48 mpg.... damn....

plenum leak, take off your air cleaner (need a flathead screwdriver) and hop on top of your motor and with a flashlight (and mirror if you dont wanna sit on your motor) and open the butterfly valve on the throttle body and take a look straight down, if there seems to be an exhorbinant amount of oil there, prolly a plenum leak. im lucky and to my knowledge havent had the problem yet. but i do check about every oil change... or if im really bored at work. the gasket gives when the steel pan and aluminum intake heat and cool at different rates giving the gasket a chance to get sucked into the intake and allowing oil to get sucked from the valley into the intake.

hope we can help you track it down.

 

Ok I'm gonna go against the grain and say that it isn't the death flash or a plenum gasket problem.

If a bad plenum gasket was bad enough to cause your truck to lose this much power, you would have a host of other problems too such as burning huge amounts of oil and lots of pinging. Your truck has neither of these problems, plus you said it sometimes runs like a raped ape, a bad plenum can't fix itself.

Its definately not the death flash because the death flash doesn't just happen. It either comes from the factory with it or it's put on by a dealership when the truck is serviced. Again you said that it sometimes runs great with lots of power, and again, the death flash can't fix itself.

By your description, I'm 99% sure it's a clogged catilytic converter. This will cause severe power loss because chunks of the cat block the exhaust flow making the truck bog down because the motor can't breath. The reason it runs great once in a while is because sometimes those cat chunks shift and allow the exhaust to flow better.

Have your cat checked by a muffler shop. Or you can take a hammer and tap on the cat, if you hear things rattling around in there, your cat is shot. You can also measure the temperature before and after the cat. If the temperature is significantly higher before the cat than after, its shot.

Good luck

EDIT: AW CRAP! I just re-read your first post and I see you've already replaced the cat. Sorry about that. It still sounds like an air flow problem to me though.

 

Sounds like you need to adjust the transmission kick down cable.
I don't think its the plenum but they all go so you might as well do it.
Have you routed the spark plug wires properly? I only mention that because there have been
how-to's on the original setup to keep the truck from misfiring.
How many miles? Have you considered pulling the injectors and having them cleaned?
I'm tired so thats all I can think of.

 

First guess. Plenum gasket, explains oil consumption and loss of power.

2nd guess. Grab a multimeter and test the TPS voltage. Backprobe the center wire with the red prong and hook the black prong to a suitable ground. Set the multimeter to read hundredths of a volt (0.00V) At KOEO, work the throttle on the passenger side of the TB and check for an even sweep from the base number to the top number on the multimeter with no skips or hangs.

If the base starting voltage is below .60V, I'd get a new TPS sensor. I like mine set between .70 and .75 volts. If the TPS voltage is too far out of spec, it won't read the correct throttle plate travel and could fool the PCM into pulling power as the PCM believes the plates aren't as open as they should be. If you have a scan tool you can verify that the throttle is reading 99-100% of it's potential travel.

I had a bad one once that read only 80% or so of the plate's true travel, and it was impossible to reach WOT. A bad TPS could also explain some of your crappy shifting issues.

 

Wow it sounds like alot of you guys are guessing and really have no ideas or even cought some of what this guy is saying. It is simple he says he has to pedal it to get it to down shift and every so often it has power and takes off. It sound to me it is deff. a trans problem with the kick down cable or the valve body. I'd take it and have the trans checked out. For one the plentum gasket if it was making power lost would be making the truck run like *** all the time and not clear up unless you change the plugs from oil build up. The "death flash" everyone is talking about is a bunch of crap and would cause the problems this guy is having. CSXfan is right with the trans or Cat. problem I'd check those two things first but I'll bet it is the trans messing up.

 

I thought the trans shifting was electronically controlled in our trucks by things like the throttle position sensor and govenor pressure sensor and soleniod?.....at least thats what fixed my shifting issues....but its supposed to throw a code right? Mine did.
You didnt do anything else like get larger tires or anything right....lol sorry had to ask. in either case watching the rpm's would give an indication of tranny issues.

 

Only overdrive and TC lockup are electronically controlled

 

Even if you put a new cat on it,if the old one was trashed,material could stop up your muffler.