1993 Toyota 4Runner Something in engine making it run rich

I've had just about everything that can be repaired, replaced, or rebuilt done to this 4Runner. It ran great the first 5 years I owned it, but has had major gremlins the last 5 years. I had the engine totally rebuilt last year, to the tune of $4000+ because an engine rebuild place in SoCal rebuilt it wrong (also to the tune of $4000+ by the time they had nickleled and dimed me for everything they could think of). It was running great after the last rebuild. I also had a new muffler and pipe installed. So what's the problem? The catalytic converter (3 yrs old) committed suicide, blew a hole out of its side. Pieces of the Cat wound up in the (1 yr old) muffler, so I just had those replaced at a custom exhaust shop. The owner informed me that something in the engine is making it run rich, so it's gumming up the works and it'll destroy the Cat sooner or later. Unfortunately, the car is so old there's no way to scan it for codes. I called a local import mechanic who told me to not even bother bringing it to him unless I had $500 as the problem could be caused by any of 5 different things and he'd have to replace all of them to figure out what was causing the engine to run rich. GAH! Needless to say, I'm not made of money. My question: What would be the most likely culprit? The O2 sensor was replaced a couple of years ago (unless the creeps in SoCal charged me for it and didn't replace it). Where should I start? I hate to put any more money into the car, but I also don't want to ruin the engine after paying that much to have it rebuilt. Do I just have to bite the bullet and scrape $500 together for the expert to figure it out? Thanks for any suggestions and help you can offer!