Car in sig. Just wondering if anyone could point me in the right direction of where to start to get it to pass. Readings where"

HC=41 (limit 53) CO=0.10 (limit 0.28) NOx=829 (limit 614)

Tech said it was most likely the EGR system. Does that seem to make sense looking at my numbers?

 

It does. High levels of nitrides of oxygen are formed at high combustion chamber temperatures. The egr blends in some spent exhaust gas to cool the chamber temperatures. I know it sounds contrary as exhaust gases are hot but the they're a lot cooler than the combustion temps.

The egr is operated by vaccum so check to make sure the vacuum line is intact and installed. You can check for vacuum supplied to the EGR solenoid while the engine is idling but won't be able to check the output as the egr isonly supposed to function when the engine is under a light load...as in cruising down the highway. Anyway, if you have vacuum to the solenoid and the lines are intact to the EGR then it's possible the egr is stuck closed. To check, make sure the engine is cool and press the diaphram on the EGR unit. If you can push it in then remove it and clean both the pintle on the EGR ans well as the port it plugs into. Before reinstalling it, apply vacuum to the port and look for smooth operation.

Good luck!

 

NOx can also be high when the engine is running lean, but the HC and CO don't look unreasonably low.

NOx is "oxides of nitrogen" and EGR doesn't cool down the combustion because the exhaust gas is cooler... it cools down the combustion because the exhaust gas displaces air and the contained O2, reduces the O2 partial pressure, which slows down combustion, and the PCM supplies less fuel, so there's less combustion to start with.

 

Thanks, Injuneer, for straightening out my post for the OP. You can't tell from my post, but I actually do understand this.....LOL

 

I went through this years ago. Pacesetter LT's with catted y and stock cam failed on NOx. The location of the EGR tube on the Pacesetters doesn't provide enough back pressure for the EGR valve to open. The EGR on these motors is a back pressure sensing valve. Put all the vacuum to it you want and without back pressure on the valve it will not open. I ended up manually lifting the valve during the test to get a pass...........its all about who you know.............

 

This is the first I've heard that. That's not to say you're wrong...I just never heard it. That said, I went out to my garage and took an egr valve off the shelf and applied a vacuum to it...the diaphram snapped back like it was a solenoid. What is the vacuum for if not to open the valve? Why, if it was a 'blowback' system, wouldn't it open any time?

 

Unfortunately, the only person I knew that worked at an emmisions station packed up and moved to Florida. Would it be possible to support it in a slightly open position?

 

It'll run rather poorly at idle and on accelleration if you do that. As Injuneer explained, the EGR allows spent gasses into the chamber diluting the fresh, and combustible, fuel/air charge leading to low power just when you need it...like on acceleration.

Check the thing before you do anything else. If the diaphram moves freely when you press it then look elsewhere, otherwise fix things that are incorrect before trying to rig it to pass the test...

 

It requires both vacuum and backpressure to function. See the link for testing and operation http://shbox.com/1/4th_gen_EGR.html

 

It seems to move VERY freely and makes quite a hiss when depressed. Haven't tried depressing with the engine running yet to see if it causes the engine to stumble.

 

Apply a vacuum to the nipple and make sure it works..if the diaphram is torn it won't move when you apply a vacuum. If it doesn't move then you need a new one. If it does move the problem lies elsewhere...possibly where 96flame outlines.