Hello to all. Long time reader first time post. I have a 99 F-150, 4x4, 5.4L, SCAB, automatic, K&N filter, Flowmaster muffler w\2 1\2" pipes single exhaust. I bought the truck in 2001 w\46k miles and the mods were already there. Here is the deal. The #6 COP went bad @ 122k miles. Since I had not had a tuneup, I decided to go the whole thing while my local Ford dealer fixed the COP. New plugs, PCV, fuel filter, crankcase ventilation valve (not sure what that is). Before the COP failure & tuneup I was getting 15.5-16 mpg every tank. After the tuneup my mpg dropped to 12.5-13.5 mpg and developed a shiver @ idle. I have taken the truck back twice. The first visit I was told the PCV valve hose was not connected properly. That fixed the shiver, but not the mpg problem. The second time back I was told they hooked up their flight recorder and did a test drive. They told me I should be getting 17mpg highway and they can't find anything wrong. They did mention that the MAF was reading a little below the factory voltage and that it was dirty. They attributed that to the K&N filter. My odometer/fillups gravely differ from their statement. I know these trucks are not known for good mpg, but why the sudden drop after a tuneup? I drive 30 miles (highway 55-65mph) to work oneway daily. Have every since I bought the truck. I don't have a heavy foot as all gear shifts are between 1800-2000 rpm's. Any comments will be greatly appreciated. Sorry for the long post. Wanted to get as much info as possible Thanks in advance

 

You may try reading in the technical section how to clean a MAF wire. Be careful, though they are fine.

I had a similar experience with my Ford dealer, with my first and last tune up I will ever get. I have since learned that these vehicles just don't need a tune up in the traditional sense. Plugs, wires, cleaning out the throttle body, and sensors is about all you can do. IMO I would never run an K&N filter, but I cant say that is your problem. I just prefer to change my paper filters when they need it.

Hope you locate the cuase.

 

Just guessing here but whoever made those mods before you bought the truck may have put a chip in the ECM (computer). That chip has to come out before you get a tuneup. If the chip stays in then the Ford tuneup will reprogram the ECM incorrectly and screw things up like mileage and performance. Good Luck

 

Programming the ECM isn't part of the tune-up procedure. ECM programming is done per TSB's on the vehicle. Ford reprogramming the vehicle with a chip in it doesn't matter because the chip would override the ECM tune before and after a reflash. I doubt he'd have a chip it in and not know it because he'd have to run premium with a chip in it.

 

I had a JET chip in about 4 years ago when the MAF sensor failed and I took the truck to Ford. I forgot that the chip was in and when Ford replaced the MAF sensor they told me they did some kind of quick tune (no plugs) and the truck ran like sh#@t. I pulled the battery cable and the chip and left it overnight. Replaced the chip and cable and the truck ran fine. One year later I pulled the chip just to see if there was any difference and the truck ran even better. I hate it when I get fooled into thinking that some gizmo is going to magically add HP. My experience for what it's worth.