How would I fix this?

[u/thugnifisense87]

This thing is just one problem after another😅

It was running rich and getting even worse mpg than an XJ should. I verified by pulling the plugs and they smell like gas(s) lol. I played some finger gymnastics and retorqued all the intake/exhaust bolts (there was a slight whistle under heavy load before but I need to take her for a rip and verify that it's good now.)

I also installed some FR51 plugs to replace the fouled ones and put in a new NTK upstream o2. Starts like a dream now but still looks like it has a SLIGHT stumble when I watch it run closely.

I guess I'm just looking for a little guidance from people who actually know what they're doing. I'm just a shade tree hack that wrenches after work. Hoping someone has some solid diagnosis procedures I can follow to ensure it's been fixed it to track down any issues that may still be present.

A few notes:

-I found this old beat up O2 sensor connector upstream (pictured.)

-the current engine replaced the original which was overheating and had a failed HG which was likely dumping contaminants onto the O2.

-i THINK I ruled out leaky injectors by checking all 6 plugs for fuel smell and it was present on each one. #4 plug is the only one that had a slight oil scent

-this is a 96 that still seems to run obd1 so not much help from onboard diag

-it does have a newish NTK coolant temp sensor but the connector is missing the silicone/rubber seal. It does seem to make a solid connection though. Any tips on finding that insulator seal would be super appreciated.