1999 Ram - won't go over 70MPH
 

OK, I have a 1999 Ram 5.9 4x4 and recently it began acting really sluggish on the highway. In fact it will not go over 70MPH. If I step down hard on the gas it will just hold at about 70. If I really quickly step down I can get it to down shift and push the RPM to 4K. When I do I get a P0300, P0304 and P0306 codes. Same ones always. It runs fine up to about 50 and I do not notice any real gas mileage problems (12MPH) in town. I am on my third tranny and I think it is working fine. Intake manifold was replaced back at maybe 40K. I checked the MAP and have voltage that changes smoothly when I pull vacuum to 20mg. I just thought you guys might have some ideas. I hear so many possibilities that people try and really never get any results:

New plugs, cap, rotor
Tranny sensor
Diff sensor
MAP
Throttle sensor

One I do not see often is the cat converter. Could that be plugged?

Thanks all

clogged cat is common. to test, temporarily remove front O2 sensor to relieve pressure.

welcome to DF.

I will try this today and get back to you. I would guess these sensors are rusted in pretty bad?

Try removing it when the exhaust is hot. Sometimes that helps.

It came out easily am I believe we have resolved this problem. Runs great now and I believe the converter ispretty clogged, as even with the o2 sensore out I would guess I still have some back pressure. I will get the Cat replaced this week. Any suggestions on that?

Check the plenum, and see if that isn't why the cat clogged in the first place. (See stickied thread at top of this forum.) If your plenum is blown, fix that FIRST, otherwise, you will just be eating another cat......

That just sounds wrong......... you get the idea though. :D

you've got several options.

if you don't have emissions testing, or if you do and you think you could pass the tailpipe sniffer test anyway, you could gut the insides from your original. there's a million threads about this, including avoiding CEL.

if you want to do it yourself, you can buy a universal from the internet and weld it in. or you can buy a higher priced, hi-flow, which might be more hype than substance. or, you can also buy complete cat/pipe combo's that can be bolted in without welding and little to no cutting.

or you can take it to a muffler shop and have them weld in a universal replacement which they supply. prices can really vary on this so shop around.

the earlier years were 2 pipes in and 1 out.
the later years were 1 pipe in and 1 out, with a Y pipe from the manifolds.
i don't know which the 99 is, look at yours and see.

Intake manifold replaced way back so I am not thinking that is contributing as much as just time. I have no symtoms of the gasket leaking. I have two pipes in and one out. Likely will just go to muffler shop and have it replaced by welding it in. I would have to cut it out to punch it out.

Check the plenum anyway. :D If they just replaced the gasket, and that was it, it may have failed again.

What do you mean by "intake manifold relaced?" New kegger, air gap?

If plenum gasket was replaced using the orignial pan and bolts, do not rule this out a as a possibility. It is worth checking it out as others have mentioned.

You lost me on the "no signs of gasket leaking...two pipes in and one out" as well. Have you removed TB and looking into the intake with a flashlight?