Emissions: NOX just a little too high

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Fastback86
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Emissions: NOX just a little too high

Post by Fastback86 »

Went to get my car smogged today and she just barely failed the NOX test. Two years ago, this car passed with flying colors and I haven't modified it since, so I'm not sure where to start.

What I know:

The PCV was disconnected, an oversight on my part but not NOX causing.
It was 95 degrees out today.
The CAT was brand new two years ago, replaced it just before the test and should be fine.
Timing is set at 10 degrees.
I have a low-temp thermostat.
I think the front manifold is cracked.

The numbers:

15 mph: Max Allowed: 1148 My Car: 1432
25 mph: Max Allowed: 978 My car: 1294

As I understand it, being 95 degrees out today certainly didn't help, so next time I'll bring it in right when they open. I've also read that turning the timing back a few degrees will help too. I don't care about getting the NOX way down, just enough to pass the test. Obviously, the exhaust manifold needs to be fixed, but if I can get past the test now I'll worry about it later once the registration is settled.

Any thoughts?
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Re: Emissions: NOX just a little too high

Post by The Dark Side of Will »

EGR is what keeps NOx down. Is your EGR fully operational?
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

As best I can tell, yes. The valve is physically operable and the vacuum lines have been replaced with aluminum tubing, so there shouldn't be a vacuum leak. Idle is right where it should be. The car throws an intermittent code 32 on when cruising on the freeway, but it hasn't done it in a long time and it always goes away.

I can't remember if the low-temp thermostat is a 160 or a 180, but either way I'm going to replace it with the stock 195 degree and, if I can find my damn timing light, retard the timing a few degrees before I take it back for the retest. I'm also planning to go back first thing in the morning when it's cooler outside, see if I can get a better fuel/air mix than in today's 95+ temps.
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Fastback86
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

Good news and bad news.

Turns out I did, at some point, swap the thermostat back to the stock 195* unit, but it was also stuck open. New stat fixed that. Set the timing back to ~4* advanced (so damn hard to see those timing marks) and took another run at it this morning when they opened and ambient air temp was about 25* cooler.

Good news is, I was able to bring the NOX down. Bad news is, not enough. I now pass at 15 mph, having brought my NOX count down by nearly 400 ppm. Unfortunately, I'm still failing at 25 mph. Blowing 1119 ppm, or 131 ppm over the 978 ppm limit. Everything else is passing. HC is way below limits, as is CO2. All I have to do is get that NOX at 25 mph down a bit more and I'm golden.

Anybody know any more tricks? I can try turning the timing back a bit more, but I don't know how much it will help. The car was warmed up to proper operating temp before I smogged it, but I can't say for certain the CAT was properly lit off. It's also possible I didn't drive it enough for the ECM to relearn its fuel maps with the new timing before taking it in.

Thoughts?
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Re: Emissions: NOX just a little too high

Post by AkursedX »

Would octane boost help? Or does that just lower HC's?
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Fastback86
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

Don't know...anybody?

It's also been suggested that my 1986-vintage EGR may well still be functional, but not working as well as it should. Would like to be sure of that before I start throwing money at it in the form of a $45+ replacement valve.
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Re: Emissions: NOX just a little too high

Post by The Dark Side of Will »

More likely for your tube to be clogged, if the valve at least partially works. Sometimes you can't make a good diagnosis, but you narrow it down to one system, spend some money and it works.
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Re: Emissions: NOX just a little too high

Post by Shaun41178(2) »

will adding drygas help with the nox? I heard people doing thta trick. I guess the alky in it burns a bit cleaner and you just need it to pass.

I think its funny they are worried about your car and the pollution it produces over all the pollution and air quality from the fires.
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

I've got a bad feeling it's the solenoid - of course, the most expensive part in the damn system. TK advised I try manually holding the RPMs at 2500-3000 while feeling the EGR to see if it opens (with a glove on, of course). Not only did it not move, but there didn't appear to be any vacuum at all coming out of the line, or the solenoid for that matter. Unplugging the lines didn't affect the engine at all.

The EGR tube itself is fairly new and is the flexible unit from the Fiero Store, so I doubt it's had time to clog up.
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Fastback86
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

Shaun41178(2) wrote:will adding drygas help with the nox? I heard people doing thta trick. I guess the alky in it burns a bit cleaner and you just need it to pass.

I think its funny they are worried about your car and the pollution it produces over all the pollution and air quality from the fires.
Guess they don't want me contributing to the mess.
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eHoward
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Re: Emissions: NOX just a little too high

Post by eHoward »

Bringing back the timing helps. Not sure how much more you can take out though.

Fresh cat also helps...not sure how many miles are on yours after two years or how hard your car is on them.

you could also try cooling your intake air /engine bay with dry ice somehow.

good luck. I've been there.
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Re: Emissions: NOX just a little too high

Post by The Dark Side of Will »

Good point, Howard.

Is it an aftermarket catalyst? Aftermarket units have *much* less catalytic material in them than the factory cats do; they're usually done in 2-3 years.
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

The CAT is two years old, don't know how many miles. It's a daily driver, so probably a lot, several thousand for sure. Put it on right before I went for smog last time because the time before that it just barely passed, though I don't recall what the issue was back then 4 years ago. Would a new CAT bring the NOX down? I still need to take a closer look at the EGR system and see if I can ID any problem there. Anyone know if those "put alcohol in the tank and it'll burn cleaner" tricks actually work or if they're urban legends?
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Re: Emissions: NOX just a little too high

Post by eHoward »

I've done the run to home depot, buy a gallon of alchohol thing , and it brings the NOX UP and hydrocarbons down.

New cat would surely bring NOX down, but I can't say how much.
Fastback86 wrote:The CAT is two years old, don't know how many miles. It's a daily driver, so probably a lot, several thousand for sure. Put it on right before I went for smog last time because the time before that it just barely passed, though I don't recall what the issue was back then 4 years ago. Would a new CAT bring the NOX down? I still need to take a closer look at the EGR system and see if I can ID any problem there. Anyone know if those "put alcohol in the tank and it'll burn cleaner" tricks actually work or if they're urban legends?
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

UP? Wow, maybe I'll rethink that one.
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Re: Emissions: NOX just a little too high

Post by Fastback86 »

Third time's a charm. I've been driving it regularly since I remembered I needed to get it smogged and I dropped some Chevron fuel system cleaner in there and topped it off with premium, then took a long drive to get the cat good and lit off and it passed well under the limit. Funny story, the Check Engine light came on on the way home.
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Re: Emissions: NOX just a little too high

Post by Blue Shift »

Haha congrats. I remember a Fiero 2.8 refusing to pass on NOX, and it was the vacuum control solenoid to the EGR valve itself not behaving. We replaced it (in the smog shop, amusingly enough) and it passed fine after that.
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