Alright, so I'm chasing a damn demon.

I have a 1996 XJ 4.0 AW4 - 283k miles
Mods are
- IRO TB spacer
- K&N Drop In Filter
- Magnaflow Cat and Catback
- 5-90 Battery Cables

Tune up was done about 8k miles ago, last oil change was 2k miles ago. Tune up was full - plugs, coil, cap, wires.

None of the mods have been recent, all over 3k miles old.

Sunday on my way to hockey I was getting a slight hesitation and a bit of surging under throttle. Sub 3k and it would pulsate but it wasn't all that bad and it wasn't the entire drive (12 miles). I got out of hockey and left and the problem seemed a little worse, but I was able to get home.

Did a little research (no CEL) and figured it was the TPS. So I replaced that, no dice. But it wasn't really doing it as bad or as often, so I figured it just needed to learn.

Then yesterday it was bad. CEL came on, P0140 I believe and so I replaced the front o2. I know it was probably the wrong one to replace, but who knows how long it'd been on the car. Put the o2 in the car, if anything it's worse. It hesitates to start, runs awful and it will surge and pulse like crazy. Also now throwing a P0138 code.

Had to get it towed home.

Things I do know
- Fuel Pressure at rail is 48psi (tested)
- Front O2 is brand new
- TPS is brand new
- Header has a crack in it (I have a new one in the basement)
- HCV is leaking (I think I'm going to bypass it)

I guess my question is - does this sound like something that would be caused by a cracked header? I have pretty well eliminated the TPS, O2, and fuel pump as the issue. A rear o2 wouldn't cause this issue, it doesn't affect drivability.

Anyone have any input as far as what I should possibly look for? Or what may be wrong. Looking for some advice. Luckily I have 3 days to tear it down before I have to be back at work. But I want my XJ back.