Poor Gas Mileage 2004 Mercury Grand Marquis
I’m also experiencing poor gas mileage now in winter. 2004 Mercury Grand Marquis GS with 102K. I fill up once a week, around 40-50 miles worth of travel to and from work, short trips both ways. I usually get 8-11 MPG average. Today I filled up, I wound up getting 6.96 MPG! Piss-poor consumption. I’ve been told it’s due to the very short trips making it not get up to operating temperature and thus running inefficiently. Mechanically it’s in tip-top shape, has no CEL or codes, all live data is great (LTFT and STFT between 0.8% and 3%) so I just accept that it is whet it is. FWIW, I do use the remote start a lot and sometimes warm it up about 10-15 minimum, so I’m sure that is definitely lowering the overall MPG.
Idling your engine for 10-15 minutes every day will absolutely destroy your gas mileage. You're wasting approximately 3L of gas for every hour you idle. That's 3/4 of a gallon, give or take. 5-7 days of that, and you're definitely wasting away 2 gallons of fuel doing nothing. Not to mention idling in traffic and stopping and starting while city driving. https://ecomobile.gouv.qc.ca/en/ecomobilite/tips/idling_engine.php#:~:text=The%20estimated%20fuel%20consumption%20of,litres%20of%20gas%20per%20hour.
Stop warming up your car for 10-15 minutes and start driving it after a minute or two. It warms up faster and you are netting some mileage while burning the gas, the car's not sifting there doing nothing.