poor gas mileage

[nightcop]

Did a 4.0 swap in my 88 ranger 4x4 last year. motor complete rebuild new sensors and computer changed also. was getting poor mileage changed out 4.10 gears for 3.73 gears and still only getting 12 mpg .. truck runs great.. at a lost now not sure what else to do but maybe have computer reset to match truck.. any help or idea's

 

[RonD]

Pull the spark plugs and see what the tips tell you, slight tan color of white electrode means good complete clean burns are happening.
Black residue is rich burning, not all fuel is burning so a lower MPG than it needs to be.

If plugs look good then you may be at best MPG you can get with your setup, or you have a fuel leak, could be in the filler tube or vent tube, fuel line or return line.

Dirty plug tips could have many sources.

MAF sensor tells computer how much air is coming in to the intake so is the main sensor to set fuel/air mix.
Dirty MAF can cause rich mix, this is easy to clean.

O2 sensors are speced for 70-100k miles this sensor is used to fine tune the MAF set mix.
No real test for an O2 sensor on a OBD I system.

Fuel pressure regulator(FPR) on the fuel rail has a vacuum line attached, a small leak in the FPR diaphragm will cause fuel to be sucked into the intake, lowering MPG.
Remove vacuum line and smell for fuel.

Injector tips can get dirty so fuel drips instead of sprays, this lowers MPG, can of Seafoam or other gas treatment once a year can clean and prevent dirty tips.


Engine temp needs to be 200-220degF for best MPG.
This would be just under 1/2 on the temp gauge.
Thermostat should be a 192 or 195degF to maintain 200degF minimum temp.
Some often use a 160degF or 180degF t-stat which would lower MPG and is of no benefit for engine, 200degF keeps oil cleaner(SAE reports) and MPG up higher