1995 XJ6, 4.0 US market car with EGR and Air injection.

The car has flaky idle and cracked exhaust that I am in process of replacing. It had a sizable exhaust leak and the air injection pipes were unplugged from exhaust manifolds. When I lift my foot off the accelerator the car produces a popping sound and backfires. I don't see a check engine light on and I have not tried to pull any codes yet.

The car currently has autolite plugs and aftermarket coils. I am swapping them for champions and used OEM coils, just to rule out the obvious ignition issues.

Popping/backfiring usually means unburnt fuel making it into the exhaust. My suspicions are:

- Plugs, coils and wiring to the coils
- Exhaust leak meaning too much air gets to O2 sensors fooling the ECM into thinking that the mixture is too lean, so it gets enriched too much
- injectors sticking open
- exhaust valves sticking (How easy are they to inspect with manifolds off?)

Once I removed the exhaust manifolds the engine became more eager to rev up. Should I suspect plugged cats or is the Jaguar exhaust supposed to be that restrictive? If I put a manometer instead of the O2 sensors, what readings should I expect?

I also discovered that the previous owner completely blocked off the EGR by using a custom gasket with no hole between the EGR pipe and the exhaust manifold. Do these cars run better without EGR? I am considering exorcising EGR and air injection, at least temporarily to eliminate them as suspects. Do I need to get PROM chips with Euro or some other non-US calibration? I suspect that the fueling and ignition advance maps are different for cars without EGR, and for a good reason.

Suggestions on course of action would be greatly appreciated.