Recently my 1998 12V failed the Colorado Diesel Vehicle Inspection because of excess exhaust "opacity" (smoke). My numbers for this lug down test were:
@60 MPH 78% opacity;
@50 MPH 56%;
@40 MPH 48%;
@zero shift (whatever that is?) 2%.
***? This truck had always passed inspection on the first test. The kind lady told me to change the air filter and bring it back for a no-charge retest. I run a Napa Gold air filter which was many years old and dirty but the back pressure thingy on the air filter housing had not moved so I always assumed the filter was not clogged. Also performance and fuel mileage were normal. Anyway, I changed the filter and the retest showed minimal opacity, 3%, 6%, 6% and 0%, essentially no smoke at any load. What was the lady's clue for a bad air filter? Look at the first test numbers: "Zero shift" I guess is engine idle so there was enough air provided to burn the minimal fuel. At higher loads (lug down) the air filter did not pass enough air to burn the increased fuel which results in smoke.
My maintenance tip: Change your air filter when it looks dirty, and don't rely on the stupid air inlet back pressure gauge.

 

Wow , the drop in the % was massive , who knew right ?

 

My gauge used to work but no longer does. I think I disturbed the grommet where it seals to the filter box, or the seal on the box half no longer seals well after so many years. That probably was once a good indicator but age, heat, and constant compression take a toll.

 

You can get those at NAPA and they are not that expensive