Poor gas mileage and losing oil somewhere

I have a 2002 Ford 150 Supercrew truck which used to get 17mpg in the city and 22mpg on the highway when I purchased it (new). Over the past few years the mileage has decreased dramatically. At 100,000 I had a tune-up, fuel filter change, transmission flush, oil change (oil and air filter change), front transaxle oil change, and shortly thereafter new tires. I was told by repair shops that nothing could be done about the poor mileage unless a “Check Engine” light was on to aid in diagnosis. This past weekend my gas consumption increased quite a bit: on Friday I had a full tank of gas and on Monday I had less than half a tank left after about 60 miles of city driving. I used about 15 gallons. So, today, I took the truck to the dealer where I bought it and they hooked up a computer that displayed the real-time and average gas mileage while the vehicle was driven. On the highway, the average gas mileage was 18.5 mpg and in the city it was 16.3. The city mileage is very close to the sticker claim of 17mpg, but even the highway average was not too far off from the sticker. Here’s what’s really weird: when I’m driving on the highway, my gas mileage is better, as I would expect, than the city mpg, yet the computer numbers indicated the opposite, and the computer numbers were very high compared to what I am experiencing. I am not leaking gas from the tank. I don’t drive like am maniac and I don’t tow anything. I haven’t used four-wheel drive since last winter, and when I do, I expect the mileage to go down.

Secondly, about every two weeks I am putting in a quart of oil. There is no sign of oil leaking on the engine or burning - the dealer mechanic agreed. I am stumped and so are they. Any ideas?


Bill Dickinson

I have a hard time seeing a full sized truck getting 17 MPG in the city and 22 MPG on the highway. The revised estimates are 14 MPG city 19 highway for a 2WD with either the 4.2L or 4.6L. What you’re getting is pretty close to that.

I’ll agree that your “before” miles were unusually good and the reduction you’re seeing could possibly be explained by changes in traffic conditions, temperatures, local fuel blend, etc.

The oil consumption is more problematic. First off, how many miles do you drive in the two weeks it takes to consume a quart of oil? If you have a long commute, it could very well be an acceptable level of usage. As for where it’s going, if it’s not leaking it’s gotta be burning and on modern cars the pollution controls will clean up oil-burning smoke, so you need to be burning a LOT of oil before you see any blue smoke these days. Unfortunately, the two symptoms of lowered fuel economy and increased oil consumption could well point to a piston ring problem. Your next step probably needs to be to get a compression test. Just out of curiosity, how many miles are on this engine?

Thanks for your comments. The engine is a Triton 5.4 L v8, and yes, the sticker claimed 17mpg in the city and 22 mpg highway which I got for the first 3.5 years.

I asked the mechanic about doing a compression test and his response was: " You shouldn’t have to worry about that until you have about 300,000 miles on the truck". I have 118,000 miles on it. In two weeks time I drive 400 miles - a mixture of city and country driving. I hadn’t considered the affect of the pollution controls (masking the burning oil smoke) so the compression test seems to make sense. I’ll take it to a different mechanic this time. Thanks for your comments.

Should and do or do not are not the same thing.