I have 51,000 miles on my 1997 grand marquis and suddenly the mpg has dropped considerably. The dealer replaced the "mass air flow sensor valve" cleaned fuel injectors and still no improvement. Any advice. Salvatore

What is the average miles per gallon you are getting? And is the vehicle equipped with Automatic Temperature Control?Edited by: MercAdvisor

I am getting about 14 MPG. Very little city driving. Yes, I have automatic temperature control. Salvatore.

Do you use the auto temp control or do you manually make your settings if you use the auto it will switch the AC on and off while driving and can draw down your MPG.. Your mass air flow sensor could be dirty but shouldnt cause much of a MPG decreaseand check your air filter

Thank you I will try manual control. The mass air flow sensor has been replaced and air filter is clean. My highway MPG is showing from 22MPG to 26 on the instant economy reading and in city it shows 20 to 21 MPG. When I calculate the gallons used versus miles travelled I average out to 15 to 17 MPG. Just a few months ago I actually averaged out to 23MPG. I want to mention about 2000 miles ago I was having torque converter "rumble" and tranmission was flushed out. My problems began after that. Perhaps you can give me more advise. Thank You. Salvatore.

Actually 17 mpg in the city isnt terrible it is actually average for those vehicles especially as we get into the colder months. I dont know where you live so maybe you dont get cold and that may not be an issue.. As for the transmisison rumble the that is a common concern for the GM/TCAR it happens as the fluid breaks down and causes the rumble between the last shift into overdrive.. Unless the transmission is not shifting into overdrive now that could definatly have an effect on your gas mileage significantly dropping it. Make sure you Overdrive off light is not on and or flashing... Did they replace the Mass Air Flow sensor or Idle air control valve? I see you said mass air flow valve it is only a sensor just making sureEdited by: MercAdvisor

They changed the "mass air flow sensor valve. Now that you mention it, I am getting an erratic shifting and it feels like the tranny is in 2nd gear and sometimes shifts into drive at 35 mph or higher with normal acceleration. The dealer mentioned to me that the fluid could be breaking down. Would you suggest I change the tranny fluid just to be sure? Salvatore I live in Syracuse, New York and it does get cold up here. Thanks again.

If you just had the fluid changed 2000 miles ago no i wouldnt recommend another transmission flush however you may want to have it looked at to see if it is shifing through all the gears

Allright I will have them check out the transmission and see what they find. I read somewhere on the net that if you disconnect the battery cables and change the battery the computer will have to reprogram itself. I am 84 years old and never heard of it before. Any truth to it. Thank you, Salvatore.

If you only change the battery the computer will hold its program and usally will for a long period of time.. That is just a myth... UNless there was some type of interference or the computer needs a reprograming for udpated software reprogramming is not necessary..

That you for the info. I will let you know how I make out with the tranny. Salvatore.

Better fuel economy can be acheived by turning off overdrive until you reach approx. 65 mph on the freeway then turn the overdrive back on again. It should be easy to remember just turn it of every time you shift into gear.

The default position for the Overdrive is "ON!"
There is NO WAY that turning the Overdrive "OFF" will improve your fuel economy on the FOMOCO 4-speed Automatic transmission.
The transmission is controlled electronically, and knows exactly which gear it should be in according to the engine speed and load.

Your had your transmission "flushed out". After doing so, your fuel mileage decreased noticeably.
Problems such as yours are the reason why you should NEVER FLUSH the Grand Marquis transmission !!
The "Tranny Flush" is a way for Dealers and Shops to make money very quickly by eliminating steps in the "Transmission Service" procedure.
They win, YOU lose !!
Your Marquis' auto transmission is properly serviced by removing the transmission pan, cleaning the internal magnet, changing the internal filter,
and drain & re-fill the the Torque Converter.
Trans Pan Gasket is RE-USABLE, and requires NO SEALANT.
The complete job requires 12 quarts of MERCON V -type trans fluid and a MotorCraft (OEM) transmission filter.
The Motorcraft filter comes with NO gasket, so the original gasket is cleaned with WD-40 and re-used.
If your original gasket is damaged, replace with MotorCraft gasket ONLY !! NEVER use a cork gasket.
Changing the Trans Filter at 35,000-mile intervals is all that's necessary to keep your transmission shifting smoothly and without slippage.
If appearance of the trans fluid is bright red, and has no "burnt" odor, it's not necessary to drain the Torque Converter at every filter change.
For those who do their own work --
When trans pan is off, it's a good time to install a B&M Drain Plug Kit -- saves work and messy cleanup on next filter change.
For snappier shifts, I installed a B&M Shift Improver Kit, and for long life, a B&M Transmission Cooler.

A 15 year old thread from the dead.