Dear the smog man, or anyone else that can help...

I have a 1991 Civic Dx that recently failed the smog test in San Francisco. Here are the readings:

Low speed emission results (15mph)
-------------Max Allowed-------Measured
HC(PPM)------- 130------------21
CO (%)---------0.80-----------0.08
CO2 (%)------------------------15.2
O2 (%)--------------------------0.0
NOx (%)--------835-----------934

High speed emission results (25mph)
-------------Max Allowed-------Measured
HC(PPM)------- 105------------18
CO (%)---------0.69-----------0.09
CO2 (%)------------------------15.2
O2 (%)--------------------------0.0
NOx (%)--------774-----------750

This was a retest, I had failed it just a couple weeks before, with almost identical results, both times the low speed NOx is too high (1108ppm the first time)

I read a similar thread that suggested a new Cat.

https://honda-tech.com/zerothread/1794590

However in my case, my O2 numbers are zero, and my HC numbers are low, suggesting that the Cat is working fine, right? The low HC would suggest a LEAN mixture, right? and this would cause elevated combustion temperature and thus high NOx, right?

So whats my problem?

I do NOT have an EGR valve, so this easy fix is out.

Ive visually checked my vacuum lines for leaks or cracks.

Im sure the timing is fine, as the car does not idle rough or ping or knock at all.

The temperature seems to be fine, the gauge is always in normal range.

Im thinking of replacing the Oxygen sensor, reasoning that it might be causing too lean of a mixture if its not working right. Would the low HC and CO readings mean that the Oxygen sensor IS working correctly?

I dont really know where to go now, as there doesnt seem to be an "obvious" answer beyond an EGR valve, which this car does not have. Im zeroed in on the oxygen sensor, but not sure about this....

HELP!

 

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Im sure the timing is fine, as the car does not idle rough or ping or knock at all.

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Your sure? Have you actually checked it? A couple degrees can make a big difference.

 

No I have not actually checked it, I was just assuming...

On my smog results, it says I passed for 16BTDC

 

set the timing back a degree or two and do a tune up and u should pass, you didnt fail by much at all. also maybe a slow o2?but do the tune up first

 

I did a tune up between the first fail and the retest. I changed the oil, topped off the coolant, replaced the spark plugs. This dropped the NOx from 1108 to 934 ppm, but I still failed again.

Why do you think its the timing?

 

i work at a kragens n when i went to referee my crx i used CRC GUARENTEED TO PASS cost like $7 and i dropped my nox from 1102 to 856 try it and ull pass no prob.

 

umm..maybe u need to change the o2 sensor..just a suggestion

 

High NOX is typically a lean mixture...and high combustion temps as a result. Could be O2 sensor...or it could be dirty injectors as well. Put a can of BG44K in the tanks and drive its tail off for use up most of the tank. I'll lay odds you pass it after that.

 

Great job.

Your HC is at a good level. Not the problem I don't think. It is a little low, I would like to see 35 - 40.
CO is up a very little. This has to do with poor combustion. Might be a clogged injector. Injector cleaner for this and it might change the HC too.
Nox... Is carbon build up raising combustion temps by raising compression and or pre-ignition. Can be timing like was said. But I have found that a fan in front of the car when testing is a must. (I hope the tester used a fan. Required over 72degrees)
So check timing and stay on the retarded side just a little If at 16 bump it to 17 or 18 but don't go past 2 degrees +or- spec.
Decarbonize I also would suggest, with Seafoam thru a small vacuum line at 2000rpm.
Run about 5oz. thru it and shut off the motor for 15min and repeat.
With all of this run 87 octane at the test without any additives. So if you run the injector cleaner get it out of the tank before the test.
O2 reading is good so the cat is not the problem. An O2 sensor maybe??? I would try these other then see or if you haven't replaced it do so. It's only $40 or so.
Also run your tire pressure at 35psi. You will have less rolling resistance and could help for a couple of ppm's.
That is all I got, Good luck