86 Maxima, failing NOx. Here’s what was done. What next?

Can't guarantee this will fix it, but I had the same problem on a Toyota 4Runner right before I sold it. New cat solved the problem (It had an el-cheapo cat on it as well) and it passed with flying colors at 215k miles.

Was the car run at least 10 miles before test? It needs to be hot when the test is made.

After browsing through recommendations from this and other groups here is what I've done so far:

1. Did a Lambda Calculation
   formatting link
   Lambda 0.9889; Air / Fuel Ratio 14.54This appears to be within acceptable range, and says that the mixtureis not to lean

1. Basic EGR valve check: manually squized the diagraphragm which caused engine to chocke until release Checked the presence of vacuum on the hose going to the EGR valve (finger check) Checked the operation of EGR by revving the engine - the diaphragm moved up with higher RPM and back down when throttle was releases

1. Checked O2 sensor by checking the self diagnostic codes on the ECU. Hanes manual says the lights should blink at least 5 times within 10 sec at 2000 RPM in normal ECU mode. They do. I have not been able to detect any error codes in self diagnostic mode either, although I am not 100% here I did it right. Also, I read somewhere that good air/fuel indicates that O2 is probably OK

1. Visually checked Cat converter. It is present ;) and has no rust on it. It is a universal type, seems like an inexpensive type, not clear when it was replaced - could be as long as 10 years ago, but the care was gently used.

2. The thermostat: I did not replace it, seems like a lot of work on this car. BUT the car does seem to run a bit hot(er), at least perceptually and here is why: it has a shorted fan controls so the fans are always on, yet the temperature needle shows slightly above 50% scale most of the time even in 40 degree weather. There is also a bit of burning smell after the car has been on ran on a highway, but that could be due to some oil sipping on the manifold. The car DOES NOT overheat though. I also read on this forum that if you have a lambda indicator, the engine temperature is adequate. Can't say I understand this, but person seamed to speak with authority ?

Now the question: is it reasonable to assume that I have a faulty Cat Converter and if I replace it would be the end of my problems? The money is very much an issue here! I can afford to gamble $120 to get an "Eastern" muffler from

Thanks for your suggestions.

Max N

Put premium fuel in it and it will pass..

You think that will retard the timing enough to pass? What about dumping an octane booster on top of it?

I am just curious how much can Cat Con compensate for some problems in the system that cause the excessive Nox. In my case to go from 3.85 to 3.0 -- that's almost 25% reduction!

Yes and now: it was driving and or idled for at least 25 minutes prior to the test, but it was not driven for 10 miles since this was city driving. I have mixed feeling about getting the car hotter for the inpsection -- on one hand it will make the cat converter hotter, which is good. On the other -- it may make the engine hotter, which is bad for NOx. If there were a way to make cat hotter w/o overheating the engine ....

Are you thinking this will retard the timing enough to reduce NOx sufficiently? I might as well dump a bottle or two of octane booster instead, no?

What was the effect of installing a new cat? How much can I expect it to reduce the NOx by?

Sorry, I am not following: if less exhaust gas gets recircuilated back into the cylinder, that would cause HC to increase? I though it would burn better!?

Not for 10 miles, but for 20+ minutes city/idle. My concern is that in order to get the cat hotter (highway driving), I would have to get engine even hotter still, thus increasing NOx...

Well, he explained. Not all cylinders would be getting the same mixture. Most of the sensors that are controlling fuel mixture are responding to what's happening to all of the cylinders which are supposed to be all getting the same amount of exhaust recirculated.

But EGR is the place to look since the whole purpose of having EGR in the first place was to reduce NOX emissions. As far as I know the catalytic converter is not helpful in reducing NOX and may even add to it. Nitrous oxide is not really a product of combustion its just what happens to some of the nitrogen in the air when exposed to very high temperatures. EGR makes the fuel burn slower (like an octane boost) which means the nitrogen doesn't get quite as hot.

-jim

Again put premium fuel in and the NOX will drop a BUNCH. You're probably having inaudible detonation which makes the NOX go up. Retarding the spark has the same effect if the engione has a timing adjustment, which these do. Or you can go buy a bunch of parts chasing this if you want to..

It doesn't retard the spark, it reduced detonation which is a major cause of NOX.

No, that won't do it and could likely cause other problems. You need to run that tank out and fill up with 93 or retard the spark 5 degrees or so and then retest. I bet light dotonation is causing this problem for you

Thanks Steve, I hope you're right!

I don't think you got it either. Obstructed passages to some cylinders would reduce the amount of recircuilated exhaust to them, and the fuel would burn better, with less HC, and hotter. NOx would go up, HC down.

Ahm, no.

The current plan is to put a can of Techron into a tank of 94 gas, run it till less then 3/4 left, then go take the test again (total cost $10) If that fails -- spend $120 on a new cat and pay $30 for another emission test. Thanks for your advise.

Consistent with the test report.

Except on the cylinder[s] that are receiving [too much]EGR, those cylinder[s] will show an increase in HC, consistent with the test report.

It's a known pattern failure on many Asian automobiles that use small EGR passages to individual intake runners. Is it a problem on -your- vehicle, hell, I don't know, like I said, it's been a while.

Your 86 Maxima uses a three way convertor, so yes, it reduces NOx, but I wouldn't condemn the cat until I did some more testing...

Thanks Neil. I get it now. Based on Haynes manual, the diagram shows one large passage from EGR to

the intake. I have not figured out how to remove the EGR valave assmbly yet though to doulbe check -- the manual simply says "remove", so I must be missing something. I may swing by the dealership (I need to pick up some minor parts) and look at the print out diagram and then do the disassembly. Thanks for the tip!