Decreased fuel mileage question

I have a 97 Dodge Dakota that the fuel mileage has went to half of what I have gotten in the past I have run a code test and nothing has shown up faulty. Im getting black smoke from the tail pipes which indicates
unburnt fuel . I have replaced the plugs and wires and the air filter is clean and no obstructions in the air intake hose but the problem still continues . My normal fuel mileage in the past has been 17-19 mpg hwy but now it is 10 mpg . My question is there a sensor problem that would not show up on the code test ? The engine is a 3.9 and auto trans . Any help on this matter I would greatly appreciate.
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On Thu, 10 Jul 2008 12:17:59 -0500, snipped-for-privacy@webtv.net (Leon Taylor) wrote:

I had a similar problem a few years ago on a different model. All at once black smoke and the mileage tanked. Took it to a dealer and they replaced the oxygen sensor and changed the oil, on the chance that all that extra fuel had diluted it.
New oxygen sensor, smoke gone mileage back up. I didn't check for any codes nor did I get a check engine light, just a black smoke cloud.
Jerry O.
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Check the operation of the front oxygen sensor with a scan tool. A failing O2 sensor can cause both the problems you decribe.
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also make sure with the use of a scantool that the map sensor is working correctly as well as the the coolant temp sensor
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