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SMOG passed but NOx ...?

157k mileage
1987 Acura Integra

It passed SMOG but the NOx readings are:
542 295 536 (2 yrs ago 448) at 25MPH

Only different was I was driving for roughly 2hrsin town before the SMOG compared to last time.

What makes NOx to deteriorate? What might be a preventive maintenance?

You might want to have your EGR system checked/cleaned.

What causes the NOx numbers to rise is the slow deterioration of the catalytic converter. That 2 hr drive done before the emissions test was a good thing. If you got the catalytic converter well heated up; got it onto the wheel dyno ASAP; and the tester did a pretest warm up, that is about the best you can do.

If your Check Engine Light is not lite, the EGR system is probably working okay. If I were to trouble shoot this high NOx problem, I would make sure the EGR system is operating optimally, check the base ignition timing, and verify that the engine cooling system is working well. Then I would consult with an emissions specialist to determine if replacing the catalytic converter would be cost effective. I would wait until I actually failed the emissions test before I proactively pursued this.

In your case I’d strongly suspect that the catalytic converter is reaching the end of its useful life.
The converter core is a ceramic honeycomb coated with a platinum-palladium (an alloy in the rhodium family). When the coating is heated by the exhaust, it strips the NOx (which is nitrogen atoms bonded to oxygen atoms) or its oxygen. The output of that first stage of the converter then becomes nitrogen (of which air is 77% anyway) and oxygen, which can then be collected by carbon monoxide (making carbon dioxide) and/or released as oxygen.

The problem is that only the NOx molecules that come directly in contact with the heated metallic coating undergo this transformation, and no matter how clean an engine operates, there will always be some carbon (soot) in the exhaust. The carbon over time coats the platinum-palladium, reducing its effectiveness, since the coating prevents the NOx molecules from coming directly in contact with the metal. Thus, the efficiency drops over time.

By all means check the EGR system out. That allows a wee bit of inert exhaust to be drawn into the intake under load, thus keeping the cylinder temps from becoming hot enough to ramp up production of the NOx in the first place. If cylinder temps spike, NOx production increases.

But also start saving for a new cat converter next year. Or in two or three years, depending on how long it takes the converter to get below the efficiency required to keep the NOx levels low. You might never need the money, but at least you’ll have had time to prepare in case you do.

What are the limits you busted? If you’re “on the bubble,” a simple tweak might get you passed.

As a general rule, rich mixtures produce high CO and HC levels; lean mixtures produce high NO(x). If you’re OTB, and have plenty of wiggle room w/ CO and HC, perhaps test with car slightly cooler coolant temps for a richer mixture…also real gas will be richer than E10 at equivolume basis.

If you’re way high across the board, you need a tune-up. (For cryin’ out loud, who still smogs 28-y.o. cars?!? I thought PA was strict, but you get a pass at the 25-year mark!)

P.S. check for intake leaks and cracked vacuum lines.

Rhetorical question from meanjoe75fan but for others who may be wondering, there is some kind of answer. 1973 is the last real non-smog year. After that year, most cars had catalytic converters. No smog checks at all in Maine. We have enough trouble up here with the weather, the roads, the governor…

If you got your smog sticker, what’s the problem?? It’s a 27 year old car, just drive it…

Just planning
I will look for an opportunity to repair them if it is an incremental repair.

TSM … good description of how a cat removes nitrogen pollutants. I think it used to be that cats only removed carbon pollutants, like HC’s and CO’s, but now they have 3-way cats, the third is the nitrogen function. Does that sound right?

Anyway, in reading your post, I was wondering if a cat was marginal, if you could wash out the oily carbon deposits to return it to serviceability, at least as far as for nitrogen products. Conceptually I mean, the same way you remove grease stains from your laundry, just remove the cat, pour some water/detergent solution through it, then dry it out? Do you think that would work?

Not recommending this, but anyone here every tried something like that to restore a marginal cat?

Unfortunately there is no way to ‘clean’ a catalytic converter. Any deposits that could be washed off would be burned off in normal operation. The surface of the platinum palladium is very hot and burns off any deposits. Over time the active sites on the catalyst wear out and get coked up with unburned junk. About the only thing you can do is put a new air filter in, make sure your PCV and EGR valves are good and take the car on a nice ride on the interstate for an hour. That will get the catalyst good and hot with plenty of oxygen to burn. Almost like blowing the carbon out of an old car with a carburetor. But this will only make a very modest improvement if any at all.

Not quiet, George. The separation of the nitrogen and oxygen is the first stage, and it’s what makes it possible to clean up the unburned hydrocarbons and the carbon monoxide. It provides the oxygen atoms necessary for the unburned hydrocarbons to “combust”, an operation often referred to as the “second burn”, and for the carbon monoxide to grab another oxygen atom and become carbon dioxide. The NOx separation is the key to the other processes.

There are additives that claim to clean the catalyst, but I’m unfamiliar with any that actually works.

Sincere thanks for the compliment, by the way.

Were the HC and CO #'s well below the limits? If so the cat is probably OK.
The OBD system on this car doesn’t check for EGR flow.
After 27 years there’s a good chance the EGR system is partly clogged.
My '88 Accord passed the treadmill test well below the limits at 210K miles and 20 years.

Clean the EGR. I have heard of a product called Cataclean. They say it can help a marginal cat pass. Isaw it on an auto show, Wheeler Dealers, and they passed the smog test after using it.

Also, consider cleaning the throttle body as well. The Asian cars of this era used a two-stage vacuum line system and a vacuum modulator to operate the EGR. The vacuum is ported on the throttle plate to provide vacuum at half throttle on one port and full throttle on the other. If these ports get plugged up, the EGR will not function.

I discovered this on my '92 Celica. I managed to get it to squeak through emission for a couple of years with marginal NOx numbers after cleaning the EGR, but found I was getting very little vacuum to operate it. I took of the throttle body and had to use 2 cans of throttle body cleaner and some wire to pipe clean the vacuum ports to get all the built up gunk out of it. NOx numbers were at an all time low after that and have stayed low.

The only time my 1984 Toyota pickup ever failed its California smog test was because of high NOx two or three years ago. I eventually traced it to a vacuum leak. The distributor vacuum advance had developed a leak in one of the diaphragms resulting in a lean mixture. Regardless of the vehicle, especially when they get old, vacuum lines or vacuum operated devices can leak. I would check for vacuum leaks first since they can be relatively inexpensive to fix although there are exceptions.