# Irritating Smog Check!



## 1996nissanlover (Apr 22, 2004)

To my surprise, I just failed the smog check on my well-maintained 96 Altima last weekend with NO2 at 480 ppm slightly higher than the California standard around 460 ppm.

The tech charged me $60 in addition to the smog check fees to fix the problem but finally I found out the only thing he fixed was the cracked hose about 2" long adjacent to EGR valve.

Since this very small repair cost me so much money, I wanted to understand how EGR is working but found neither good reference or correct part description, not to mention the small hose adjacent to this EGR. Can someone points me to a good reference and price on EGR and hose?

Finally, hearing my story of this small repair, my friend's comment on the smog check is that someones are making big bucks from smog check because the repair doesn't seem regular so that it is difficult to set a fair price, though the check does improve the air quality. Any comment?


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## Cooperman (May 18, 2005)

New EGR valves are in the vicinity of $100-150. Not sure about the hose.

An EGR valve works by taking exhaust gas and recirculating it back into the enginge to cool the temperature of combustion. Exhaust gas is hot, but much cooler than the combustion temperature. The higher the combustion temperature the more NOx is produced because nitrogen in the air, while normally very inert, at high temperatures reacts with oxygen to form NO and NO2 - both of which contribute to photochemical smog.


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## jserrano (Oct 27, 2004)

^ Nice explanation. To add in some of mine if I may, basically it works like this,

During some operating conditions, typically when the engine is rev'ing towards 2000 rpm, the ECU controls the EGR/EVAP solenoid valve to allow a vacuum signal to reach the EGR valve. It goes from the intake manifold->EGR/EVAP solenoid->EGR-BPT->EGR valve. The EGR valve lifts off it's seat to let exhaust gas go inside the intake manifold. An EGR temperature sensor feeds back information to the ECU about the exhaust gas temperature that is recirculating into the intake. That is one way the ECU knows when to turn off the EGR/EVAP solenoid valve at the right time.

A Backpressure Transducer (BPT) is connected to the EGR valve vacuum source and it also senses the exhaust pressure. When the exhaust pressure is low it bleeds off the vacuum from the EGR valve so that it won't operate. When the exhaust pressure is high it let's vacuum reach the EGR valve to operate as normal.

Hope this makes some sense.


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## 1996nissanlover (Apr 22, 2004)

*Do I need to replace spark plug and fuel filter?*

Thank both for explaining how EGR works.

Since NOx is caused by high temperature in the combustion chamber, do I need to replace spark plug and fuel filter to lower the temperature?

Thanks!


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## jserrano (Oct 27, 2004)

1996nissanlover said:


> Thank both for explaining how EGR works.
> 
> Since NOx is caused by high temperature in the combustion chamber, do I need to replace spark plug and fuel filter to lower the temperature?
> 
> Thanks!


I would suggest four things,

1. Check ALL the cylinders' pressure with a engine compression gauge.
2. Flush the coolant system and also make sure all the components are working within specs.
3. Make sure the EGR system is working properly.
4. Make sure there isn't high backpressure in the exhaust system.


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