87 5KCST High HCs

[John Larson <jdlarson@ix.netcom.com>]

Hi. A rich mixture produces High HCs, (as does a really lean one) and
water out the tailpipe. First thing you need to do is a compression
test, and you don't mention that. A cheap thing, often preventing the
throwing of parts at the problem. While we're doing cheap things, have
you disconnected the O2 sensor to check the effect on the mixture, or
checked it with your volt meter? How about the Fuse, the one that looks
like it's in a spare position? These are basic things to check, and can
often save time and money, and may not apply here, but should be
checked. Obviously, you've replaced the O2 sensor, but did you check the
old one or just replace it?
A good cat will eat a substantial amount of HCs, and some CO, but it's
efficiency decreases with age and miles. It works better when fully
warmed up, especially when old and tired, much like myself. You don't
say what the CO reading is. Even a new cat has it's limits and won't
make up for a dead cyl.
In closing, I suspect a compression problem with the #4 cylinder, sounds
like a burned valve, although it's not impossible that the injector is
dumping gas into that hole. Sorry this kinda rambles, it's late and it's
been a long day.
HTH, John