Car Won't Smog-High NOx emissions at 15mph

So I went in to smog my 95 Blazer the other day, not expecting any problems, but it seems that my NO emissions at 15 mph were too high, so I failed. Anybody have any information that could help me figure out what to check so that I can fix it! Max NO at 15 mph was 501 ppm, mine was 667 ppm. Everything else was in spec. I'm thinking EGR valve, and I want to know if a nut kit or leaky regulator in the plenum could cause this too while i'm working on that. Any help would be appreciated... I have to have this smogged soon because it's pay-your-money-to-the-government time for my car again.:banghead:

to bad it's not N20, laughing gas or you'd have a money maker there. here's the scoop on NO emmissions:

NO (PPM): Oxides of nitrogen (more commonly called NOx) are odorless gases that help form smog, and give smog its characteristic brown color. NO is produced when temperatures in the combustion chambers exceed 2500 degrees Fahrenheit. Excessive engine temperatures could be caused by a lean fuel mixture, by retarded timing, by carbon buildup inside the combustion chamber, or by a malfunctioning engine cooling system. The function of the EGR System is to reduce NO. Unhealthy levels of NO emissions result in a vehicle failing its Smog Check. NO is measured in parts per million (PPM).

Exhaust Gas Recirculation (EGR): The vehicle's Exhaust Gas Recirculation system reroutes exhaust gases back through the intake manifold to lower engine temperatures and, in so doing, reduces NOx emissions. The EGR functional test is part of the two-speed idle (TSI) test, but is not necessary for the ASM (BAR-97) test, because that equipment tests for NOx.

Taken from your states web site: http://www.smogcheck.ca.gov/StdPage.asp?Body=/Geninfo/OtherInfo/Vehicle_Inspection_Report_Glossary-May_1996.htm

They should have the web site on the inspection form.

thanks for the info, but the smog station didn't do a functional test of the valve. In order to do that, it requires a scantool, and replacement of the EGR valve is going to be a pain in the A$$. Oh well, as long as I know that's the only possible cause of that, then i guess i'll have to change it.

Before replacing the valve, I'd check the EGR pipe for pinched/crushed areas. Then I'd clean out the EGR passages. If it's passing everything but that one test, I'd say the EGR system is functioning, but it may not be flowing properly. You haven't done anything to the exhaust system that would reduce backpressures, have you?

Before replacing the valve, I'd check the EGR pipe for pinched/crushed areas. Then I'd clean out the EGR passages. If it's passing everything but that one test, I'd say the EGR system is functioning, but it may not be flowing properly. You haven't done anything to the exhaust system that would reduce backpressures, have you?


corning is right, try removing the EGR valve assm and blowing it out with compressed air. Carbon from the engine can lodge against the pintle. Use a screened gasket when reinstalling the EGR to keep carbon particles out of it.

Don't replace the valve, it doesn't need to be replaced.

Take it off, spray carb cleaner or throttle body cleaner through it and dissolve all the carbon and spray cleaner into the two holes in the engine.

Get a new EGR gasket and reinstall.

There is no tube on a 1995 4.3L EGR valve. Just a valve under the intake and it is easy to swap in and out.

Well other than a high-flow cat, 3-inch piping, and a hi-flow flo-pro muffler, no i don't think there's anything that might reduce backpressure :-) . But yeah i'm going to try tb cleaner on that valve. Only problem is it's a pain to get to, and i don't want to remove the intake plenum. I already ordered the screened gasket from rockauto, and I'll try that. As far as increasing backpressure when at low speeds or idle, is there a possibility of installing a heat riser valve, like one used in carbureted engines to heat the intake air on cold start. Not a thermostatic one, but a vacuum-actuated one. That would increase torque while allowing the low backpressure flow at higher rpms. any opinions on that? because I really don't want to change the exhaust back to stock or anywhere near it.

This is not a backpressure problem. This is caused by higher combustion temperatures from restricted EGR.

Also, go and get a bottle of Redline Complete Fuel System Cleaner and throw all of it into the fuel tank before you fill it up, it will clean out the injectors really we..

Rasied my Neon 7mpg and my Blazer rebiunded 3mpg after treating it.

It could be a backpressure problem.. It takes some backpressure to push the exhaust gases in. We just got done studying this in class.. But I'd yanl off the EGR valve and make sure everything's clean. I agree with the screened gasket, especially since EGR passages tend to be nasty...

It could be a backpressure problem.. It takes some backpressure to push the exhaust gases in. We just got done studying this in class.. But I'd yanl off the EGR valve and make sure everything's clean. I agree with the screened gasket, especially since EGR passages tend to be nasty...

It could be, but I highly doubt it unless his muffler is completely shot.

Easiest way to test is put the back pressure tester into the O2 sensor bung before the cat. If there is no back pressure, then you have no exhaust problems.

But you need a little backpressure for both typed of valves... No more than 1.5psi. But it really all depends on whether it's a positive or negative backpressure valve. Look on top of the valve, and the last letter in the code will be a N or a P(N=Neg. P=Pos.). If it's a positive pressure valve, it relies on exhaust backpressure to open the control valve which in turn allows vacuum to open the valve, otherwise you'll create a lean situation. Aftermarket exhaust can wreak havoc here. With a Neg. BP valve, at lower engine and vehicle speeds, the negative pulses in the exhaust system hold a bleed valve open. When the engine and vehicle speed increase to a preset value, the negative exhaust pressure pulses decrease, and the bleed valve closes. Under this condition, if control vacuum is supplied to the diaphragm chamber, the EGR valve is opened.

It could be, but I highly doubt it unless his muffler is completely shot.

Easiest way to test is put the back pressure tester into the O2 sensor bung before the cat. If there is no back pressure, then you have no exhaust problems.


Got the muffler only a year ago, so there's not a problem there with the muffler, got the cat less than a year ago, and my car is the same model as yours so if you don't have a problem with egr with aftermarket exhaust, then neither should I. Going to clean out EGR valve today. I tried before, but not with TB cleaner... Used MAF sensor cleaner because I didn't want to damage any electrical parts

How long has it been since a tuneup?

only about 3k miles... and i'm changing the oil and filter again in about 500 to 1k miles

what did the tuneup include?