High NOx is caused by really hot combustion temperatures, often due to a lean running condition. However, lean running also usually means that your CO and HC numbers will be very low, and you said yours were above average. So you should look elsewhere for the cause of high combustion temperatures first.

Items to check include:

1. EGR function. Working under the hood, observe the vacuum operated rod that opens the valve while you open the throttle, increasing RPM to around 2000 or so. The rod should move back and forth, opening and closing the valve as you move between idle and cruise RPM's. If this checks out ok, then see if the EGR valve is doing its job by removing the vacuum hose and attaching a vacuum pump to the valve. While the engine is idling, apply vacuum to the valve. The idle speed should drop noticeably, perhaps even killing the engine. If idle remains the same, then the valve is either obstructed with carbon and needs to be cleaned, or you have a bad diaphragm in the actuator.

2. Ignition timing. The more advanced your timing is, the hotter the combustion. If it's already adjusted to spec, then backing off the timing a few degrees will probably lower your NOx enough to pass, since you are very close to the threshold. Only do this if you are desparate to get your plates renewed and you have not found any other culprits, as gas mileage and power are reduced by retarded timing. It may also increase your CO and HC, but in all likelihood not enough to fail the test.

3. Carbon in combustion chamber. This increases compressing ratio, resulting in hotter combustion and increased likelihood of spark knock. If no other obvious culprits, run a can of GM Top Engine Cleaner or Sea Foam into a vacuum line (cost = $6). The process is spelled out on the can. This will not hurt anything and may help it run a lot better anyways if it's got a lot of mileage.


If none of these things is the problem, then look for evidence of lean running. Items to check:

4. Oxygen Sensor. Check that its voltage output fluctuates roughly between .2 and .9 volts. If it is stuck above .5 volts, this signals to the ECU that the mixture is rich, to which the ECU responds by leaning out the mixture. Of course, if your mixture is not really rich like the sensor says (as your emissions numbers seem to indicate), then your sensor needs to be replaced. On the other hand, if the sensor is stuck under .5 volts, this signals a constant lean condition. If this is so, the sensor is probably telling the truth, and I would look at the following items to see if they are causing a constant lean condition:

5. Unmetered air entering the intake. Can be caused by cracks or holes in the bellows between AMM and throttle body, vacuum leaks, loose clamps on bellows. Try retorquing the intake manifolt bolts to spec.

Other helpful suggestions:

6. The tip regarding premium gas is correct; it burns slower, resulting in lower combustion temperatures.

7. Also try using an alchohol-enriched gas (i.e., one with at least 10% ethanol). Alchohols burn cooler that regular gas, so this will help with NOx. As a bonus, it also lowers HC and CO because it has a higher oxygen content as well.

8. As long as it is above 60 degrees outside and/or the car is fully warmed up, the missing Thermostatic Air Cleaner hose makes no difference at all. I guarantee you, that is not your problem as long at it's missing. (As the previous poster said, if you put one on, it could cause a problem only if the switch is stuck in the hot air position, but even then, it is unlikely that this would make enough difference to cause the car to fail the test.)

Hope this helps you to find a solution quickly!
John ('86 245, 150K)