1994 Toyota Pickup high hc on smog test failure

PTURN

MEMBER
1994 TOYOTA PICKUP
4 CYL
2WD
MANUAL
174,000 MILES

This is the second time it failed for the same reason. HC max 60 PPM measured 74 PPM. First time fixed by adjusting the air intake adjustment screw. That was 4 or 5 years ago and it has passed smog a couple times since then. Will the adjustment screw fix the problem again or should I be looking for another or bigger problem. If this will fix the problem where should the adjustment screw be adjusted to?

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings