Hey guys, (gals), I have a '90 f-150 4x4 thats been gone through and over more than once. I have a '93 351 Roller motor (stock and Old Air System) With Headers, true duals (no x-over), o2 in place, egr in place, Smog can and tubes removed but pump still in place, Jacobs juice box, Bosch V-powers, Connected to an E40D running 3.55Ls and 10.50x31.50x15's. I run Regular Unl. 87oct always and use napa 10w30 and filter.(incase some one wants to know)

The issue is I am getting terreble fuel mileage in town driving. Terreble mileage as in a full tank average less than 7.5 or 8 MPG purly in town. The reason Im confused is the fact it used to run over 10-12 MPG in town only about a year ago. I run a 16.44MPG avg at 60 on purly higway. I have ran just under 16 mpg before in wind but typicaly I average at that or higher whether the AC/Defrost is on or not.

The truck runs perfect, my air/fuel was between 12.8 to 13.1 average across the board when I had Turbo mufflers, which I have changed to 4" Round Magnaflows, and I don't rod it a bit in town. I have replaced the pressure regulator in the past and my most recent oil change (1/4/09) there is no sign of fuel smell, or lack of power. I also just replaced the air filter (it was time but not terrible).

I havent replaced the fuel filter in about a year or so and the PCV in about the same. But aslso have only put half my normal annual milage (around 19k down to 11K). other than these obvious things what could or would cause this thing to run so good yet just drink and drink while in town but completely turn around on the hiway?

Any valuable ideas or information that anyone could contribute would be great. Im stumped, and dont know what to do. Even with gas down under 2 bucks a gallon I cant really and dont want to afford filling a 35 gallons system only to be dissapointed. Thanks for your help and if you have any more detail questions let me know.

-Sean

 

I would thoroughly check every inch of the fuel system for anykind of leaks espeacially around the fuel pump and gas tank.My truck had a leaking gas tank and I never smelled any fuel.I put a good tank in mine and my mpg returned.I hope this piece of info helps out.

 

guess I should of added that also. Both tanks have been replaced recently and are perfect, the pumps also have been replaced and are both working the same and right. Minus the front sending unit. Thanks for the input but this truck also has no leaks at all.

 

dump the bosch plugs.

 

Try checking the throttle position sensor under the throttle body,it could be off enough to tell the computer to give you too much fuel at lower speeds.

 

The bosch plugs upped the milage over 2mpg over autolites. and another 1 or so with the juice box. I think ill be keeping those. I also have recently replaced the TPS for a bad idle and no auto rev. all seams to be working correcly there but I'll keep it in mind. Thanks

 

You have a wideband O2 sensor? Will the motor achieve 14.7 under light cruise at any speed? If it doesn't then it's not going closed loop. Have you pulled the codes?

 

with no other changes?

hard to believe.

i suggest motorcraft coppers.

 

What thermostat are you running and is the mileage the same on both tanks?

 

Is the exhaust 4" or just the actual muffler body?

4" exhaust will kill low-end and cause the mileage to suffer.

 

Granted it will kill low end but why do you say it will hurt mileage? A free-flowing exhaust system will increase your MPG.

 

I dont have a wideband guage, these numbers came from when i dynoed it. No codes are present, although I have a feeling the ECM is done (replaced 3 times) We have tried to scan it in the past 8 months a couple of times just to see with a Snap On computer unit. No go just searches for ever.

And yes the mileage increased that much with the plugs, jacobs box. On top of already existing mods.

Both tanks have the same milage and checkvalve is inplace and working as if i empty one tank and use the other the tank remains empty.

I talked to ford today and my guys says could be MAP, could be plugs needing replaced(only at 30k miles) many sensors and such, but doesnt explain running perfect and getting good on hiway. expecially where I can not smell and rich fuel at idle or around town driving, and carbon build up isnt too obvious to create attention.

I could understand if maybe its just to a point in its long life of wanting to give up and maybe its just me, but anything under 10 mpg in a 5.8 in town seams pretty redicoulous.
Thanks for you input and I guess we can keep trying

Sean

 

with ONLY the plug change i doubt very much +2 mpg.

 

O ya and as for my exhaust, I have headmans to 2.25 inch pipe into 2.5 adapters into the 4" Magnaflow rounds back to 2.5 and into the 2.25" which head 45 deg out the back and into 3.5 SS. tips.

 

I had the jacobs box first and then the plugs, it also woke up the bottom end will lay a nice scratch. I agree with you that only the plugs wouldnt Improve milage more than maybe a mile or less, but were not talking about only plugs either.