Failed emissions inspection, NOx

So, surprisingly, my 1990 Acura Integra failed the Maryland Emissions Inspection. Too much NOx, 3.2 with an acceptable amount being 2.5 (I forget the units). What is the most likely cause of failing only the NOx? Is there something simple I can do myself that will probably let it pass the reinspection? I really don’t want to pour alot of money into it. According to law I must spend up to $450.

How long has it been since you changed the sparkplugs? the air filter? the fuel filter? You’re pretty close right now. The EGR valve is probably the actual problem but changing the three things above will get you back under the limit.

Couldn’t hurt to change the oil and oil filter either. Make sure the engine is run for at least 20 min. before you get the test, don’t shut the engine off if you have to wait in line.

All filters and the plugs have been changed in the last year or so, and the car does not get much driving. One thing that may be contributing is a “peculiar” cooling system. It tends to drive warm in warm weather and cold in cold weather. Sometimes, I’ll be driving along and temp. gauge is pegged at stone cold, like when you first start up in the morning and will drift from cool to cold as I drive. If that is caused by a bad thermostat, could I inadvertantly be driving the engine too hot, or is the temp. gauge sensor in the engine itself?

It sounds like the engine isn’t getting up to its normal temperature. That alone could increase the NOx values. A faulty thermostat is almost certainly the cause.

So the probe that connects to the temp gauge is measuring the temperature of the fluid within the engine block, and not in the radiator? Unfortunately, when the test was conducted I had to sit in line for about 20 minutes and the gauge was registering fairly normal at that time.


NOx is produced at high temperature and pressure… The following problems would raise one, the the other or both:

Carbon build-up in combustion chambers, raises compression ratio. This causes higher combustion temps and pressure. Given the cars age, and that it is not a mile out of spec, this would be my first guess.

Overly-advanced ignition. Same issues as above. Ignition is sometimes advanced a little attempting to gain a few HP. This sometimes works because ignition may have been retarded in order to meet the NOx numbers.
Vacuum leaks. Depending on location this could cause one or more cylinders to run lean. Very lean engines run badly. Just-a-little lean engines run OK but make lots of NOx.

Dirty injector. One or more cylinder may be running lean for this reason. Techron injector cleaner additive is recommended by several car and motorcycle manufacturers as a periodic maintainance item.

Malfunctioning EGR. The whole point of EGR is to dilute the intake charge with already burned exhaust gas, in order to lower Nox emissions. Carbon can build up in either the EGR valve itself, or in the tube between the valve and the exhaust system. In addition, there are a few bits that control the EGR that have to be working, but I’m not familiar with your particular car.

Finally, catalytic converters break down NOx and use the O thus produced to burn stuff like CO and HCs. So after 15 yrs your catalytic converter may just be worn out. An engine in need of a tune up (cylinders misfireing occasionally) can really stress the cat and shorten life.

Note: Check the emissions laws in your state. I didn’t follow what eventual outcome was, but CA in particular was proposing some very draconian measures if you failed a second test…

NO[sub]x[/sub] goes from almost nothing to sky high as the mixture passes 14.7:1 air fuel ratio. Rich = almost zero NO[sub]X[/sub], lean = lots of it
Typical causes include:
[li]engine running too lean A lean misture runs with a higher combustion temp. Anytime the combustion temp goes over 2500F NO[sub]2[/sub] is formed[/li][li]missfire Not all the fuel is burned, making the effective mixture lean see above[/li][li]bad cat If the cat is bad, it won’t convert the NO[sub]x[/sub] into NO[sub]2[/sub] and O[sub]2[/sub][/li][li]**cat running too cold **The cat has to be good and hot for it too work. idling for 20 minutes is not the best.[/li][li]lazy O[sub]2[/sub] sensor If the O[sub]2[/sub] doesn’t switch fast enough the front part of the cat won’t get hot enough to start the reaction.[/li][li]dirty fuel injectors Can cause a missfire, see running lean above[/li][li]plugged or inop EGR system Designed to prevent NO[sub]x[/sub] if it is bad, NO[sub]x[/sub] will be present[/li][li]engine overheating engine temp too high, combustion temp will also be too high, can lead to missfire and or temps above 2500 F.[/li][li]over advanced timing Creates too much pressure in cylinder, too much pressure = too much temp = No[sub]x[/sub][/li][/ul]

In California we measure NO[sub]x[/sub] in parts per million (PPM). The limit for my daughters 89 is 700PPM. It soundls like your car is measuring in grams per mile. do you have a print out that gives units of measure? In any event you are about 28% over the limit and it can be a bitch to get it down that far. I know guys that have struggled for days trying to get a car down 10 PPM.
My suggestion would be to change the plugs, and take the car for a good run down the highway, then straight to the test center. Try to find one that does not have a line. Do not turn off the car while waiting
Restest the car. If it passes whoo-hoo. If it fails, I suggest a good shop with the proper tools to diagnose the problem.
In some areas money spent by the car owner trying to fix the problem themselves does not count toward the cost cap. I suggest you check into this before you spend too much, and find that the dollars you spent don’t count. YYMV of course.

Nice post Kevbo
Vacuum leaks! :smack: Yes check for vacuum leaks, probably the bigest single cause of NO[sub]x[/sub] problems