I have a 98 1500 5.9 auto, will rev up fine out of gear but accelerating it starts to bog above 2500 rpm and while driving even with the pedal to the floor it wont downshift enough to get the rpms above 3,000 and again bogs. The tranny slips seconds as well. On top of that It also starts hissing and idling rough when warming up but it clears up after I rev it. I know the intake manifold gasket was recently replaced as well and I recently installed new plugs and wires. Not sure where to start as the rest of my trucks are 7.3 idi's and have little experience with dodges and gas engines.

 

Sounds like under load, it's got issues? I'd check the Catalytic Converter first. If you have a vacuum gauge, plug it in, then increase RPMs. See if the needle slowly drops. Other than that, I'd need more data...

 

Hissing and a "recently replaced intake manifold gasket" makes me wonder if that wasn't done right. You can check by spraying carb or brake cleaner where the intake meets the heads. If the engine speed changes there's a leak somewhere.

 

Double check the spark plug wire routing.

 

Thanks for the help. I'll take a look at the cat, does have 180k on the clock good possibility. As for the intake gasket it was done by a local shop but I will be checking that as well. Double checked the plug wires to, all good there. Check engine light came on this morning to, I'll read it when I get home today hopefully that will give me a clue.