Failed smog check with high HC, CO and NO at low speed only

My 1991 325iC with 201K miles on it failed the smog check. I had done the following beforehand:

- replaced spark plugs
- replaced air filter
- replace fuel filter
- refurbished the fuel injectors (taking them out and having them cleaned) beforehand.
- Ran Guaranteed to Pass for one tank

The values for HC, CO and NO at 15 mph are all too high, the values at 25 mph are all fine.

I have since then

- replaced the vacuum hoses
- changed the oil
- added an oil pressure additive
- did an engine flush
- pumped up the tires.
- ran CataClean to clean the cat.

Did a retest (pre-test) and the test results were still the same.

Latest values

15mph: (all failed)
HC 164 (max 108)
CO 0.82 (max 0.71)
NO 1374 (max 767)

25mph: (all OK)
HC 24 (max 83)
CO 0.13 (max 0.58)
NO 227 (max 706)


Of course the car is old so perhaps the catalytic converter is bad. But what else could be the problem?

Forgot to mention: the car leaks a lot of oil, but as far as I know not internally.

Thanks for any input,

Joop Kaashoek

I think your HC is causing the high CO as well.
Could be vlave stem seals, as you have it under high intake vacuum conditions at idle and low load.
Could also be gunk in the zorst. Have you taken the vehicle for a long hard spin after doing all this ? To burn the exhaust clean...

NOX can be brought down by adjusting the timing at idle.
Adjusting the fuelling to run lean at idle might just push you through on HC and CO values, but can kill the values if you run it lean enough to miss.

i would replace the oxygen sensor if fitted. leading cause of poor economy and poor emissions in my experience

I passed the smog check today, after replacing the oxygen sensor, and changing the valve cover release hose to the official one instead of the silicone version I had put on, as that one had the tendency to collapse.

The O2 sensor was one of the main culprits, when I tested the old one it did not appear to show the right voltage levels at all.

Anyway, problem solved. Thanks for your input, gentlemen! Luckily I did not need to do big repairs like fix my valve stems, or replace my catalytic coverter....