[s-cars] Emissions and O2 sensor musings

[Gabriel Caldwell]

The last paragraph is an interesting one.

Hard Nox
NOx is a tough one. It forms when nitrogen (78 percent of the air we breathe) combines with oxygen (21 percent of the atmosphere), which can happen only at very high combustion temperatures. The trouble is, many of the things that were initially done to cut HC and CO involved dramatically increasing the heat inside a car's cylinders. That heat created plenty of NOx. Fortunately, it was found that only the peak temps (over 2500°F) produced this smog-promoting gas, not the long, hot burn that oxidized the other two pollutants. Introducing Exhaust Gas Recirculation (EGR) knocked the top off the temperature curve, thus putting NOx within bounds. One of the main reasons electronic engine management systems were developed in the late 1970s was to allow a 3-way catalyst, which adds rhodium to the 2-way cat's platinum and palladium to further diminish NOx.

The first things to suspect are an inoperative EGR valve (perhaps the vacuum line is cracked, or the internal diaphragm has ruptured) and carbon-clogged EGR passages. Try applying vacuum to the valve at idle. If the engine starts to run roughly or stalls, the valve is okay and the passages are at least partially open. Remove the valve and dig out all the deposits you can.

There's a good chance that the catalytic converter is no longer capable of reducing NOx, but determining that requires high-tech diagnosis best left to a professional. There's another distinct possibility that you can deal with, however: heavy carbon deposits in the combustion chamber and on the backs of the intake valves. These can raise the compression ratio enough to cause the hot flash that forms NOx, and will also interfere with the carefully engineered swirl pattern of the incoming mixture that makes it burn evenly.

Numerous treatments have been devised to avoid the huge job of removing the head and manually cleaning the combustion chambers, piston tops and valves, but the only one suitable for the do-it-yourselfer is chemical treatment. The most basic approach is to pour a suitable additive into the gas tank. A more thorough job can be achieved by adding detergent- or solvent-based cleaners directly into the intake stream.

Too much heat is another thing to think about. Is the engine running hot? Is the intake air overheated due to a jammed Early Fuel Evaporation valve, or is a Thermostatic Air Cleaner door in the snorkel stuck closed?

Ignition timing that's too far advanced can result in detonation that causes NOx to rise. A switch to premium fuel may reduce it. A final note: Moisture-laden air can lower NOx output by as much as 30 percent, so you might consider visiting the emissions station on a rainy day.

Gabriel Caldwell

-----Original Message-----
From: Mark Pollan [mailto:mark.pollan at mci.com] 
Sent: August 10, 2004 2:02 PM
To: s-car-list at audifans.com
Subject: [s-cars] Emissions and O2 sensor musings

Hi all:

 

The patient is a 95.5 S6 with MTM1+ chipset.  In April I failed emissions
due to high NOX.  Today she passed with "flying" colors.  Only difference(s)
was a new O2 sensor (old one only had 20K miles on it), full set of Samcos,
oil change with Techron in the last tank before change, and I drove her hard
for about 10 miles before going to the emissions station.

 

Below are the results of my two tests:

 

                                    HC (GPM)                     CO (GPM)
NOX (GPM)

State Limit                    1.0000                          20.0000
2.2000

Failed Results               0.5230                          4.4715
5.0118

Pass Results                 0.0915                          5.5075
0.9592

 

Any thought out there as to which of the above "changes" from FAIL to PASS
my have driven these results?  And on that subject, is there anybody willing
and able to test my old O2 sensor?

 

Like I said, just ramblings and musings so if there are no takers that's
cool but I sure am curious.

 

Regards,

 

Mark

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