NO CEL. the car is strggling to accelerate off the line at WOT, it will not break the tires loose and gets kinda choppy off the line. the car also is having issues when above 3,000 RPM almost cutting out then going and repeating worst at WOT. the car has a bad Muffler but is currently patched. New gas cap (AC delco this time) as well as being recently seafoamed have not solved the issue. I Have had P0420 in the past but seafoam fixed it.... However the car does stink always has. not burning excess amounts of coolant or oil. tranny service with new fluid did not help. Paul cannot replicate it. it is becoming more and more constant. from your experience what could be the issue.

running 9.2 psi vacuum at idle and -24 degree timing at idle. according to ECU the vacuum goes to near 0 at WOT is this normal?

 

I had a P0420 show up after changing out my Cat a while ago. Basically its saying that your front and rear O2 sensors are reading too similarly.

I got rid of it by putting in a spark plug defouler at the rear O2 sensor which takes it out of the stream of the exhaust and forces it to read differently than the front O2 sensor.

If you are having acceleration issues could it be that your Cat is plugged? I had those same issues in the beginning which is why I had to swap out the Cat. My acceleration was TERRIBLE I mean I couldn't roast the front tires for anything and the acceleration was REAL hesitant at times.

*EDIT* I meant to say is it possible your Cat is plugged LOL

 

Without knowing more, I'm throwing two thoughts out there (they are nothing more the guesses of where you could start to look):
1 - Failed Cat - When it's really acting up, take a look under and see if the cat is glowing red, if so, guarantee that is the issue. If it's not, it still may be a problem, more work would need done.
2 - Old/worn out spark plugs. Sometimes the problem begins a little at a time and progressively gets worse. If the plugs are over 100K miles, change them. AC Delco or NGK is the recommended plugs I would go with.

 

Plugs are a year and a half old. so should be ok unless i killed them with seafoam.

CAT does not glow. I'm having success with using electrical tape to tighten up some vacuum lines. it appears the PCV valve is too loose in the crankcase creating a vacuum leak. I'm going to try to get a new grommet as the PCV valve is less than a year old. the grommet looks new but damaged. when paul put in the new PCV it would slide up it hasn't in awhile. i taped around PCV and the valve slides up again.
*not perfect but better*

The cat may as well be changed in the spring when i do a new muffler to help me with smell. so hi-flow cat and flowtech afterburner muffler in the spring

 

I had the same issue with my PCV valve being loose, and wound up buying a new "kit" that had the map sensor bracket as well. The guy before me had broke the map sensor bracket.

When my Cat was plugged it had horrible acceleration. I didn't notice any glowing but when I hit the gas to WOT from a stand still it would like "chug" a little and then accelerate. I even had a transmission code show up with mine. Had a transmission shop tell me the tranny was toast, changed out the plugged Cat and VOILA' no transmission code AND my acceleration came back.

 

EVERYONE wants me to go cat-less around here. I ordered a new PCV valve crankcase ventilation grommet it was 3 bucks at autozone and ill get it tomorrow should solve most of the issue. IF NOT ill go crazy on that stupid CAT

Car has started chugging off line but taping up near the PCV seemed to fix it. only time will tell

 

Maybe just a vacuum leak then? I would have done without mine as I live in a state where we don't do emission testing but I couldn't find a shop that would eliminate it for me. My nephew told me afterword that they could have done it at his shop but its a big fine if they are caught doing it.

 

well with the new gasket the car feels much healthier pulls much stronger but still has the problem on occasion specifically once warmed up. vacuum and timing are much more stable and idle is much smoother. this confirms that the engine it self is healthy. ill try some normal dry gas next fill up before trying cataclean.

 

I say its a bad cat! my 03 ss was chugging,no pass gear terrible mpg. I also checked for "glow" which it did not, however it was the cat, 240$ later ran like a scolded dog!! is your gas mileage crappy??

 

The EGR passage is probably plugged \ restricted