Jaguar Repair: 84 XJ6 loss of power, continued, fuel pressure regulator, quick snap

Question
Howard,
I made the necessary modifications to the front catalytic converter to improve airflow, but still note that a gauge affixed to the manifold reaches way up to 7+ PSI when the accelerator is "stabbed". Is this still evidence of the second cat being blocked, even though it seems to run great now? I even took it out on a divided highway tonight at 70 + MPH, and had no problems. At the end of each "stab" of the throttle, there is a cough, but not quite backfire at the exhaust. Is that indicative of a clog still present, or perhaps related to the bad fuel pressure regulator? My new one arrives tomorrow, so I will install and continue to test.
Thanks so much!
Michael

Answer
The high RPM and high load is what you need to look at for back pressure as opposed to a quick throttle movement. A quick snap of the throttle sends a charge into the exhaust that has a hard time pushing the long column of exhaust in all the pipes and mufflers out so it can be normal to see a jump of the pressure when the throttle is snapped.  

If your pressure regulator is still not giving you a pressure boost when you open the throttle (which makes vacuum go down and fuel pressure go up to act like an accelerator pump in a carburetor.) That can be a source of a "lean" cough.

Howard