My 94 Dodge 5.2 has a very werid issue, it will runs great most of the time, but when on the highway and when towing, the power goes away.
The power can come and go, while towing a car yesterday 200 miles, the truck ran just ok, not good, and got 12.2 mpg. On the trip home towing only the empty trailer, the truck had lost even more power and used WAY MORE fuel.
The plenum has been checked, but that would not allow the truck to run ok at times and run terrible on the hightway and while towing which is where the power loss accures most of the time.
Ant thoughts?
MArk

 

I had an almost identical issue on my 97 5.9.. only evident on longer trips, towing, etc.. After troubleshooting a handful of other issues, mine turned out to be a clogged catalytic converter. Do you hear any rattling coming from the exhaust by chance? You can slide under it, and hit the CAT a few times to see if you hear anything inside that could shift/move/expand under heavier exhaust heat and flow, causing a partial clog.

Since changing my CAT, I get full power again, better fuel mileage, and lower temps... hope it helps!