I recently went for a smog check on my 1994 Accord LX and it fail because of high levels of NO emissions. At this point I don't have the Check Engine Light on. The results of this test are shown below.

%CO2 %CO HC (PPM) CO(%) NO(PPM)
TEST RPM MEAS MEAS MAX AVE MEAS MAX AVE MEAS MAX AVE MEAS results
15 mph 1918 15 0.2 61 27 18 0.51 0.1 0.22 490 165 1092 fail
25 mph 1844 15.1 0.1 36 16 12 0.49 0.09 0.21 511 128 841 fail


What I did next, after reading in this forum, was cleaning the EGR ports and apply a good dose of carburetor cleaner spray. I then drove the car on the freeway for 10-15 minutes and went back to the test station. Here are the second test results.

%CO2 %CO HC (PPM) CO(%) NO(PPM)
TEST RPM MEAS MEAS MAX AVE MEAS MAX AVE MEAS MAX AVE MEAS results
15 mph 1816 14.8 0.1 61 27 23 0.51 0.1 0.13 490 165 830 fail
25 mph 1853 14.8 0.1 36 16 16 0.49 0.09 0.12 511 128 644 fail

The levels of NO improved but the car still fail the test.

I wonder what else can I do to pass the test. I discarded the option of changing the catalytic converter since the levels of HC and CO are low and within the average levels. Any help will be appreciated.

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Are the spark plugs due for a change?

Download the 94 shop manual from one of the links posted in the Online Manuals thread in the DIY section. That will show you how the EGR system is connected and the locations of the components. There should be a sticker under your hood showing the vacuum routing as well.

Inspect the vacuum hoses throughout the EGR system. Also make sure they are connected properly. The hoses for the EGR control solenoid and vacuum control valve are hidden and could be cracked/rotted.

 

Also try retarding your spark timing just a bit. That would lower NOx (but increase CO & HC).

 

Thanks for the advice. The spark plugs were replaced about two months ago. I'll check into the vacuum lines and try to retard the spark timing a bit. Do you think if the cleaning of the fuel injectors may help?

 

Cleaning the fuel injectors can't hurt. Usually when you have high NO and everything else is in spec, then the EGR is cause of high NO emissions.

 

Usually NOx plays off against unburned HC (which includes CO). If you do something to reduce NOx, it tends to increase the HC - and vice versa.

Lowering combustion temperatures & pressures will reduce NOx while increasing HC. You can fo that by fixing a condition of insufficient EGR flow. If that's not enough, you can set spark timing towards the retarded end of the "allowable" range. Got a timing light?