Replace the injector. Replace all the injectors.
When ChryCo re-spec'd half of everything for the 1991 MY, they did away with the fuel injectors that RENIX used as well. Leaking at the body seam (it's just crimped, anyhow) is a known issue with 1987-1990 XJ/MJ w/4.0, and if you search for "engine fires" here, I'm sure you'll find out how serious the problem can be. If at all possible, DO NOT DRIVE your vehicle until you get replacement injectors. If you must drive it, make your first stop picking up a 5-10# A/B/C fire extinguisher (which you should have anyhow...)
You can usually get a set of "pull" injectors from a later 302 or 305 for quite a few bucks less than a set for your 4.0 - check
www.fiveomotorsport.com or similar. They're the same injectors (Bosch, pintle-style, 19-20#/hr.) and should drop right in. Spring for a new o-ring kit while you're about it.
Again, REPLACE ALL THE INJECTORS, and if possible, DO NOT DRIVE until you have. We've lost enough XJ's to engine fires, I'd hate to add another one...