Why did my MPG drop so fast?

[imq707s]

I've got a 1997 Mercury Mountaineer (just like an X) with 70,000 miles,AWD and a 5.0 V-8. The only mods I've done to it are to swiss-cheese the air box and add a K&N air filter. All through the summer I've been able to get around 275-300 MPG out of a tank full of gas. Now in the last two weeks I've noticed that I can't even get 200 out of a full tank. I don't have a check engine light on, and I've not noticed any decrease in performance. What could be wrong? Could an O2 sensor be going bad? What about cleaning the MAF? Any ideas? Thanks. Also, would an aftermarket exhaust help milage?

 

[DesertSpive]

I think it is your O2, I responded to your post in the Main forum.

--Sean

 

[asp84]

Don't they also start oxygenating the gas around this time of year... to cut down on emissions? Just a thought... Definetly check your O2 and computer codes.

Al

 

[JoeC5]

I have a 93 that cant even get 200 miles on a full tank of gas driving at 55mph. Very soon I will have to fix this problem b/c it will pulling it out of storage, funny part is I had it checked for codes and nothing showed up

 

[DesertSpive]

Oxygenated gas definately plays it's part in low fuel economy. Keep in mind, a bad O2 doesn't always throw a code. It doesn't know its bad, it thinks it is on startup/oopen loop all the time and runs rich.

--Sean