#1....why the heck is my car so much louder in reverse? ever since i got my cat and stuff done my car has this nice rumbly growl, and you can actually feel the car vibrating a little bit if you get on it hard in reverse.....doesnt do the same thing in any forward gear no matter how hard you get on it....so im thinking something...is hitting something in reverse that it wouldnt hit in drive, but why?

#2 upon removing the oil cap from my running engine, it stumbles and the idle becomes rough, this is a symptom of a bad PCV valve correct?

#3 my gas mileage has been suffering a bit lately, the car runs very well, im thinkin maybe the O2 sensor is crapping out, but just isnt throwing a code, also the PCV valve in #2 could be causing problems, i think the plugs are fine, what are some other things that could cause bad gas mileage?

I'm wondering the same thing. I can't get any better than 23 MPG on all highway driving, and this time last year I was getting 26 average. I also scanned for codes and there aren't any. How many miles do you have?

i have 112,xxx miles

up until last week i had never seen below 20mpg on this car, no matter how hard i drove it. even all summer long with the AC on full blast id still get 21-22, and actually the highest ive ever seen is 25 if i remember right, but a lot of my driving isnt highway, and ive heard the 3.5 is somewhat inefficient in those types of conditions.

But I saw 18-19mpg last week, so something has to be up, i noticed the stumbling with the oil cap removed today when i changed the oil.

Well for the northerners, MPG killer is reformulated gas. As for the rest, change the valve and 02 sensors since those are quick inexpensive fixes.