QuestionTuned it up & took it to smog...
Passed visual, CO2, O2, CO, & NO, but failed HC (62ppm over at 15 MPH and 7ppm over at 25 MPH).
Any ideas what is the likely cause?
AnswerExcessive hydrocarbon emissions are always caused by incomplete combustion, they are caused by unburned fuel, this may be because of an engine misfire or some other reason for one or more of the cylinders to not burn all of the fuel and instead dump it into the exhaust.
Since the tune up was already done I would recommend having the engine compression tested, there may be a leaking valve or another reason for a loss of compression in one of the cylinders, this could also be caused by an intake air/vacuum leak or something more serious like a blown headgasket.
