99 Yukon failed emissions test.

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Baszboy

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99 Yukon 5.7
Old girl failed colo's emission test yesterday, twice.
Failed due to high NOx-gpm. After First fail I changed out the egr valve (internet info pointed at that) Second test still a fail but it did have a lower NOx-gpm figure, just not enough to pass.
My code readers generic, just shows the codes once you have an engine light on.

Trucks got 169k miles, replaced plugs wires and cap last year, ac delco parts. She runs good and does ok mileage wise.

So any good ideas as to where to start looking for my problem? Internets kind of a bust, could be 20 different things or 10 different sensors. Not wanting to throw a heck of a lot of $$$ at it buying parts till it passes! Or spend a week trying to figure it out.

Anyone have a good scanner that can pin point the problem?? Denver Co area.
Or any good ideas as to what to look for?
 

Erik the Awful

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NOx is a sign of running lean. Make sure you have a clean air filter, then check for vacuum leaks, then check fuel pressure.

If you can get your hands on a scan tool that shows you data stream, that will give you a better picture. I use a BAFX OBDII bluetooth dongle that I bought off Amazon for $25 connected to the Torque app on my phone, but my truck's an '89 with Sniper EFI, so I don't know if it will communicate with the later Vortec PCMs. The $200+ tools are great if you use them often. I don't.
 

Schurkey

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NOx is a sign of high combustion temps. NOx does not form if combustion temperature is kept lower than a certain threshold.

High engine temp makes high combustion temps more likely.
Lean fuel mix makes high combustion temps more likely.
Heavy throttle makes high combustion temps more likely.

Yeah, malfunctioning EGR is the go-to for excessive NOx. EGR reduces peak combustion temps. The exhaust tube from exhaust manifold to intake manifold, or the manifold itself could be plugged. The EGR valve could be defective, or the control system for the EGR valve could be broken.

The passages, the valve itself, and the control mechanism for the valve seem to be about equally responsible for problems. Ya gotta test everything.

But, as said, faulty EGR is not the only thing that makes for high NOx. Does your vehicle have a 3-way catalyst? If the catalyst has failed, you're getting no post-engine treatment. Running hot? Low fuel pressure? Engine out-of-tune?
 
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