Poor mileage and running rich

[Clint09]

First ever post for me here. I’ve been struggling to get my USA 97 16v running properly. The most obvious symptoms are poor mileage (18-21), very rich exhaust at idle, a code for bad catalytic converter, and an occasional misfire. The previous owner told me the catalytic converter was al busted up inside so he hammered it all out and it is now gutted. I Have been testing different things and it seems that the EGR doesn’t work properly. The EGR passes the stall test but when I rev the engine (with rear wheels turning) doesn’t seem to move the EGR diaphragm at all. After more research I learned that the EGR needs the CAT in place to function properly. So I have plans to reinstall a CAT but I am wondering if this could also be the source of my rich exhaust issue or is that being caused by something else? According to a scan tool all other sensors are reading fine. Also what would be a good option for a replacement CAT?

OBD2 USA rule #1 (mine) is dot not worry P0421 cat codes, until the engine is fixed (EFI and all)  CAT errors are normal if the engine is sick, and is.
18mpg is horrid.
ok gutted, and break ups are common and so is that act, if not in smog testing state no worries there.
The P0400 error (DTC) will be endless as will the cat error P0420 , P04xx errors endless all, no Cat,
the cat missing on these cars, kills dead EGR so EGR errors P04xx hit. all 100% normal for missing cat or rodded out.
EGR uses cat back pressure to measure engine loads,  to inject EGR Gasses at the correct rate
The Cat misssing means the EGR mode will be dead, how can it not be dead. in this case.
but lets forget all the above for now... focus on spark and fueling, if compression is 180psi all 4 cylinders, done WOT.


The scan tool is handy but you need to look more carefully.
Rich means, 2 things, (besides gross misfiring bad spark, parts, ever do full spark tune up?)
1: fuel pressure is too high. (FPR dead as most are this old) (need keyon pressure  and idle pressures to know) no fuel leaks out vacuum nipple FPR.?
2: 02 sensor fibbing, or just the #4 exhaust tube heater crack sucking in air making the o2 lie all the time.
QOD:
1: is the o2 in closed loop at HOT idle.
2: is the O2 sensor scanned, swinging at hot idle.
3: is coolant at 180F for sure in scans of ECT at 180f or above?
4: drive car lots with O2 unplugged, Ignore DTC, errors, and get better MPG and end black exhaust smoke and raw fuel stink?

lots more but this is a good start.

It has new plugs, wires, cap and rotor within the last year. fuel pressure is around 45psi, compression is mid 170s on all cylinders, Fuel pressure reg is new and tested working and not sucking fuel through the vacuum line into the intake, I did notice the #4 manifold pipe is cracked. I didn’t realize the cracked manifold would suck in air and mess up the o2 sensor. I’ve unplugged the o2 sensor and drive for a while but didn’t notice much difference. Maybe I’ll try that for a full tank of fuel and see if the mileage changes.