As described above, I thought it was the header gasket, so I took the header out which was fully intact, and replaced the gasket. No change whatsoever. I can't see exactly where the smoking gun is because it's behind the engine and the billows are large, but it appears to be right above the header on passenger side. There was oil residue on top of the header heat shield as well if that helps narrow it down. Fuel economy is way down and way too much CO is getting into the cabin and my CO alarm goes off after driving for more than 5 minutes, so I keep getting CO poisoning and it sucks. Please help me diagnose so I can spend another 16 hours of hard labor working on it instead of breathing fumes.
