Help - Smog Test Results Analysis

Air-injected? If so, check the plumbing that controls when the air injection is active. Mine was vacuum-operated, and with a crack in the vacuum line, was failing miserably on hydrocarbons at all speeds, no matter what I did to the beast. Finding and replacing that length of vacuum line completely cured the problem. Also look at the air pump (if any) and plumbing between that and the point where the air gets injected.

A bit on the lean side when slower. Lots of places to check. Air leaks in air path, EGR leak-by (carboned pintle), or just the catalytic converter wearing out.

- Bill

Bill - Can you explain how a worn out catalytic converter would explain the high HC level at 15mph while the 25mph reading is fine?? Also how did you determine that the 15mph reading shows the truck is running leaner than at

thanks, Paul

Your results:

Lower speed (15 mph) shows higher RPM (lower gear) than higher speed (higher gear). Lower speed shows HC MEAS(ured) 118 just under the MAX 121. New catalytic converter can bring this down because it will have fresh chems. Higher RPM means more gases through the pipe at higher velocity. This means those gases will not have the same time-exposure to the catalytic so more HC get out the pipe. On the other hand, the EGR process of recycling exhaust gases has an effect on combustion temperatures and it also causes a leaner mixture which creates higher HC's. So if it's malfunctioning, it could create the same result.

- Bill