What could cause high HC and CO, but low NO and CO2 in a smog test

Question

Despite my confidence in having a (relatively) new catalytic converter and a new O2 sensor, my lovely 1994 BMW 325i Convertible got flagged as a "Gross Polluter." The test results are summarized below:

CO2   O2    HC    CO    NO
13.6  0.1   333   2.00  ~80

Essentially, HC and CO are far too high while NO is fairly low. Also, CO2 is low, indicating lean conditions.

What are some possible causes, and some ways to investigate them, that could cause the test results?

My current thoughts are:

• Compression Loss
• Fouled Plugs
• Faulty MAF
• Excessive Oil from PVC

Answer 1

The supplied gas values suggest a rich run condition and possibly a non-functional catalyst. Using a mixture calculator for the supplied gas values shows that the mixture is 6.4% rich.

CO elevated is the key indicator as it is not present in lean combustion in significant amounts.

NOx is produced in significant amounts in lean combustion and requires high gas temperatures usually on seen under loaded conditions. Little is produced at rich mixtures.

CO2 is maximized at stoiciometric mixture. So low CO2 can mean rich or lean.

FWIW in my vehicle emission testing region (Seattle WA) this engine would pass a tailpipe emission test.

Refer to the graph below for with help with gas value interpretation. The left side of the chart is rich and the right is lean.

Of the list of possible failures the MAF is a possible cause of a rich condition. Fouled plugs would result in much higher HC due to misfire. Low compression would likely be corrected for by a properly functioning fuel control system including an accurate MAF reading. Oil in the combustion chamber does not greatly affect combustion gas values until it gets to excessive amounts. Failures of the PCV system do usually present as lean run problems.

Answer 2

The most likely culprits are clogged fuel injectors, carbonized intake & exhaust valves. The best solution is 1 oz. Marvel Mystery Oil (MMO) for every gallon in the gas tank, and a fresh oil change with 20% of the oil substituted by MMO.

"Rich" and "lean" become too confusing because both can happen at the same time. When the injectors are dirty, they do not atomize fuel effectively. Micro-globs of gas pass through the combustion chambers unburned -- effectively creating a lean condition in the chambers -- so that the O2 sensors add more fuel to the mix, which in turn produces a rich mixture in the exhaust and uneven flame in the chambers.

The cars EEC attempts to compensate by vacillating between more and less fuel every few seconds, which will also confuse the values on a scanner.