I have a 98 1500 4x4 318 in the first 4 gears its has great power but once I get into 5 I have no power I lose speed and have to shift back down to 4th I have a lot of lowend power but no top end any ideas for this

 

That's how the trucks are. 5th is overdrive you won't have much power in overdrive.

 

Fixed your plenum yet? How many miles on the truck? Still on the original timing chain and gears?

 

Along with everything the others have said;
What size tires do you have? What gears are in your axles? You can look on the metal tag on the diff cover for your gear ratio.

 

5th gear? I've never had one with 5 gears. All the ones I've had were 4 gears. With lockup after that. But all mine were 360s. Not 318s. From my experience thou. Once my truck is in lockup 4th it's gutless. With 265 75 r16 tire and 3.55 rear end. Basically For flat road only. Great for gas mpg on long road trips thou. I live in PA which Is alot of mountains. I rarely drive with OD on. As soon as I start my truck I hit the OD off button. U should have one of those. Also most tags for rear gear ratio can be found under hood if you look up. if it's not gone

 

JR,
First off PA don't have any mountains it has things called hills. Now out west here we have mountains. Big ones too that you have to drive up and down and don't have to shift out of OD unless you're towing something.

The OP has a manual transmission, you know the one that you have to think about when to shift and stuff. It don't do it for you. That's why he has 5 gears and not 4.

 

I live top of cresson mountain over 2000 feet elevation. I don't consider that a hill. And yes I've owned 3 petal trucks.

 

Hehehehehe, 2,000 ft is a foothill. I live at 2,250 ft and I'm near the bottom of a mountain. Relax dude I'm just yanking your chain.

If you've owned a 3 pedal truck before then why didn't you know he had a stick? And I suggest you open a thread of your own to discuss why your truck doesn't have any power.

 

No I have not changed the plenum original timing chain a little over 183600miles I think 3:55 gears tires are 36-14.5-15 but it even did it when I had 33-12.4-15

 

In 5th gear I am losing speed even on flat land