1986 Toyota Corolla Dropped Nox Emissions- But HC's are now

ISOD

MEMBER
1986 TOYOTA COROLLA
4 CYL
2WD
MANUAL
141,000 MILES

Okay so here's the deal. This is the 1986 4A-C Corolla engine in a Sport SR5 SOHC. I failed the first emissions test with the main problem being the Nox. I was at the 3000's and needed to be at the 1.2 thousands. My mechanic cleaned my EGR valve and I almost got into range with 1.4 thousand NOx. However I was almost passing HC's with a 250 without EGR cleaning (needed something like 150 hc's to pass.) Now I have 100 more HC count totaling 350 HC's. Mechanic is telling me to replace intake manifold gasket due to leaking fuel causing HC'S to rise. He then said the Nox would drop with a new O2 sensor. This car is actually in a very operable condition. Just very old parts. Let me know what's up guys. Thank you very much. You guys are doing something simple and straight to the point and this is highly appreciated amongst the community. Thanks a million.

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.