Running Richo or low gas mileage? Try checking fuel pressure

[Reejerey]

So I went to have my truck inspected and emissions tested the other day, passed safety with no issues, but failed on emissions with high HC and CO, both nearly 10 times the allowed level. I've had a check engine light coming on intermittently for the past year, so I checked my codes and found code 173 as the only code, HO2S Stuck rich. The rich o2 reading was expected with the high emissions, so I preceded thru the diagnostic flow chart to the point of checking the fuel pressure, which was supposed to be 35-40 psi, but I was running over 70 psi. Checked for vacuum to the regulator, and had good engine vacuum at the regulator. Checked for A pinched return line and found no issues, so I replaced the fuel pressure regulator. Now running at 35 psi fuel pressure and no more codes, and passed emissions on the recheck.

 

[Opera House]

That is quite an unusual failure. I've cut these open and they usually have seat wear to one side that causes them to stick open or the more common leak to the vacuum side. The only possible mechanical failure this could have had to cause high pressure is some debris that plugged the small valve seat port.