Failed smog

Hi All,

I have a 1987 IROCZ 5.0L with tune port injection. It failed the NOx test. Prior to the smog test, I replaced the Cat. Everything passed with flying colors except for the NOx test at 25MPH load. It was allowed 700 but had 1904. The funny thing is that it passed easily at the 15MPH load with 201 measured. I have checked the vacuum to the EGR while running down the road and it is fine (10 -15 in. HG. under load). I did all the testing for the EGR valve and EGR solenoid and they all pass. Is there some other component that will cause high NOx? Any help will be greatly appreciated.

02 sensors?

I think there is only one (driver's side). I have not replaced it. But I am not getting any codes showing a lean or rich mixture from the computer. Would the O2 show a high NOx at 25 MPH but a low NOx at 15 MPH?

check your air filters and try and clean out your throttle body a little. Sometimes if your motor isnt getting enough air it can do that.

ya, do that and run injector cleaner in it and try it one more time, i here it can reduce numbers big time.

where do you live that you are having emissions test run on a '87? here in arkansas i can already get an antique tag for my car haha

I replaced the air filters and cleaned the throttle body. I'm going to try the injector cleaner and run 91 octane (help keep things stay a little cooler in the combustion chamber). The only thing that throws me is why does the NOx have a high reading at 25 MPH and a low reading at 15 MPH.

drvngstorm05 - good old California. Where they keep making the smogs harder and harder. I thinks it time to move to Arkansas where life is good!

life is good and cold here...

high levels of NOx i believe is evidence of running lean correct?

Yes lean is one of the factors. I know the EGR is used to cool the combustion chamber (usually is the main cause of high NOx when it is not working). Maybe the O2 sensor works fine under the the 15 MPH load test but can't keep up during the 25 MPH load test and thus gives a real lean mixture.

egr actually increases intake temp... john told me a while ago that some cars actually got better emissions w/out the egr...

cleaning the injectors will prolly alleviate the problem, don't know if it will solve it but it should definately help... if all else fails put bigger injectors on it w/out tuning it, then it will definately run rich lol

A fuel problem (or spark for that matter) will result in a HC out of limit.

NOx is stricly the EGR and AIR systems. The EGR on these I believe is a negative backpressure type so vacuum testing gets tricky.

Did you check the vacuum hoses out? 25 mph is when it s.b regulating the most and it's not.

I followed the book on all the testing of the EGR and EGR solenoid. I checked all hoses for leaks and everything is good. I could see the diaphram of the EGR moving and the EGR drops vacuum when the car is started (as stated). I have not tested the AIR system (it has a smog pump). Will this contribute a lot to the NOx if it is not working properly?

i wouldn't think so... simply because NOx involves a lean air fuel mixture... air pump adds air to the exhaust to completely burn up any excess fuel... if the air pump wasn't working then you would have rich exhaust...

NOx is formed by high combustion temperatures. A lean A/F ratio (in the cylinder, not the exhaust) can cause this. A hot running engine can also cause this, as can over-advanced static ignition timing. Carbon buildup in the cylinders can cause this, too. The exhaust reintroduced to combustion via the EGR system is designed to cut down on this pollutant by reducing peak combustion chamber temperatures. Air injected into the cat is also supposed to lower the emissions by catalysis. Two things I would check are:

#1, the EGR system as everyone else has touched on. Take the valve off and check to make sure the pintle moves freely. Sometimes, carbon can build up in the area around the pintle to the point the thing won't move. Lightly tapping with a hammer should free it up. Also, you can do a quick and dirty test on the diaphragm by depressing the pintle as far as you can and sticking a finger over the vacuum line. If the diaphragm (and pintle) immediately return to the no vacuum position, and you don't feel vacuum on your finger, then the valve is suspect. Also, check the EGR solenoid and the vacuum lines to ensure they all properly work.

#2, the smog pump (AIR system). Make sure the pump works! Make sure the electric valves operate properly and that all vacuum lines are in working order. Double check the check valve on the line dropping down to the cat to ensure it isn't blocked by carbon buildup. If no fresh air from the AIR system makes it down to the cat, the cat can't do its job with the NOx.

You should not have a mixture problem if the EGO sensor properly works and there's no service engine soon light (trouble code) relating to the EGO/mixture. Remember that a rich mixture will lead to early demise of both the EGO sensor and the catalytic converter - so if you have had a problem in the past with a rich mixture, you might need to change both of these.

NOx is caused by increased cylinder temps. EGR reduces cylinders temps by introducing inert gases into the cylinder. These inert gases can't burn, so overall there is less of a "bang" in the cylinder. Higher octane fuels will only worsen the problem if the EGR flow is blocked. These fuel's will burn hotter and cause detonation. If you can see the EGR valve moving, I'd suspect a blocked passage. The EGR passages can get blocked pretty easy, especially on older vehicles. Removing the EGR valve is the only way to be positive the passages aren't unblocked, and a replacement gasket is around 3 bucks. Be careful not to let any chunks of carbon enter the intake manifold if it requires cleaning. Also, inspect the EGR tube, in between the EGR valve and the exhaust for crimps. The AIR system only provides the catalytic converter with oxygen to reduce emmisions. Only a secondary cat or a "tri-cat" can reduce NOx numbers.

NOx is caused by increased cylinder temps. EGR reduces cylinders temps by introducing inert gases into the cylinder. These inert gases can't burn, so overall there is less of a "bang" in the cylinder. Higher octane fuels will only worsen the problem if the EGR flow is blocked. These fuel's will burn hotter and cause detonation. If you can see the EGR valve moving, I'd suspect a blocked passage. The EGR passages can get blocked pretty easy, especially on older vehicles. Removing the EGR valve is the only way to be positive the passages aren't unblocked, and a replacement gasket is around 3 bucks. Be careful not to let any chunks of carbon enter the intake manifold if it requires cleaning. Also, inspect the EGR tube, in between the EGR valve and the exhaust for crimps. The AIR system only provides the catalytic converter with oxygen to reduce emmisions. Only a secondary cat or a "tri-cat" can reduce NOx numbers.

Pretty much what I said. The "tri-cat" is installed on every vehicle beginning with the '77 model year as required by law. Any cat with a AIR tube on it will be a tri-cat. The only reason for the air is for catalysis of the NOx within the converter. Generally it won't affect other emissions unless the engine is rich running or misfiring and the presence of HC is abnormally large - in which case the cat will burn itself out. The air injected at the manifolds will be all that is required for a normal (dual-cat) to do its job on the HC and CO.

BTW for everyone who might wonder what an "EGO sensor" is, it's Exhaust Gas Oxygen, a fancier way of saying oxygen or O2 sensor.

Thanks silicon212,

I followed the steps you listed in #1 & #2. Everything worked fine. So I took it to a pre-smog station and it passed. Next I took it to the test only station it it passed there. So I think the failed smog may have been a glitch it the test. At the pre-smog station with the EGR disconnect the NOx were around 2000ppm. With the EGR connect the NOx were around 400 ppm. So the EGR is really the main player when it comes to high NOx. I'm not sure how much the lean mixture plays since the car pinged really bad (something I need to fix now). Since it pinged so bad I thought maybe the O2 sensor might be leaning it out (but no codes set). But even with the lean mixture, it still passed easily.

Thanks everyone for you help.