I have an '89 F250 4x4 302, M5OD and I am getting 7 MPG ). I have dual tanks but the rear tank is never used. No leaks and the rear tank is dry immediately after a drive - inserted a hose to the bottom and it came out completely dry. I'm an old guy and don't understand EFI that well but I did pull the codes.

KOEO code 11-11.
CM codes
33 (EGR valve opening not detected),
53 (TPS circuit intermittently failed above max. 4.5 v.)
87 (voltage not detected on fuel pump monitor while engine was running).
KEOR code
44 (thermactor air injection system inoperative - right side).

Do any of these codes explain the terrible gas mileage?

 

TPS = Throttle Position Sensor. That is one of the many issues you have. Does your truck jerk while your driving? This senses your throttle position and sends information to the computer to adjust how much fuel and air is needed at a given time.

If your EGR valve isn't opening then that's another issue because this recirculates the exhaust gases back to the cylinders to burn any unburnt fuel and reduce NOx emissions. This can also bring down MPG if I'm not mistaken.

Does your truck still have a smog/air pump? It's mounted below the idler pulley

 

Are you sure you don't also have the cross flow problem between the tanks?

 

Thanks for the quick reply, ChristopherN. Yes the smog pump is on the engine.
The truck does stumble just a little on acceleration, but only when first pressing the pedal. It is almost like a very slight bog.

I don't understand why I get all clear on KOEO and the 3 codes in Cont. Memory.

 

I don't think so because I took the truck for a drive and immediately after shutting it off, ran a hose into the bottom of the rear (unused) tank and the hose came out dry as a bone. If I was getting return to the wrong tank, I would think I should have detected some gas in that rear tank.

 

codes 33 53 should be addressed. when was the truck last tuned up with new plugs, wires, cap/rotor and fuel filter?

 

Everything mentioned has less than 1k miles on it. I probably should check the timing though, as that wasn't done at that time.

 

Do any of these codes point to what would make a truck that seems to perform pretty decently only get 7 MPG. If it ran terribly I could understand but it doesn't seem too run that bad. I know that a bad EGR would reduce mileage slightly but not that far would it?

 

No the EGR won't have that big an effect on fuel milage. Are the tires stock size or oversize? The most accurate way to calculate milage is to divide miles driven by the volume of fuel needed to refill the tank, but that assumes you have an accurate measure of miles driven and that won't happen with oversized tires unless the speedo is recalibrated. Another possibility that may or may not throw a code is the O2 sensor, if it hasn't been changed recently it's worth a shot. And also inspect the fuel pressure regulator to make sure it's not leaking fuel into the vacuum line.. if it is it needs to be replaced, and while you're in there check fuel pressure, the system should build and hold about 40psi with a few cycles of the key(engine not started), and 32-35psi with engine running and vacuum attached to the regulator. If the pressure drops immediately after building you may have a leaking fuel injector, or if the pressure never gets this high the pump is weak or isn't getting enough voltage(corroded connections). If the pressure is higher the regulator could be bad or you could have a blocked or pinched return line.

 

Tires are stock size and I'm pretty sure the O2 sensor has not been changed in quite a while,at least from the look of it. I haven't owned the truck for long , at least in terms of miles driven, so I don't really have service records. I'll check the fuel pressure and regulator and see about an O2 sensor. I rather suspected that the O2 sensor could be part of the issue but when no code showed up for it I was confused. Thanks for the replies and any other suggestions anyone has.

 

Checked the fuel pressure today and got the following:
Key cycled but engine not running, gauge show steady 40 psi after about 6 key cycles. Engine running, the gauge showed steady 32 psi. Now the question; the engine was still cold when I did the test, will this matter? If so I can retest tomorrow. Any reason to believe that the fuel pressure would drop as the engine warmed up?

 

Engine temp should have no effect on fuel pressure, and your numbers look good.

 

I would be looking at that Throttle Position Sensor.

The idea is that when the ECM detects a WOT (wide open throttle), it goes into open loop and starts lengthening the electrical pulses to the fuel injectors in order to cause more fuel to be sprayed into the engine.

This provides more power under heavy loads or high RPMS, and dramatically increases the fuel consumption rate while it is occurring.

Also, the code is telling you that the TPS was above max... meaning that it is reading abnormally high and a high voltage being read through the TPS means a wide open throttle which causes the computer to start commanding the massive overuse of fuel.

Low voltage (0-.5) through the TPS means closed throttle, 4.5-5V means wide open throttle.

It makes sense when you stop for a minute and think about it.

There is a way to test the TPS with a voltmeter, but if the problem is not continuous, the better option might be to swap with one from the junk yard.... New ones can be rather pricy.
They're pretty quick and easy to replace.

Also please check to make sure that you have a proper vacuum level in the intake.... if there are vacuum leaks, the MAP sensor will detect an incorrectly high pressure and assume that the throttle is open more than it is in reality and command the computer to enrich the mixture in order to compensate.

Good luck!

 

And yes, a faulty TPS can cause the awful fuel economy you're experiencing.

 

TPS is next on the list. The fuel pressure tester kit that I borrowed from AutoZone has a nice little multi-meter with it and my manual describes the test procedure. Should get to that today. Already bought the O2 sensor so it might go in today too. Thanks every one for the tips.