Failed Smog II - too much NOX

[thx247]

Well, I didn't pass Smog today. My NOX emissions are too high. Roughly 1800ppm when it should be less than 800ppm. This was only on the 15mph test though, the 25mph test the car had only 60ppm.

HC's and CO readings were fine.

I have checked the timing, 20btdc. I started checking my EGR valve. How much vaccum should I be reading on the EGR vac line on a 91 DOHC motor at 2,000rpm? The EGR valve does move, but very slightly- not near as much as my other car with a 98 DOHC.

I'm going to compare the two motors asap and see if I can figure out if my EGR is not getting proper signal or if something else is broken. Or more preferably, can I add something to the tank to get a lower temp burn? I'm on a tank of 91 octane right now.

[cdlong]

sounds like it's time for a new catalytic converter. the egr is for HCs mostly.

[thx247]

Uh are you sure?

The cat does clean NOX but the EGR is specifically to lower exhaust temperatures which lead to high NOX emissions. Replacing the cat is an option I have, but the EGR system should be looked at first.

[cdlong]

how would sending hot exhaust gas into the combustion chamber lower cylinder temperatures? the EGR sends the unburned hydrocarbons back into the cylinder to be burned.

there are probably other things you can do, but i don't know any off the top of my head.

[thx247]

Thanks nitekids...


The EGR reduces cylinder temps by adding mass to the intake charge. The added mass takes more energy to heat up, thus reducing exhaust temps. NOX only forms when exhaust temps go over 2300F or so.

[fleaf]

check to see if all your vacuum lines are sealed. Might not be it, but it could be a possibly

[thx247]

this isnt really one of those things I can spray carb cleaner around and listen for is it?

[cdlong]

for vaccum leaks? yeah, you can do that. it should work, it'll make the engine rev higher if you hit a leak.

The EGR reduces cylinder temps by adding mass to the intake charge. The added mass takes more energy to heat up, thus reducing exhaust temps. NOX only forms when exhaust temps go over 2300F or so.

you're right, but the book i'm looking through says it works by diluting the mixture, kind of the same thing, but not quite. the mass in the cylinder is determined by how long the intake valve stays open. all the EGR does is put less combustable stuff in the combustion chamber. i stand corrected, you learn something new every day.

you might try retarding the spark timing slightly. that has an effect on NOx.

[thx247]

yeh I wanted to try doing that, but the stupid tech will fail me on timing then =(

[Phax]

Originally posted by thx247
...NOX only forms when exhaust temps go over 2300F or so.

I thought that pistons started melting at around 1600F. Most EGT gauges only go up to about 1800F.

[HiPSI]

Originally posted by Phax
I thought that pistons started melting at around 1600F. Most EGT gauges only go up to about 1800F.

lol, is your egt probe mounted inside the cylinder? if so that's a neat trick. when the combustion process begins the cylinder is under extreme pressure, and pressure=heat. as the piston moves down, the pressure in the cylinder drops, as well as the burn begins to finish, and the valve opens decreasing cylinder pressure as well as it's pushed out on the exhaust stroke. all of this together (at least for a basic non-scientific type of answer) drops the temperature drastically between the point that you have full ignition on the charge (right around TDC) and when it hits your EGT probe in the exhaust manifold;)

[Var]

The timing is supposed to be 15BTDC not 20...UNLESS the dual cam is supposed to be 20. I have single cam. The EGR may have a lot to do with your high NOX readings. Have your smog shop do a functional test on it, it only takes 20 seconds and a decent shop wouldn't charge you for it. Or you can do it yourself if you have a vacuum hand pump. It sounds like it may also be your cat cause the readings are pretty high. It could also be your 02 sensor or you car may be running hot.-->cooling system.

[thx247]

yeh I have a vac guage here and it reads vac but I dont know how much it should be reading. My 98 EGR valve moves a lot more, I have not had time to compare and contrast though =( Christmas OWNZZ ME.

About pistons melting, temps in the engine are not all that hot really. Even when the spark ignites the fuel/air mixture in the chamber, the flame never actually touches the piston or cylinder walls. If it did, the temps would be extremely high very shortly. A thin layer of unburnt fuel and air basically protects the chamber walls from heating up.

[Var]

not a vaccum gauge. a vacuum hand pump. Pump the egr and see if it works when vacuum is applied to it. The car should start idling really bad or stall when you do it. That means the egr is passing enough spent gases through it. If nothing happens you need a new egr valve. Also like i said if timing is supposed to be 15 and is set at 20 that will raise NOX too, but not all the way to 1800ppm.


@ 2500 F NOX levels start to rise quickly. Before that there is a little bit of NOX but low amounts and most of it is taken care of by the EGR and the cat. The flame does touch the piston as far as im concerned and the valves and the combustion chamber. what else would explain carbon buildup?

[thx247]

I have the vac hand pump, I'll play with it on the 26th when I get some time again. Timing is supposed to be set for 20btdc on the DOHC motors.

I remember reading about flame fronts and whatnot awhile back, I'll go check into why carbon is deposited. I imagine its because you can't fully burn everything in the combustion chamber, and inevitably some gets left behind and deposited by the high pressures in the chamber.

[curbsurfer]

your thinking too hard. it is the catalytic converter. a new one will solve your problem even if your egr is bad.

ive worked at an exhaust shop for years that was right next to a smog shop. if you want to pass smog , just put a cat on it. if you want to research it , analyze it, take it apart, rebuild it, whatever. you will still need to put a cat on it to pass smog i bet.

