Bill Ball

I'm helping another local owner sell his 91 S4 with only 35.5K miles on the clock. He has a buyer! Today I took it to smog in preparation for the sale and it failed NOX. Here are the numbers.

15MPH:
RPM 1410
CO2 13.1%
O2 2.8%
HC 22PPM
CO 0.01%
NOX 869 FAIL (limit 767)

25MPH:
RPM 1262
CO2 13.2%
O2 2.6%
HC 20 PPM
CO 0.01%
NOX 922 FAIL (limit 706)

The car runs terrific. Smooth, stable idle, great throttle response, quick acceleration. No miss, no hesitation. Bone stock except RMB. Had a 60K service at 27K miles 5 years ago.

Where would you start?

Looking at other threads uncovered with a search for high NOX, other people have examined and changed in no particular order:
- find and repair vacuum leaks (leans mixture) - doubt there are any, but will check.
- replace MAF (out of calibration can cause lean mixture) - may swap mine in
- replace O2 sensor - hmmm...fair idea; probably original
- run injector cleaner or replace injectors figuring one or more are plugged - injector cleaner is simple; doubt plugged injectors based on the way the car runs, but will check
- check cam timing - OK, need to check timing belt anyway.
- air pump - make sure working; paradoxically disconnect the diverter vacuum. - that's confusing.
- replace cats - uhhh, well, I do have another set to loan for a test from a car that passed, but at 35K I doubt that's the problem....high NOX only would be odd, although a plugged cat did that on my Durango. My impression is the original cats are very durable, so this is way down the list.
- higher octane gas - slower, lower temp burn? - I can get 100 (R+M/2).
- spark plugs (copper single electrode)
- check compression - if high (>200, indicating carbon) try to blow it out

Some of these things are low cost and simple; others not. Give me some thoughts.

I need to find a smog station locally that doesn't charge if you don't pass!

heinrich

Bill, start maybe looking for oxygen and fuel. Either. So leaks anywhere.

Adamant1971

Not that I suggest any shady solutions. But in the past I have gotten several Rotary powered vehicles (notorious polluters) to pass by adding .5L of methyl hydrate to a quarter tank, take test, pass with flying colours, fill up to dilute.

GregBBRD

Pretty lean.

I'd start at the O2 sensor and see if it is putting out the correct voltages and fluctuating.

If that is working, pull a plug and check the style and heat range.

Vacuum leak would be next. Vacuum pods for HVAC could drive up the NOX that much...and mileage has nothing to do with those going bad.

snoz

I think it should be sent to the midwest (my place in particular) for extensive further testing.

Bill Ball

Thank you, Greg. BTW, after it's smogged it will be going to an Anaheim resident. I've already told him how lucky he is to have your shop in the neighborhood.

Bill Ball

I assume this works like ethanol in fuel, burning a bit cooler. I'll keep it in mind.

danglerb

I'd swap the MAF with a known good one, do an Italian tune up.

JHowell37

http://www.nichols.nu/tip766.htm

If you actually google "high nox emissions" this is the first link that comes up. I assume you know that Nichols.nu is a 928 site.

Bill Ball

Yeah, I'm going to check the MAF output with Theo's program (I think it shows MAF voltage live). I do Italian tune ups regularly.

John Speake

In the UK they have recently introduced a tailpipe measurement of lambda at the tailpipe for cars registered after '93 (these are cars fitted with cats). These 928s always show weak on lambda even when everything is OK. The problem is the airpump pushing air into the exhaust that fools the tailpipe measurement. They pass OK if you remove the vacuum pipe to the airpump BOV and cap the loose pipe.

Mongo

I see a small percentage of O2. Last time I had that, I had an exhaust leak after the cats due to a loose pipe clamp.

The fact that hydrocarbons are so low on those readings tells me the cats are working. Like the other guys have said:

-MAF swap or replacement
-O2 sensor (couldn't hurt)
-Fuel injectors (replace them with Ford Racing orange tops for cheap Part No. M-9593-C302 *part may have -01 at the end*) Don't forget Roger's O-rings!

Best price I have found for them below..
http://www.mustangsunlimited.com/ite...1=M9593C302+01

On a side note, these are single pintles, which are discontinued. I figure since he is selling the car, these will do just fine for stock replacement.

WallyP

From Wiki..
"Since 1981, "three-way" (oxidation-reduction) catalytic converters have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that in effect require three-way converters on gasoline-powered vehicles. The reduction and oxidation catalysts are typically contained in a common housing, however in some instances they may be housed separately. A three-way catalytic converter has three simultaneous tasks:
1.Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
2.Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
3.Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O
<snip>
Three-way catalysts are effective when the engine is operated within a narrow band of air-fuel ratios near stoichiometry, such that the exhaust gas oscillates between rich (excess fuel) and lean (excess oxygen) conditions. However, conversion efficiency falls very rapidly when the engine is operated outside of that band of air-fuel ratios. Under lean engine operation, there is excess oxygen and the reduction of NOx is not favored."

The HC and CO reactions are closely related in operation, while the NOx is different. As several folks have said, a lean condition will cause high NOx while not impacting HC and CO reduction.

Jim Morton

Bill:

IMHO, there is a huge clue in your emission results in the stated O2 content... O2 should be near 0 pct., suggesting a leak in the system if O2 is measurable relative to the other gases.

$0.02

Speedtoys

Bill: Fire up the intake system pressure tester I gave ya..

You'll find the leak..