Just purchased an '03 3500 6 speed a week ago. When I bought it, the guy told me it was due for an oil and fuel filter change (every 5000). I put a few hundred miles on it pulling a 2300lb bumper pull horse trailer, when suddenly it lost all its "go". I changed the oil and filters the next morning, and I've got most everything back, but I'm not getting the fuel mileage I was before. Could I be losing a pump?

 

Congrats on your purchase. these are fine trucks and arguably one of the best engines ever put into a non comercial vehicle.

need more info about your problem. How much has mileage dropped? Power is not related to your oil change. any funny noises? maybe you should have a reader connected to the diagnostic port to see if there are any codes indicating a problem. Might be intermittant if it was low on power and came back, like a bad sensor or short in wiring that comes and goes. just some thoughts.. start with diagnostics and go from there....

 

Sorry, I should have said the truck has about 120,000 miles on it. The previous owner showed me a new fuel pump located on the frame rail in front of the fuel tank and said he replaced the factory 50gph pump with a 100gph MOPAR part. I replaced the fuel filter when I did the oil change, thinking maybe it was clogged, and thats when power came back. The mileage has dropped from near 26 to about 20 unloaded, and from 21 with a 2300lb trailer to about 11. I'll get the codes read in the morning. It seems to rev freely to redline in the first 3 or 4 gears, but hesitates in the last couple.

 

Okay, its kicking out a P0628 code; fuel pump A control circuit low. Is fuel pump A the tank pump or the lifter pump? Do I need a new fuel pump, or just a relay?

 

When I substituted my twin Holleys hangin on the frame rail for the stock pump attached to the filter cannister I got that code. The ECM is looking for a load attached to the wires that led to the original fuel pump. If it doesnt see that load then that code pops up. I ran a seperate wire for my pumps because that whimpy wire assembly to the stock pump didnt look like it would handle what I was asking it to do... so I had nothing attached to the original pump harness, hence the code. So... I fooled the ECM into thinking something was attached drawing power, like a fuel pump, and never got a code after that. What did I attach?
A yellow coated tail-light bulb. Yellow because the bright white was too distrating under the hood at night and yellow because I had one extra. If you do like I did and cut off the harness connector fromt he original lift pump (if you can find one) with a bandsaw and solder the bulb contacts to it, it'll plug right in and look and work great, if no connector then just solder a couple of wires to the bulb and push them into the mating connector of the harness wiring.
If none of the above applies to your situation, check the pump to make sure it's working up to snuff. If it starts failing, it'll be just like the original and set codes and cause your truck engine to drop in RPM at the higher loads, (higher speeds or when pulling)
I have several pics in threads I've posted on the board, you can go search and see what I've done.
db