[Var]

Originally posted by curbsurfer
your thinking too hard. it is the catalytic converter. a new one will solve your problem even if your egr is bad.

ive worked at an exhaust shop for years that was right next to a smog shop. if you want to pass smog , just put a cat on it. if you want to research it , analyze it, take it apart, rebuild it, whatever. you will still need to put a cat on it to pass smog i bet.

You are probably right. but you never know with cars

[thx247]

The only thing I dont have extra is a cat. EGR and all that other stuff I do have. Im going to see what I can see today

[DriftDragon28]

Hey guys after reading this i felt that i needed to stoep in and put in my .02.

Just so you know where Im comming from I AM A SMOG TECH. EA tech to be exact. that means they let me play with the dyno and the NOx gas.

if your car failed for nox then u most likely got an EGR problem of some kind. most likely it has some clogged lines that need to be cleaned. its gonna be a pain in the ass. You could throw on a new cat and pass. get the cat installed that morning and drive (on the freeway) to the smog place. you should. i repeat should pass with flying colors. a new cat will hide ALOT of problems. thats why if i replace a cat on every car that fails i can loose my license.

NOx is formed when combustion chamber teps reach 2500 degrees. (no the pistons will not melt) things that will raise the temp up that high include, advanced timming, lean mixture, high compression, malfunctioning EGR, high boost, increased load, and cheap gas.

so to fix it make sure your timming is dead on, your not running lean, (injectors could be clogged or dirty) compression souldnt be a problem nor should boost.

my suggestion to you is run high octane, and make sure your EGR is working. you could also try to de-carbon you engine. try a local auto store they should have what you need to do that.

Good luck and any more questions just ask.

[thx247]

I'm in a time pinch right now so I threw on a cat yesterday. I would play with the EGR stuff but I don't have the time to mess with all the vac lines controlling the valve. I need to get this car registered and street legal asap.

I will tell you guys what happens tomorrow when I try and smog it again. I hope it passes but I don't like this way of fixing the problem.

[thx247]

new cat did not pass the car. NOX levels are still too high.

[formeRiceR]

I have the same problem, the emission tech told me that the CO2 or the NOx levels were too high (don't remember)....I think my car is running really rich, a shop screwed around with my TPS so I think that might be the it. Would a rich mixtures poduce higher levels???

what do you think guys?:bash:

[thx247]

NOX is formed by lean conditions possibly, not rich

[Var]

Originally posted by formeRiceR
I have the same problem, the emission tech told me that the CO2 or the NOx levels were too high (don't remember)....I think my car is running really rich, a shop screwed around with my TPS so I think that might be the it. Would a rich mixtures poduce higher levels???

what do you think guys?:bash:

Running rich would cause your CO and/or HC level to be high, not CO2.

THX247...what are your new numbers for NOX?? They should have dropped dramatically but if they didnt your cat was still good. The egr test is very simple, like i said if you know what you are doing it takes 20 seconds. If not it may take you 5 or 10 minutes. It may save you some money if you figure it out on your own. If you want to bring it by my shop in san bruno ill test it out for you for free. Let me know.

[thx247]

Sounds good to me. Toss me an address/Phone #

[email protected]

We did a pretest, the numbers were crazy high imo, higher than the actual test. When they were doing the test I saw the numbers dip down to 600 for the 15mph, and 42 for the 25mph, but that was only after it was running for awhile.

The numbers they printed out for me were 1929 for 15mph, and 1342 for 25mph. These seemed to be from the beginning of the test, but the shop didnt try and sell me on fixing it there, so I don't know if they numbers are honest or not.

[Var]

Hmm it sounds like they didnt heat the cat up enough. I'm sending you some info through the email address you left .

[manuelku]

I just failed 2 pre test yesterday, my reading was HC and CO too high, NO passes.

I was told my sparkplug wires are bad, cause my car misfire, and the O2 sensor needs to be changed. I will have the car replaced the O2 sensor and wires the first thing tomorrow then go to the smog place again to have it smog.

[Ghettokracker71]

I was going to suggest 02 Sensor but I see someone else already did that,and the vacumme leak checking is also a good idea...makes me happy I don't live in an emissions controlled area.

[crxcess]

Did you ever find the solution to this problem? maybe the egr vacum source? I have the same exact problem....I changed egr, cat, decarbonized combustion chamber, spark plugs, fuel filter, cleaned and checked egr and transducer for proper operation, only things left is the egr solenoid or vacum pump? 15 mph 1300 25 1000 NOx

[thx247]

I passed the test by running in 1st gear. 2500 rpm barely got the car under the wire. Other things you can check are timing (as if you hadn't done that already)

There is a few laws on the books to help assist in cars that cannot pass. You might be able to get an extension.

[DuffMan]

People, stop telling him to buy a new cat. If his HC and CO are fine, thats not the problem. NOx is usually an EGR or other similar type of problem.

[crxcess]

I am not implying that he should change the cat. I am trying to find the solution to my problem. I have already changed almost everything emission related, minus a few pain in the arse things. I think maybe the egr solenoid might be the culprit. But its between the head and the firewall, about 1" of space there to work in.

[thx247]

if you are going remove EGR I would almost say remove the intake manifold lol. No space to work on that thing at all.