ust got an 03 dakota. V6 4.9l 4x4 manual transmission. Driving home I learn that the truck has a hard time accellerating uphill and loses momentum at high speeds uphill on the highway. Truck runs smooth, no noise from the engine, no shaking, rattling or vibrating. Its had the muffler / exhaused repaired, the transfer case and chain replaced, and fluids checked. What could be the issue?

 

I will move this to the 2nd Generation Dakota Sub-Forum.

 

Mine is the same way the 3.9 lacks bottom end so get use to it if it wors then before do a tune up pulgs,wires,cap,rotor button

 

She's not a speed Demon unfortunately

 

if its a manual, you just downshift to 4th. i usually have to do that when driving up hills on highway, so i dont loose my momentum. although you probably already know that, just throwing it out there.

 

I had the same issue when I bought my 2003 2wd with the 3.9. It would not get out of it's own way. I put in new plugs and wires. It was better. I put in a new cap and rotor, now she runs like a champ. I wasn't even able to use the Cruise because it would keep dropping into the passing gear at the slightest hill. Not anymore.

 

The other possibility is a plugged converter. Throw a vacuum gauge on it and check it out.

 

^^^ I second a plugged converted, with a 4.7 (durango) and 5.2 (dakota) in the family, both clogged the cats somehow after 12-13 years of life, well My 5.2 couldnt pull the trailer like it used to, soo started with a tuneup, then eventually it went worse and had a hard time unloaded going up a steep off road hill normally climbed with ease, after a lot of guess work I decided to measure back pressure, it was high. dont remember how much but still, anyway I swiss cheesed the pipe and it ran great till I could get the $15 in pipe and the cat fixed (bolt on instead of welded).

Next, moms durango started having problems, wouldnt run good once hot, had a miss, changed plugs, 2 fouled. Then one day on the freeway I punched it, trying to pass a semi. Me going 75, semi 65, started going into a hill and it downshifted as expected but once hit 3.2k rmps started violently shaking, was like F*** soo I deicded to go all or nothing, went to the floor, powered out of the shake but still ran horrible at 4k, Forgot to mention check engine light came on sometime then. Then power loss major, had to pull off the freeway, she was overheating, missing, everything, Looked underneath for a oil and coolant leak, saw something glowing, the cat was glowing bright red, almost white!!!!! I figured injectors were stuck open causing a dump of fuel, nope, took to dealer this time because of no 4.7 knowledge, cat was 93% plugged, they said unheard of, backpressure should have stalled out the engine.

 

Could be a bad plenum. I know atleast for the first gens the plenum would go bad and what you described for the lose of power uphill it might, but i dont know if the second gen magnums have the same problem. Look through your throttle body into the intake manifold and see if there is any oil. if there is check to see if the PCV is not clogged and if not the plenum is bad

 

turn off the overdrive function when going up a hill and hear the 3.9 do work!!!!!