I have a 08 Dakota 4.7L V8 with 71,000 miles that has been having intermittent performance issue lately. No CEL at all. MPG is steady at 8.3 which even for a V8 seems low and horrible and never used to be that way before. The truck just doesn’t seem to have the power it used to have and under hot and humid conditions it gets worse. Not to bad in colder conditions. Power issues extremely noticeable when accelerating to go up a hill or getting onto a highway after slow down or stop. Does anyone have any ideas where to go from here. Also I ran a live data test with my scanner while driving to see if anything is out of range, doesn’t seem like there is but dose anyone have, or can anyone with a similar truck in good running condition post a copy of their live data set, so I have a good baseline to compare my results too and see where my issue might be. Any help is appreciated thanks.

 

Is the fuel good? Gas thats degraded will still run, but deliver poor economy.

 

The fuel in the tank is good. Initially I thought the same thing however I have gone through several tanks of fuel with this problem still occurring. I am going on 3 or more months now with this problem. The truck is still drivable to get to and from work but the performance and the fuel mileage continues to degrade.

 

I'm getting about 13mpg city, 19mpg highway in my 2005 V8 4x4. All the stop and go traffic kills my mileage, though I'm pretty surprised yours is that low. I mean, the 4.7 isn't exactly known to be super efficient, but still. Lotsa things to inspect. Air filter, spark plugs, TB cleaning, dragging brakes, tire pressure, toasted u-joints... alignment...

 

I used to get 12MPG around town but I haven’t seen that in a while. I did all the stuff you mentioned above in addition to a vacuum check, compression check and a fuel pressure check with everything checking ok and yet the problem still exists.

 

Hot and humid kills performance anyway, but that it runs better in the cold could be a clue. How dirty are the injectors? What are your fuel trims like? Tried a TPS sweep to see if it has any dead spots?

 

I never took my injectors out to look at them so I don't know how dirty they are. I did do a TPS a sweep and there is no dead spots in that. I droved and recorded my live data to see fuel trims when the truck started losing performance while driving. The whole recording was 28 frames, and the truck remained sluggish from frames 1 through 14 and then went into Open Loop drive at which point the truck regained all its power. Below is the 3 second time frame of data right before the truck started to gain power.

 

Interesting. Frames 1 and 2 look pretty similar but the 3rd frame shows a drop in voltage from the O2 sensors, which force a massively rich mixture. Odd, that the top fuel trims are different from the bottom fuel trims. I know you tested your fuel pressure - what's your leakdown like? Only ask cuz I checked mine today and think I might having a leaking injector.

... side note, just noticed that RPM went from 2200 to 3000, and throttle position closed?

 

I noticed that myself. I’m starting to suspect that I might have an ECU problem. Cause nothing else makes sense. Especially without a code being thrown. As far as the leak down goes I don’t remember what my leak down was, I just remembered I was at the right pressure But I can try and throw a gauge back on my fuel rail this weekend and see what the leak down is after it’s pressurized.

 

Crazy. Wish I had any more suggestions for you. I guess it would just depend how friendly you are with the parts cannon. Diagnostically, I'm out of ideas.