Help: rough idle, no power under load, exhaust sounds louder and choppier both low and high speeds
So I've got an 09 Focus. It has a steeda cold air intake with the restrictor removed, FSWerks 4-1 header and 2.5 inch stealth exhaust, and tuning by Tom.
Tonight I drove downtown to go eat dinner with my parents. When I started it up in my garage it sounded a little strange but thought it was all just from the echo, and it drove fine. When I got out of dinner after it sat in the street for a couple hours, it was maybe 20 degrees out (nice n' cold), and when I started the car up I noticed it sounded really funky. The exhaust note was deeper, sportier in fact, and the idle was choppy- almost like a very modest aftermarket camshaft.
Sometimes when I start my car up the AFR (I have a sensor mounted on the flex pipe) will progressively drop down to 10 and the engine will stumble. Never really knew why, but if it happens I just kill the engine and restart it, and once I get it driving it stops being an issue- but it did that this time, so perhaps it's relevant now. As per usual I got it moving and expected it to iron out the problem, but it didn't. I had very little power, and when I drive it to be honest it sounds like my friend's modded Subaru STI. But it's definitely lost power.
I considered a few things. Given there's a sound change, maybe my header is cracked. That might explain the AFR gauge and the loss of performance if the AFR is not reading properly from a crack (however the ECU's AFR sensor is on the header itself, while my AFR gauge sensor is at the flex). But I popped the hood and I don't see any steam coming off it (had my girlfriend rev it up for me) and I don't hear any new hissing noises. An exhaust leak anywhere else shouldn't cause performance issues, so that's out.
A friend I called suggested maybe my vacuum line- but why would it sound different? I haven't looked at that yet.
Driving it to be honest it feels like a misfiring cylinder. But I have no CEL thrown. I'm wondering if maybe that could have changed the sound too, but I can't decide if the car sounds LOUDER or just DIFFERENT. I used to run it with the "race" exhaust (no resonator) when I first put the header on, and it is definitely not as loud as that, so again I'm leaning away from an exhaust leak.
Is there any component that might cause a misfire without throwing a check engine light?
The car just made a 2200 round-trip drive to Colorado from Chicago over Christmas... while I was out there I cleaned and re-oiled my air filter, I had some issues with it running rich at first but it normalized and I hadn't had a problem with it all month til now. Replaced my spark plugs maybe 2 years ago but they supposedly were good for 300,000 miles. Would a coil pack throw a CEL?
Could my TB or MAF sensor just be dirty?
Any ideas or suggestions of what to check appreciated. By the way, the header has been on for over a year and it was parked outside all last winter... if anyone was wondering about it just being something from the cold. I've left it in my garage tonight so hopefully it just starts up magically tomorrow with no issues ;)
2009 Focus Coupe SES 5-spd MTX
Steeda STS, Steeda SRI, FSWerks Race header, 2.5" flex, and 2.5" race exhaust
Tuned by Focus-Power (Tom)