I have a 98 Ram 2500 5.9L 24V. Mechanically, the truck is in good shape (it looks like a Mad Max reject). After 100 miles of sustained high speed driveing, 70mph or so, it gradually looses power. If I leave the fuel cap off it much better. With fuel cap on I can hardly sustain 60mph eventually. Is there a vent somewhere that is blocked?

Kurt

 

Look on the top of your fuel tank. There should be 3 connections. Hoses run to 2 of them and the other one should be open. If it has a black or red cap on it, take it off. You can also see if it's blocked by cleaning around it, then connect a piece of hose to it. Take the fuel cap off and listen there while you blow into the hose. See if you can hear the air.

 

I believe as well there is a pressure & vacuum valve in the fuel cap as well(at least on a chev diesel there is. if its faulty it can/will callapse the fuel tank quite easy, (good thing the dodge has a plastic tank). FWIW, my vent on my tank(as described above) was plugged solid with dirt/mud, as the previous owner lived on dirt road

 

you would think a truck that old would not still have the cap?

 

I can just see the top of the tank. It looks like there may be something on the third connection. Hard to tell for sure. It's a full tank so I can't drop it just yet. Thanks for the tips though.

 

You dont need to drop it, look in from the pass side

 

I had a Dodge in work do the same thing to me, i went to the local auto parts store and got a new fuel p because i could not suck through the old one. Good diagnosis and idea be that you took off the fuel cap to see if it got any better