perrys4

My car passed its most recent testing but the guy mentioned my NOX levels were a little high. I posted this question on SOCAL 928 board and no one including the all knowing Dr Bob had a decisive answer. Even the tech editor at the Mag. I work with didnt know for sure.

I have heard all the things that cause high NOX levels, What steps can be taken to reduce them if it seems all else ( cat ) is functioning properly??

Levels Max allowed My level
15mph

HC (ppm) 108 26
CO ( % ) 0.71 0.02
NO (ppm) 767 598

25mph

HC (ppm) 83 14
CO (%) 0.58 0.02
NO (ppm) 706 673

I drove the car hard and got it real hot before the testing. They hooked it up within literally seconds of arriving. I run 91 octane and car runs great. Precision Motorwerks does all the work on it. It is well maintained.

perrys4

Couldnt figure out how to edit first post so I am doing this

15mph

HC (ppm) allowed ( 108 ) measured ( 26)
CO (%) allowed ( 0.71) measured ( 0.02)
NO (ppm) allowed ( 767) measured (598)

25mph

HC (ppm) allowed (83) measured (14)
CO (%) allowed ( 0.58) measured (0.02)
NO (ppm) allowed (706 ) measured (673)

Hope this makes it easier to read

Susan K Thomas

I am certainly not a smog expert (and we are not a smog station) but since the new smog laws went into effect we have seen two main reasons that 928s fail on the NOX. One is a bad mass air flow sensor, but in this instance the car usually fails grossly. The other is a slightly higher than normal operating temperature. If you can get your temp down by a needle width or two those numbers should drop...a lower temp thermostat (and replacing the rear thermostat seal) should do the trick.

Again, I'm no expert but this is what I've seen most since October. A healthy 928 in good state of repair should pass smog with flying colors. We had one today that registered 0.01 on the CO!

Hope this helps,

Susan Thomas
DEVEK

Rocha928

For what its worth. . . .

High combustion temperatures results in the engine producing large amounts of oxides of nitrogen (NOx).

Not sure if 928's have an Exhaust Gas Recirculation (ERG) system or not, but my experience in the past with American cars is that high NOx is typically caused by a faulty ERG valve.

my 2 cents

ViribusUnits

Why are you worryed about it?

You passed. Enjoy untill next year, and worry about it then. You don't seem to have excessive HC or CO, so you shouldn't be poisioning the cat.

NOx is created by too high combustion chamber temps. Keep the temps down, and you keep the NOX at bay. The next thing is that the NOx is reduced to the fullest when there is minimal O2 of that part of the catylist. O2 is a product of the reduction of NOx, and as a result, trys to "choke" the reaction off. One way to keep the temps down, and the O2 off that part of the catylist is to run a slightly richer mixture. Doesn't the MAF have a pot to adjust the idle mixture? If so, maybe the car is a little lean. My experence has been on the L-jet cars, the O2 sensor will correct for a small devation from stoch, but not for a sigificant one. As a result, you need to be very close to the correct mixture, BEFORE the O2 sensor and the ECU try to correct things.

Another way is to avoid overheating the engine.

That, or the timeing was a bit too advanced. IIRC though, the only way to do that on an S4 is if you've got aftermarket computer chips. With 20 year old cars, it's a possibility, but reletivly unlikely.

There is no EGR valve on the US cars from 80 onward, IIRC.

perrys4

Thanks for the suggestions. I am not worried, Just like to stay on top of things. I actually have two years to not worry about re-testing. Susans suggestion might be just what I need. My car has always run a little warm. Need to find out which Thermo Greg last installed in my car. Might just kill 2 birds with one stone.

dr bob

On the S4, in addition to the thermostat change, you'll want to force the louvers open and pull the fuse so they stay open, and may want to look at how to get the fans to come on a little earlier. If you don't take these additional steps, the flaps and the fan controller will conspire to keep the car warmer.

I have the flaps open, the 75C thermostat, but haven't tried any mods on the fan controller yet. It runs at the 75C spec when moving, but bumps up to the original temps when depending on the fans for airflow.

Incidentally, the mod to the fan controller should be quite simple, just adding a small resistor in series with the temp sensor signal. That would take care of both the flaps and the fan speeds.


As I pointed out on the SoCal response, the only change I've made that's still on the car since I got it is the plugs. PlusFours replaced the original copper Bosch plugs. Passed NOx with flying colors before, barely passes now. So before the next test in a year or so, I'll probably go back to the copper plugs and see if that helps.


Some other thoughts--

It only take one partially-plugged injector to screw up the NOx reading. The airflow sensor is one thing I haven't considered, Might be worth getting one from mr speake to try, and see if it's causing a slightly lean condition as it ages.

John Speake

Hi perryS4 - doesn't sound like the MAF, the CO level at all throttle openings used for these tests will be well controlled by the O2 loop.
The CO readings you are getting are fine.

As Susan says, MAFs can cause a problem, but only if they are so far out of calibration that the O2 loop runs out of range. At that point, it is usually clear the car has a problem.

The O2 loop will cope with MAF ageing of up to 4% from the correct calibration.

The idle mixture pot is non-op on a O2 loop equipped car.

The 928 does not have an EGR system.

High temp sounds the most likely issue..........

Regards,

perrys4

Dr. Bob....I have the flaps wired open. That was one of the first things we did when I took the car to Greg after I got it. They were not functioning and had other repair priorities. I think I will let the fan thing go until I try the Thermo change. Keep me informed on the plug change when you do it.

dr bob

I have 14 months to get that worked into my schedule and see the results. You'll have 9 months of head-start on your test if that doesn't do the trick. I'm really low on options, since AI've left the car almost perfectly stock. Except for that short (less that 5000 miles....) session with the Autothority chips in there, nothing has ever been changed on the engine except the plugs. All else has been Porsche or OEM replacement.



Results weill be posted when I do the next test.

Bill51sdr

I had the same kind of results upon smogging my S4. Almost no CO or HC but NOX much higher than average. The only other factor that no one here has mentioned is the catalyst. The cat midbed is most responsible for NOX reduction and I often wonder if the cats are just getting little tired as these cars age. Another way NOX is reduced in non-EGR engines is by valve overlap. This can be timed to reduce combustion chamber temperatures, thus reducing NOX. 32 valve engines have adjustable valve timing... how many of them are optimally set at this point in time? Also, the very fact that the HC and CO emissions are so low could indicate a lean running condition which is also favorable for high NOX emissions, so who knows?! Just be happy it passed