Rep Power: 0

Have a 91 civic Si with a b16a swap in it. Car was not starting a few days ago, turned out to be fouled spark plugs (all of them). Replaced them and the car fired up and ran great for about 5 days. Now the car will not start again, plugs appear to have already been fouled out. I also am only getting 13-20 mpg. ENGINE IS RUNNING EXTREMELY RICH. Here is my question

I know that this car has a bad o2 sensor (check engine light code points to it). Could a bad o2 sensor really be the cause of this poor gas mileage and how quickly my spark plugs are fouling? The plugs were coated with carbon, not oil. Engine does not leak or burn any at all.

Could a bad fuel regulator be causing this? My fuel regulator also appears to have a adjuster dial connected to it. How do I test the fuel regulator or adjust it?

What else could it be? Dizzy? Dizzy cap? PCV Valve? Fuel injectors? Fuel regulator? Fuel filter? Any help would be great appreciated, I am stumped.

NOTE: Problems appeared to begin happening after I adjusted my FITV and cleaned my IACV. Could this be related at all? Came to this forum as a lot of you out there are much more experienced than me! Please help!

 
Rep Power: 234

The carbon buildup is from running rich (unburnt fuel). A bad O2 sensor or a mis-adjusted regulator could cause this. It sounds like the fuel pressure is too high causing carbon buildup on the sensor. Cat may be bad by now too.

Replace the O2 sensor and plugs. Hook up a scan tool, clear the code, then go to live data feed on the O2 sensor and see if it's still running rich. If it is, slowly dial back the regulator until it's running right.

If you don't have a scan tool, hook up a fuel pressure tester and dial back the regulator until you have correct fuel pressure. Fuel PSI should be in the low 40's.

*O2 sensor should sweep from .1-.2 to .6-.9 volts. If it's running rich it will stay at one end of the spectrum.

All that being said, this is advice for stock performance. If you have any more mods it's anybody's guess.