Sorry if I ramble, just trying to get all pertinent information included.

I've got a 1986 F250 4x4 with the following specs:
-EFI 302, new motor with less than 600 miles
-New Process granny low four speed
-3.55 gears
-33/12.50 mud tires
-exhaust cut off behind first catalytic converter
-new sensors on the motor
-new distributor
-timing set to 10 degrees advanced according to timing light
-all three fuel pumps are new
-new fuel filter
-tank selector valve is bad, only draws fuel from the front tank

I know that a 302 is generally considered under powered for these heavy truck, but I think mine might be having an issue causing even less power. It falls flat on its face in 4th gear, acceleration is almost non-existent and loses speed up the slightest grade. To top it all off, I'm only seeing 5.6 mpg.

Any ideas on how to squeeze more power and mileage out of this truck? Or is 5.6 mpg really all I can plan on seeing in a 302 of this year?

 

Compression ratio? Cam timing? Cam? Is your timing advancing? You should be getting low to high teens depending on how it's driven. No fuel leaks or misfiring?

 

Did you pull the spout plug to disable computer advance when setting the base timing?


You could have one of the pumps hooked up backwords(electrically), this is easy enough to do if you have to splice wires. The selector valve works on fuel pressure from the in-tank pumps so as soon as a pump is engaged the valve has to switch it doesn't have a choice. If something in the selector valve assembly is stuck the active pump could be pushing fuel into the other tank too and that would make fuel milage look really bad.

 

Has the truck ever run well for you in this configuration?

I've no experience with early Ford EFI, but on other systems, I do know the exhaust back pressure plays a part in how the computer is programmed. In other words, you've made a huge change to the basic operating conditions and we mere mortals may not be able to predict how the computer will respond.

I've always said automotive computers are simultaneously very smart and very stupid. They can make complex, rapid-fire decisions, yet also be unaware of blatantly obvious things such as a thrown connecting rod (later models are better at this). So perhaps in your case, the reduced exhaust back pressure has caused the computer to react as programmed (based on what the O2 sensor sees), but unfortunately it's opposite of what you'd have wanted. Keep in mind the EFI is programmed to keep emissions low. Power output and fuel economy are secondary considerations.

I'm not suggesting you rush out and reinstall a full factory exhaust just yet. Still, I wonder about the history of this current configuration.

Is your check engine light illuminated? Any codes?

 

Pull the codes, see what the computer says is going on.

Here is one write-up on doing it at home with a test light or analog voltmeter:

Ford Ranger - Testing EEC-IV Equipped Engines

 

Truck is throwing a code 44 KOER (something about thermactor air control system problem, which im guessing is because my emissions pump is non-functional), system pass on KOEO and CM.

I unhooked the SPOUT plug to set the timing and it is advancing the timing when under throttle.

As far as cam timing, the motor I installed is a crate motor from advanced auto parts. The cams were installed when I got the motor, I'm assuming they're right.

The exhaust is cut short because I thought I had a clogged cat at one point, planning on installing new exhaust as soon as I can afford it. Doesn't seem to be effecting the motor performance, other than I get a loud pop from the exhaust when I go from WOT to idle quickly or upshift aggressively.

The reason I say the tank selector valve isn't working, is because if I switch the rear tank on, the rear fuel pump turns on but I'm still using fuel out of the front tank. (The front gas gauge drops anyway, don't know about the rear tank, needs a new sending unit)

 

How much have you driven it? The reason I ask, sometimes when those valves foul up they will pull fuel from one tank, and return it to the other. So eventually in this case your rear tank will overflow with fuel if you have driven it enough. That might be what is causing your fuel calculation to be off.

 

I put 58 mIles on it after filling the front tank, filled both tanks (did the mileage calculation on what the front tank took) and have 12 miles on this fill up. Haven't noticed the rear tank over flowing yet. Gonna drive it 25 miles to work tomorrow and 25 miles back. Might notice it then.

 

Drove the truck over the past few weeks, power level seems a tiny bit better (thinking part of the problem was older gas left in the rear tank) truck is pulling from both tanks, but is still getting somewhere in the neighborhood of 6 mpg.

I guess I'm gonna play with timing a little and see if I can't get it acting better by advancing it a little.

 

A few things I would check on/do to see if my readings are good.


Only pull & fill 1 tank. Disconnect the other tank so nothing is pulling or going to that tank.


What are you using to measure distance? If the trucks Odom could it be off with the large tires or even a gear change?
Confirm you are getting the right miles when measured.


Once you know both of them are 100% right then see what you get for MPG.
Dave ----

 

I'm basing my mileage calculations off of using fuel out of both tanks and refilling them at the same time. When using the rear tank the return stays flowing to the front tank, but I figured it won't affect my milage as long as I'm still getting total fuel used.

As far as my odometer being off, I was using my trip meter, but I noticed it wasn't acting right (would go from 9.9 miles to 9.0 instead of 10.0) so now I'm using the regular odometer. I know it will be a little off due to big tires, but I can't imagine that being 4 mpg or worse off.

 

On the tanks I do under stand your thinking but I would still take 1 tank completely out of the picture just to make it easier because even with 1 tank you can fill it more or less on each fill up and you now could be doing this with 2 tanks.


I would also find a measured mile or 3 and see just how far off your Odom is per mile then a you can adjust it when you fill up the 1 tank.


Un-till you can get the same readings every time for a mile traveled be it .8 or .7 or ? to adjust your math and get close to the same fill gals at fill up based on fuel gauge level it is all a crap shoot what you are getting for MPG.


There has to be an app fro your phone for miles driven? I know Google maps has miles for a trip. Something like that but you could set to 0 at a start and see total when you stop. Keep a note pad and mark down each trip till you fill up next time.


That's how I see it
Dave ----