fuel return line) replaced, they were brittle. Replaced the front fuel rail connector O-rings as well. Plastic PCV housing and PCV/Spring/O-rings replaced as clip that retains MAP sensor had broken off. All these parts changes lowered the LT Fuel trim a bit but not enough to get it where I expected it to be +/- 3.1%

The problem turned out to be an out of tolerance MAF switching frequency. My local mechanic put his scanner on it and saw that it was below spec and would eventually set a code. Went to a local salvage yard and picked up used OEM MAF Sensor. That did the trick! LTFuel trim is as expected -3.1% to 3.1% while on highway. The combination of both ST & LT Fuel trim on the highway is around 0%. The MAF sensor on the car was not OEM, I had replaced it with a NAPA one 5 years ago. When it comes to engine sensor electronics It's best to stick with OEM. Lesson learned.
 

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You may have just had a dirty sensor that required spraying with MAF/TB/QD Electronics cleaner. This happens often with these cars, either with PCV fumes from high mileage or aftermarket oiled filters having too much applied. The filaments will become coated and output signal will be lower, causing a lean condition (LTFT's will climb trying to compensate with more fuel at a lower MAF hz reading, and STFT's may momentarily assist at various points, depending on severity of condition).

Either way, glad you're back to good health.
 

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Thanks for info. I kept the other MAF sensor and will try cleaning with CRC MAF cleaner. I filled the tank again after driving 106 miles (highway). It took 3.488 gallons. 30.4 MPG. Also noticed considerable drop in under hood engine heat after turning off car.
 

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Thanks for the analysis of the aftermarket MAF sensor. Could you let us know what happens if you clean and reinstall the old sensor to see if that made a difference? Also how much oil does your 3.8L use? I use 1 quart to 1000-1200 miles on my 230K engine. How fast did you drive for the 30 mpg? Glad you found the problem and shared so the rest of us can learn can learn.
 

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Thanks for the analysis of the aftermarket MAF sensor. Could you let us know what happens if you clean and reinstall the old sensor to see if that made a difference? Also how much oil does your 3.8L use? I use 1 quart to 1000-1200 miles on my 230K engine. How fast did you drive for the 30 mpg? Glad you found the problem and shared so the rest of us can learn can learn.
I'll post results when I clean the aftermarket MAF & reinstall. Oil consumption is low around a quart every 2-2.5k miles. Since owning @ 13k miles, all oil changes have used Mobil 1 10W-30 and AC PF47 filter and changed around 6.5k miles. MPG speed was 65-70mph and cruise control was off. Plugs & wires need to be changed as they have better than 100k miles on them. I drive about 30k miles a year. I picked up a Tite-Reach wrench for the back 3 plugs. It was a night mare 3 years ago, especially for the plug behind the O2 sensor.

Every repair part installed is logged and car is always garaged, it costs me about $0.03/mile to operate excluding gas & insurance.
 
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