1991 Ford F150 5.0L (302ci) 5 speed 2 wheel drive?

I have a 1991 Ford F-150 5.0 9 (302ci) 5 speed, 2 wheel drive. I'm having absurdly bad gas mileage for some reason. Since owning the truck I have replaced the clutch, slave cylinder, radiator (oversized to prevent over heating), timing chain, water pump, plug wires, distributor, entire exhaust system (put headers no Y pipe, the sides are separate), regular maintenance obviously, plus a TON of things I'm forgetting… I've owned the truck for almost 15 years and I'm also wondering what could possibly be the problem for the bad gas mileage. It seems as if maybe the o2 sensor would sense that it is running lean since there's only one and the sides of the motor are separate. I've looked into it and there's no way to add another o2 sensor. Please any and all help is appreciated I've spent alot of time and money on this truck and I don't want to get rid of it.

Did it use to get better mileage? It should easily get 15 mpg on the highway. It should not matter if the O2 sensor only is monitoring 1 bank since the banks should be running the same unless you have a problem with one cylinder. If you needed an oversized radiator to keep it from overheating, what ever is generating the extra heat is causing the increased fuel consumption. Have you checked the timing? Does the compression in all of the cylinders meet specifications? Are your disk brakes releasing. If you have not cleaned them or replaced them with drum brakes, they may be dragging and killing your mileage. Is the intake system still stock and the air cleaner clean? Is the EGR valve functional? Are then any codes set in the ECM? Is the MAF clean and functional?

Hi so how about the most obvious reason a worn out old engine. That fact does seem to have gone by you with such a question. As cars age they like old guys drink too much due to worn out engines the pistons are never as good as when new the fuel gets sucked in and not burn't properly because the injectors are so worn as the high pressure through the injectors makes the holes in the injectors bigger over time and mile age so you waste fuel. Simple logic am afraid sunshine.

Not as simple as the Bears say. We don't know the mileage on the engine which could play a role but in any case nothing you mentioned seems to be connected to either the fuel system or ignition. Generally speaking those two factors play the biggest part in fuel use. On a truck that old have you checked for fuel leakage between the tank and the engine? If consumption dropped quickly that would be a likely cause, damage can be caused by dry rotted flex line, loose hose clamps or even rodent's habits of chewing on lines.

The 86-95 Fords use the eec-iv engine management system to control the fuel and timing. There are numerous sensors on these engines and some of these sensors go bad and it won't cause the (cel)check engine light to come on. The first step in diagnosing your problem always starts with scanning the ecm for codes even if the cel isn't lit. You don't need a scanner for this though, Il post a link below which will give you instructions on how to pull codes. With the mileage that your engine has, numerous things could cause excess fuel consumption. The piston tops and valves could be carbon coated which prevents the fuel from burning completely, the converter could be partially clogged, ring seal could be diminishing due to mileage, bearings inside the rear differential could be worn causing excess drag. There are several sensors that can cause mpg loss when they're defective, but no sensor should be replaced until after you've pulled codes and/or tested each sensor.Don't waste money unless its bad

The fuel pressure regulator is a common failure point with the 87-96 F-150/250/350/Bronco. The truck will run seemingly well, but it will use more fuel than usual.