Tune UPs

I took my truck in for a tune up and they only changed out the spark plugs. The mileage on the truck is 81K. It is a 2004 F150 pickup 4 x 4 with a 5.4 V8. The mile/gal has dropped off from 18 or 19 to 13 to 14. The spark plugs installed are good for 100K miles. What else could I do to help increase the gas mileage back to what I was originally obtaining?

Did you expect more to be done than just a plug change? The phrase “tune up” is too generic leads to misunderstandings and does not apply to your 2004 F150. I have the same exact truck mileage is 16mpg I figure the margin of error in my mileage computations are plus/minus 2mpg.

Why do you not tell the mechanic that changed your plugs that his work caused your mileage reduction? Are you afraid he will tell you that a plug change will not cause a mileage reduction? If he did he would be correct.

I am sure plenty of people will say “I was using XYZ plugs and the mechanic installed ZYX plugs and terrible things happened and it wasn’t until I removed those terrible ZYX plugs and reinstalled XYZ plugs that things returned to normal”

I was told that at 75 to 80K miles that a tune up was recommended. So with the mpg dropping I figured that maybe it was time for a tune up. My pickup has on average always had fairly good mpg for a truck at 18 to 19 mpg. With my mpg dropping I figured it was time for a tuneup.

The words “tune up” are essentially meaningless where modern, computer-controlled vehicles are concerned. The computer constantly monitors engine performance and adjust on the fly. All you do is replace worn parts like spark plugs, filters, etc.

There are many things that affect fuel mileage. Tire pressure, wheel alignment, weight of vehicle (including its load), ethanol content of gasoline, maintenance history, weather, driving environment, etc.

Did the mileage drop after the “tune up” or did you get the “tune up” because the mileage dropped?

It could also be a sticky thermostat, dirty air filter or dragging brakes.

Other than changing the oil filter/ air filter is there any thing else that I should change out that would affect the mpg? Again, the truck has been routinely getting good mpg until the last 5K?

Repair facilites don’t emphasize that the “tune up” era is gone. Perhaps they want to maintain a traditional revenue generator. Or perhaps they don’t want to expend the time and energy to explain this to the public.

Tell the public the real story on tune-ups they look at you with a “What the…” expression.

I certainly would get those plugs out at or before 80K, you don’t want them siezing and they do wear out stressing the rest of the ignition system.

My best 2 guesses are clean the injectors and check tire pressure. The next step would involve diagnosis with high tech stuff to determine if o2 sensors or computer is the problem.

You need to clarify something.
You state the truck got fairly good gas mileage at 18 to 19 MPG and in the same paragraph state several times the “mpg was dropping”.
So what was it dropping to before you took it in?

If the truck is apparently running fine then the plugs are not the problem and the word “tune up” is a misnomer anymore.

If your mileage has really dropped off that much then I would think the Check Engine Light would be on due to an excessively rich air/fuel mixture.

Your truck is rated at 13 MPG city, and 17 MPG highway. 13-14 MPG sounds about right for suburban driving. I have never seen a 2004-2008 5.4L 4x4 F-150 get 19 MPG without a significant tail wind on the open road, and I used to sell them. For some strange reason we get an unusual number of people posting here saying that their “brand X” car used to get 35 MPG and now for no known reason it’s getting 23 MPG. And when you look up the revised EPA estimates it turns out that “brand X” car is in fact rated to get 23 MPG. Strange.

Anyway, You could see if there are any codes stored. Somes the ECU will have a code stored but the SES light won’t come on. Also if you equipped the truck with different sized than stock tires that throw off the speedometer and odometer causing fuel mileage calculations to be off.