Hello all,

Just purchased a 1997 F-350 4x4 with a 460 in very good conditon
It has 170k miles
e40d tranny
3.55 gears
35 inch tires with a 2.5 leveling kit
exhaust has no cats with maganoflow mufflers (clamped on, bought like this)

I have a couple of problems i want to address.

First, its getting 5.5 mpg, I was expecting 8. The air horns are cut out and i drive it very easy barley ever seeing 2,000 rpm. Is there something wrong? Anything i can do?

Second, the owner i bought from just recently did a complete tune up with new fuel pumps and sending units, he was a straight up guy so i believe him. If i walk to the back of the truck on the passenger side i can smell gas. There is no leaks as i have been searching for a couple of days.

 

Third, the back glass latch is gone. What do most of you all do? Go to the junk yard?

 

One possibility for both the mileage and the fuel smell is a bad check valve in one of the FDMs (fuel delivery modules). Each tank has an FDM which includes the fuel pump, pickup, gage sender and a check valve. If the check valve in the rear tank starts leaking, then when you run off the front tank some of the return fuel goes to the rear tank. This can make it look like you're getting bad mileage, and if the rear tank is already full it will spit the fuel out, wasting it and giving you a strong fuel smell (I'm not sure where it will be coming out to be looking for signs of the leaking).

Look for better mileage on one tank than the other, and ending up with more fuel than you thought you should have after running on the other tank.

 

3:55 gears and 35 inch tires is not a good combo. Has the speedometer been corrected? How are you calculating the MPG?

 

Thanks man, i have been running off the front tank only sence i bought the truck. i will swtich to the rear and re calculate fuel mileage. Keep advice coming, and if any one knows where i can look to see if the problem Nothing Specail said is happening i would apprecaite it.

 

I calculated fuel mileage by how much fuel i put in truck by miles driven. Example, yesterday i filled up put in 13 gallons in front tank and drove 73 miles. 73 divided by 13 is 5.61 mpg.

What gear combo would help my fuel mileage? rarley see over 60 mph

 

If your speedometer is off, due to the larger diameter tires and stock gearing, your odometer is also off. Meaning you aren't getting correct mileage calculations. The larger diameter tires cover a greater distance per revolution that the smaller diameter stock tires. So you are actually traveling more miles than your odometer shows.

 

i will have to mapquest the next place i drive to see miles and watch to see if my odometer is right.

 

Beware bad check valves in both FDM's (both tanks)... my 96 had this problem. It would transfer fuel both ways.

 

How would i go about checking this problem? or replacing the check valve?

 

All of the thing mentioned so far are good points. Speedo/odometer is off (possibly), one tank flowing into another, etc...

Also as Zack said, 35s and 3.55 gears is not a good combo. It'll make the engine work harder than it should. 4.11s would be more appropriate. I know someone with an f250 with a carbureted 460 that lifted it and went to 38s with 3.55 gears, and he went from 10 MPG to 4.5 MPG until he re-geared.

 

I will have to look into 4.11 gears, Yukon?

 

Run off your rear tank until the gage is about on empty, then switch to the front tank for a while (maybe 2-3 tankfuls). Every so often switch to the rear tank long enough to see where the gage reads (the engine doesn't need to be running for this check, but the key does need to be on). If you see your gage reading higher on your rear tank as you run on only the front tank, that's a sure sign your rear check valve is leaking.

Obviously you can do the same thing to check your front check valve by checking if the front tank fills while you run off the rear, but since you said you're only running off the front tank so far it seems like your rear check valve is the first one to figure out.

If you find out that only your rear check valve is leaking a no-cost "fix" is to always run the rear tank down first, then switch to the front when you've got room in the rear for the returned fuel.

Others might have other ideas for you, but to actually fix it I think you need to replace the FDM in the tank that's filling up. Drop the tank, remove the entire fuel pump/pickup/sender/check valve assembly and put a new one in. I know you can get either the entire assembly or just the fuel pump. I don't know if you can get just the check valve.

 

I will check this out. if this is the problem i wont be able to really track my mpg, will i?

 

A GPS will also show you how far off the speedometer (and by extension the odometer) are. It'll do it in real time, without the chance of an unexpected road detour fouling up your calculations.