MPG's down from 20 mpg to 10 mpg

Sigma10 · #1 07-20-2012, 11:45 PM

Hello everyone,
I have an 03 4.2 F150. It has 198k on it and other than a couple of minor things has been a good truck. Lately though I have been suffering from horrible horrible gas mileage. The entire 5 years I have had my truck I have consistently gotten at least 18 mpg average. The last month or so though I have fallen down to 10 - 12 mpg. I am basically driving my truck and watching the needle move. I have no codes, and from what understanding I have my fuel trim lvls both short and long term seem to be running normal (obviously, no codes.) Therefore I don't know how to diagnose it. Can anyone help point me in the right direction? Thanks

jbrew · #2 07-21-2012, 01:34 AM

Disconnect the battery (Negative First), Change the Air filter, -Clean the MAF sensor. Reconnect the battery (POSITIVE first). That should help, -maybe allot!

Raptor05121 · #3 07-21-2012, 02:15 AM

And go under the truck and tap on the catalytic converters as well. I had my left side main cat go out. My truck threw a code and Ford diagnosed it to that. I was getting 7-8mpgs with a bad cat.

Also check for vacuum leaks.

glc · #4 07-21-2012, 07:08 AM

My mileage went down the toilet when I needed plenum gaskets and isolator bolts. I didn't know what was going on till it finally threw codes.

How many miles on your plugs and wires, and what brand are they?

TruckGuy24 · #5 07-21-2012, 07:14 AM

Also O2 sensors should be changed. All them

glc · #6 07-21-2012, 09:59 AM

Quote:

Originally Posted by TruckGuy24

Also O2 sensors should be changed. All them

Maybe, maybe not. Mine are all original at 157k and I'm getting expected mileage. If they are that far off to cause that bad a drop in gas mileage, they should be throwing codes and putting the fuel trims way out of whack.

Galaxy · #7 07-21-2012, 12:20 PM

Quote:

Originally Posted by glc

My mileage went down the toilet when I needed plenum gaskets and isolator bolts. I didn't know what was going on till it finally threw codes.

Hey George, what's that all about? I've never heard of this or the bolt thing? What was it causing/what code did it trip because of this?

4.6heavymods98 · #8 07-21-2012, 01:16 PM

If its your cats let me tell you I am very happy with my two magnaflow hiflow cats #94306 two will replace all 4 of your cats give your truck a nice growl and a little more power that you will actually notice. Yours MPGS might actually go up from stock as well.

But these other guys are giving you good advice already. If those are your original 02s iI would change them regardless with that many miles on them.

You could also check your PCV valve. Thats simple enough mine was bad and never threw a code.

jbrew · #9 07-21-2012, 03:43 PM

Quote:

Originally Posted by Galaxy

Hey George, what's that all about? I've never heard of this or the bolt thing? What was it causing/what code did it trip because of this?

No worries for you. It's a V6 thing.

Sigma10 · #10 07-21-2012, 08:13 PM

Thanks for the advice guys. So far I replaced the air filter, cleaned the MAF disconnected the battery and tapped on the cats to see if there were any issues. I'll run a tank and see how it goes. My hope is next weekend to probably go ahead and change the two 02 sensors on the heads. I have changed the two on the y before but not those. Something else I forgot to mention that I have been noticing is that my fuel gage is not reading the same thing when I start the truck as when I last shut it off. I havent thought too much of it being that maybe its due to being old and analog. However I got to thinkin that maybe its dumping alot more fuel than normal to ignite. Any thoughts. Is there something there thats worth looking into. Thanks again.

jgger · #11 07-21-2012, 09:39 PM

Quote:

Something else I forgot to mention that I have been noticing is that my fuel gage is not reading the same thing when I start the truck as when I last shut it off. I havent thought too much of it being that maybe its due to being old and analog.

A fuel theif can make drastic changes in your milage too!

We had someone in the neighborhood taking about 2-3 gallons at a time. Couldn't figure why all the family's cars were getting bad milage. Bought locking caps and everyone's fuel consumption went back down to normal.

Bluegrass · #12 07-21-2012, 10:53 PM

Watching the gauge is the first mistake.
Actually fill drive and fill again to get close to the actual mileage.
Then if it's to far off , begin to look into it.
If your long and short fuel trims are in order you not using all that fuel without the OX sensors detecting it.
Only loses some how before the gas gets to the motor will affect the fuel gauge and fool you.
Good luck.

57tele · #13 07-22-2012, 04:03 PM

locking gas cap

fuel filter

injector cleaner (pick one, everyone has opinions in here..lol)

drive it like you stole it... the whole tank, then check the next tank.

when mine was off....(4.6) no codes... but my math was wrong... check your math vs reality. (grocer pump discount stopped before tank is full)

try not to get here :