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Gas Mileage

I have a 2004 Grand Marquis with 161,000. I was getting 23-24mpg in town/city and up to 28mpg on long trips,till this past fall. Now I get 18-20mpg combined. Eveything is changed at the proper intervals wih synthetics. I have no indicator lights showing and my catalitic converters are fine. I use SeaFoam every 3,000 miles and a K&N airfilter is cleaned with each oil change. The spark plugs were changed at 96,000 miles. Help!

and a K&N airfilter is cleaned with each oil change.

Well it is certainly possible that unless you have been very careful cleaning and re-oiling that filter, it could have damaged your MAF (I believe your car has one and that would explain a loose of mileage and maybe power) Unless you have done a lot of engine modifications to increase power that K&N filter is not likely doing you any good and the can do some bad. The cost of OEM type replacement filters are not high. I suggest returning to the recommended filter. I also don’t see why your would want to use Seafoam ever 3,000 miles (Although I doubt if it is causing any problems) unless your driving habits are very unusual and if so, I suggest you let us know more about them.

That said, I would say you are due for new plugs and I would also do new wires.

How are you measuring the mileage? Do your numbers include more than one fill up?

You are cleaning the K&N way too frequently. They only require cleanings every 30k-40k miles. Also you’re not getting much benefit from the using the Seafoam so frequently either, once a year at most is more than adequate. It should be noted that Seafoam is known to be hard on spark plugs, if you’re using it every 3000 miles your spark plugs could be fouled by now. Don’t get me wrong though, Seafoam is an excellent product, and I’ve had success with it. According to the EPA your car should be getting about 15 MPG city and 23 MPG on the highway. The mileage you are currently getting is right on the money.

Your Car’s MPGs Will Drop A Little In Colder Weather. That’s Normal.

Also, the first of cooler/colder weather is the time to air-up your tires. If you haven’t done this lately, you may find them all a little low on pressure, also a weather thing. Bring the pressure up to specs. That could help a little. I’m not sure what your climate is like, but any extra warm-up time before driving off takes its toll on MPG, too.

And…
Car makers are aware of the problem of oiled air filters—excess oil contaminates the MAF and the rest of the engine intake tract. So, clean’em. Use an MAF (Mass Air Flow) sensor cleaner, and Throttle Body Cleaner on the throttle body bore and throttle plate.