drastic power loss - yes.
pinging - yes.
excessive oil consumption - yes.

less mpg - maybe.... the primary effect of a plenum leak is a lean condition. 02 feedback will cause pcm to richen the fuel mix up to its preset limit to compensate, but not beyond some limit. so the lean mix causes higher cylinder temp, which in turn causes pre-detonation (pinging), which means the fuel ignites before the spark, which causes the engine to produce much less power using the same amount of gas pedal, which in turn (finally), can cause your mpg to suck.

 

A leaking plenum would cause a vaccum leak. Read the FAQ in my signature if you want more info on the plenum.

 

I think I might have the same problem with the rattle, will see if I have same symtomes

 

eating oil: probably the leaking plenum like others said.

rattling sound?: Sounds like your catalytic converter is getting clog from all the oil consumption, e.g. from leaking plenum.

 

ok ive checked inside the intake manifold, its all wet on thbe side walls and bottom also the blades of the throttle body are all wet on the bottom part, still have a knock but one the mechanics said it sounds like cranck bearing, but no knock in idle only under load like if im accelerating or if i hold it at 2000rpms in park, really stuck here dont know what to do if i should even statrt with plenum or just do a 360 swap, because im not sure if the plenum gasket can fix that knock, my question is can a vacum leack caused by bad plenum gasket make a piston or something on the bottom end make knocking kind of noise? or can it just sound like bottom end knock and only be caused by vacum leack and actually come from the top? still same 3 problems i got loss of power, eating a lot of gas, and loss of oil

 

its the plenum...change that, 02 sensors, plugs wires, dist.cap/rotor, and CAT, and you should have a tought rig after

 

i still dont understand whats knocking?, and the only time it makes that knock noise if under load not at idle, just this plenum gasket can cause all of the symptoms i got?

 

The plenum leak means oil is getting into your intake, and mixing with fuel, thus lowering your octane rating. This causes the fuel not to combust properly which is causing it to ping/rattle, drink oil, and lower your fuel mileage.

Happened to me, did the hughes plenum fix, no more oil consumption, no more ping/rattle/knocking sounds.

 

IF its spark knock (pinging), its actually not the oil that causes the ping, its the additional unmetered (big word) air leak that creates a LEAN fuel mix which is more volatile, burns hotter, raises cylinder temperatures, and eventually combusts under compression, before the spark plug fires, or maybe combusts explosively. that's why its technically led predetonation, and its actually firing ever so slightly out of time, which leads to sucky power.

here's a long winded discussion
http://en.wikipedia.org/wiki/Spark_knock

 

ok thanks, it just sounds crazy to me (the knock) maybe because i never had any knocking, i will do the plenum this weekend and hopefully its the problem and not something else knocking and ill do a smoke test for the vacum leak, thanks everybody for the help and hopefully thats the problem