I have a feeling your 1991 Honda Accord turned to a gross polluter during
the smog check because of high NO (NOx - Nitrous Oxides). I'm assuming your
Honda Accord has high NO because if any other emission (HC or CO) were gross
polluter limits, you would definitely feel a problem with your Honda's
performance; maybe lack of power, maybe smoking. High NO could have caused
your Honda to become a gross polluter without you noticing any major engine
or performance problem/s.
There are several reasons why your Honda Accord could have produced high NO.
The most common is your Honda's EGR system not working properly. The EGR
valve in your Honda Accord is responsible for re-circulating a small amount
of exhaust gas back into the combustion chamber. This is done to cool down
the chamber temperature by producing an inert gas along with the regular
fuel mixture. Nitrous Oxides emissions are formed when engine chamber
temperatures rise above 2500* F. Keeping the chamber cool reduces NOx
emission.
The second most common cause your 1991 Honda Accord may have produced high
NO is a lean fuel mixture. A lean fuel mixture is when the engine receives
less fuel then is necessary to obtain efficient combustion. Lean fuel
mixtures are often due to vacuum leaks or defective fuel control. A smog
check repair shop will have to ensure your Honda Accord is in proper fuel
control. They will have to inspect the fuel injection "feedback" system and
ensure the engine computer is sending the correct signals to the fuel
injectors to either increase or decrease fuel delivery to the combustion
chambers. When the engine doesn't get enough fuel NO emissions are
increased.
