I believe that my cats are plugged on my supercharged 07 GT coupe, but how can I know for sure without going to the dealership and paying for diagnostics? The exhaust is 100% factory stock.

A little history. Earlier this year, my Vortech blower fried a bearing, so I sent it back to them for warranty repair. While I had the blower off the car, I put the stock airbox/belt back on so I could drive it while the blower was being rebuilt. I accidentally ran the car with the bigger 39# injectors for about 5 minutes, but it ran like crud and I remembered immediately that I had the wrong injectors in. I swapped the smaller, factory injectors back in, started the car and it ran great. After a little driving, I got a check engine light. I pulled the codes with my tuner and they were for left and right bank catalytic converters (too much unburned fuel). I cleared the codes, but they came back a couple more times over the next couple weeks. Got the blower back, reinstalled, and car was great.

Last week, the car started missing. I ordered new spark plugs (I use copper FRPP 3V0's for the supercharger). Two plugs were really loose and badly gunked up from the obvious blowby past the threads. Installed the new plugs, fired it up, and it purred really nice - misfire cured. Test drove and it ran great & pulled strong.

A couple days after swapping plugs, the car started making an odd noise. It's still making great power and no misfire, but it sounds like the exhaust is choking on itself as the load increases. With no load, it revs clean. Go up a hill, or give it gas and it almost sputters. Full throttle is still strong, but the sputtering is almost a hesitation at first before it starts accelerating. There's also a rattling occasionally from under the car (sounds like a heat shield rattling almost?).

I'm fairly sure it's the cats and I ordered SLP's X-Pipe with high flow cats, but they're still in transit. Even if it's not the cats, I've wanted to do an X-Pipe for awhile now, and will eventually add SLP's long tube headers and loud mouth (or power-flo) axle backs.

Any input or ideas? Am I on the right path?

 

yer on the right path with the upgrades for sure. If you can get under the car just hit the exhaust pipes with your hand and see if you hear anything odd.

 

Thanks, Glenn. I was under the car a few days ago and saw nothing obviously loose or close to hitting anything that moves, but I didn't hit the pipes. I'll give that a try.

I spoke with our service manager (who has raced/owned a few Mustangs), and my best friend who is one of the best mechanics out there (but a Chevy mechanic), and both also suspect a fuel trim issue. Adaptive fuel trim programming could have run amuck with the loose spark plugs and lean condition from a clogged cat. Both advised me to disconnect the battery overnight (which I've done just now) to reset the computer. I'll then reload my custom dyno tune. Then the hope is that I can fire up the car and it will relearn the current conditions freshly (by having no long-term fuel trims interfering). If the cats are truly toast, I'll still need to replace them, but if the cats are okay then I can drive the car without issue (and still install the SLP x-pipe that I've already ordered anyway).