97 XJ fuel pressure question

What should the fuel pressure be at idle on a 97 XJ? Mine is hovering right around 50-52 PSI at idle. When I give it some gas and rev it up it stays exactly the same. After I shut the engine off the fuel pressure quickly drops from 50 PSI down to about 20 PSI. It then stays at about 20 PSI for a few minutes with no drop in fuel pressure. Is this normal? I'm trying to track down the cause of the horrible gas mileage that my XJ gets, and I'm hoping that this is the problem.

You might be running a little rich because the fuel pressure shouldn't be more than about 49 psi. That could be the cause of your mileage drop.

The fuel pressure should drop off but I'm not sure if it should drop off that quickly.

I think somebody else on here probably knows a lot more about this issue because I've seen "check valves" installed in the fuel line to maintain pressure.