I need some help guys. I like to think Im decently mechanically inclined but Im struggling right now.

My '03 MCS "R53" has been acting up. Im currently at 178500 miles.
My acceleration has been sluggish when starting from a stop. And onve I get up to speed on the highway or interstate "55 and up" my throttle response has turned to ****. If the road starts going uphill "even a small incline" my speed will drop noticably and will not respond to throttle. I will have to downshift to about 4th and sometimes 3rd to keepfrom getting ran over.
my MPG has dropped immensly as well. I have gone from about 28- 30 MPG to around 18 MPG.

This is my daily and only car so I need some help please Im at a loss.Not sure where to start. Had a light that gave me code for cat but that just went away and not sure where to go now.

 

Going to move you to the Stock Problems/Issues section.

Long distance diagnosing is not to easy but some possibilities in no specific order: crank pulley, bypass valve or plugged cat.

 

Do you have a 'burnt rubber' smell emanating from the engine compartment? If so, your problem is likely a failed/failing crankshaft pulley.

 

A partially-clogged cat will certainly affect performance like you're experiencing but it should throw a code. First, I'd take a compression test to see if your engine is in need of a re-build. If it passes compression test, then drop the exhaust to check the cat for blockage.

 

No smell coming from engine.and just changed belt 2 mths ago when. Idler pulley bearings failed.

 

Have you ever changed the clutch, air filter, plugs. It could be several things .

 

I was finally sble to get under it last night and popped the cat with the heel of my hand. And it sounds like rocks in there so I guess the substrate broke off and is clogging it up. FML