I have a 96 Dakota with the 3.9, I'm getting less than 11mpg city and maybe 12mpg hwy if I use cruise control. After the engine warms up I lose alot of power but there are no codes stored, also I havent used any oil in 400-500 miles. So could it still be the plenum without seeing any noticeable oil loss in 400 miles or so?
I just ordered an ignition tuneup kit and msd coil from summit racing($92 inc shipping after 10% coupon), and both oxygen sensors from Amazon($80 inc shipping for both).
If this doesnt fix the problem, I'll break down and buy the Hughes Engines Plenum Reinforcement plate and gasket.
Any suggestions before I go all out? Does anyone have a step by step to replacing the plenum gasket? Although I dont really need it, I like to memorize it while I'm waiting for parts. Someone in another thread stated that they removed the intake manifold without removing the alternator and a/c compressor, is that correct?

 

Well on a v6 i dont know but on my 318 the AC was bolted into the manifold

 

I happened across a Hughes Plenum Kit on Craigslist yesterday, so after much debate(about 30 minutes), I removed the intake manifold, alternator and ac needed to come off due to two bolts under the alternator and ac compressor.
The plenum gasket was torn in two places, although I used no oil.
So far I've gained 10hp, the pinging/knocking stopped completely and the live data readings for map leveled out. It no longer smells of gas when I shut off the engine and the gas mileage is much better, I put a can of gas in the back to see how long it would take to run the tank empty. I was able to get 44 miles freeway on two gallons, nearly doubled my mileage.
22mpg on flat freeways with cruise control on at 65mph! Wow!
Oh ya, I also decarbonized the engine after replacing the plenum with Seafoam, this scared the crap out of me, it smoked like a cheech and chong festival for 5 or 6 minutes(neighbors werent happy, next time I take it to walmarts parking lot)There was ALOT of smoke.
Also replaced the cap and rotor with summit racing brand, 8mm wires, bosch platinum plugs and msd coil. I couldnt find the autolite 3923 but these seem to fire pretty well.

 

WOW! That is some great results!