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Inconsistent Idling and Help my gas mileage just went through the roof!

Hi Car Talkers!

I am the owner of a 1999 4x4 Dodge Ram 2500 Cummins Diesel that for a while ran like a champ but now I am really hoping someone can help me out here! The amount of diesel my truck has started to run through is really starting to worry me as well. I am only driving a short distance to and from work everyday and yet i am spending a $100 every other day on gas! Thats not normal at all for my truck! So anyways here are some problems I have been having…

Fluctuating idle… when in park and neutral it jumps up to 1400 RPM then drops down to 850-900 which is the normal idle for it. I have already replaced the TPS. My transmission shop put a noise filter on the ground for the TPS. That was a few weeks ago. A few days ago the check engine light came on but shortyly after it shut off. The next day the check engine light was back on. I had Autozone read the code. It said bad TPS or a short in the wiring, I was told to clean the connections on the throttle positioning sensor or (tps), so I did. I then drove it around and it seems to be driving and running right, but the check engine light is still on. Also before when i was driving at low speeds, the throttle seemed to surge and drop like it was when idling. Also, the 4x4 light does not come on on the instrument panel, but the 4x4 does engage. I don’t know if that could be part of the problem or not. I have also just changed the air filter and fuel filter, as well as run some diesel kleen through the tank. My fuel mileage has also suffered severely since the problems have started. Any advice or thoughts would be greatly appreciated!