I'm trying to find out what is causing such bad gas mileage. I have a 1999 Dodge Dakota Sport 4X4. Mileage is 175,000 The truck runs great with lots of power, sounds good, idles good has no smoke. No smell of gas fumes, and no leaks that I could find. I don't think it's the catalytic converter because the exhaust pressure feels OK and I don't smell any sulfur. I did some test, I put exactly one gallon of gas in and got 7 miles city driving and 10 MPG Highway. That's with me babying it. The MIL light is not on, the brakes don't drag the tire pressure is good. I bought the truck from my brother and he told me he got about 12-15 city and 18-20 highway. He said he does not remember when it started getting gas mileage. Any ideals what could possibly be wrong with it?

 

When is the last time it had a full tune up? You know, plugs and wires, fluids, filters, ect. ?

 

belly pan gasket can do it in a heartbeat, clogged air filter, belly pan gasket, bad O2's, belly pan gasket, old/lazy fuel injectors, belly pan gasket....

 

what I think he is trying to say is the belly pan gasket is probably what's causing it.

 

open the throttle blades and look down. If you see oil...yea, it should be dry. If not, it makes a crankcase leak, and it'll make the MAP show less vacuum. to the PCM, Less vacuum = more fuel.

 

You might also want to track the mileage over a couple tanks of fuel. You can't get an accurate number by putting in one gallon of gas.

 

I guess your saying the belly pan would be your first pick to be the most likely culprit. Never heard of a belly pan. Is that the same as a oil pan gasket? I checked the air filter and it is clean.

 

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I looked inside the throttle body and seen some oil residue, but no puddling.

 

Yeah, that's true. I did do it several times and received a different number each time. On city I got a 4.3 MPG one time, a 5.? MPG 7.? MPG. I just put down the highest readings for city and highway.