2000 Pontiac Grand Am - 3.4L NOX High

[BDBLOM]

I have a 2000 Grand Am 3.4L with 92k miles that failed e test for high NOX 2.26 grams per mile 1.5 max allowed. No trouble codes. test drove with scanner, egr working normal. disassembled egr, no carbon buildup. Ran engine with egr exhaust tube disconnected, good flow from exhaust. Commanded egr open with scanner with engine running, good vacuum and good response from scanner commands. E test diagnostics show engine running lean compared to like vehicle. Checked map sensor, ok. MAF passes performance test, no vacuum leaks, good response from O2 sensors, ST fuel trim @ 0, LT fuel trim @ -10. No vacuum in crankcase (leaking intake gasket), engine doesnt ping on acceleration. New customer and she says she has had a new cat installed along with new thermostat and IAT sensor. Checked ECT calibration and it's spot on with my infered temp gun, car has new plugs (AC Delco) and wires, new air filter and runs like a normal 3.4. Fuel pressure @ 50 psi.

Can't figure what I'm missing here. Any help would be greatly appreciated!

[ricmorin]

I have a 00 Grand Am 3.4l with 92k miles that failed e test for high NOX 2.26 grams per mile 1.5 max allowed. No trouble codes. test drove with scanner, egr working normal. disassembled egr, no carbon buildup. Ran engine with egr exhaust tube disconnected, good flow from exhaust. Commanded egr open with scanner with engine running, good vacuum and good response from scanner commands. E test diagnostics show engine running lean compared to like vehicle. Checked map sensor, ok. MAF passes performance test, no vacuum leaks, good respone from O2 sensors, ST fuel trim @ 0, LT fuel trim @ -10. No vacuum in crankcase (leaking intake gasket), engine doesnt ping on acceleration. New customer and she says she has had a new cat installed along with new thermostat and IAT sensor. Checked ECT calibration and it's spot on with my infered temp gun, car has new plugs (AC Delco) and wires, new air filter and runs like a normal 3.4. Fuel pressure @ 50 psi. Can't figure what I'm missing here. Any help would be greatly appreciated!

I would concentrate on the long term trim. 10% is a bit much. Are these trims at idle? If so, what are the trims at 2000 rpm? Leaking fuel pressure regulator or spitting injector? Do an injector balance test? Is the cat factory or aftermarket? Also, what kind of performance test did you do on the MAF?

Just some thoughts to get the juices flowing.

[BDBLOM]

It has an aftermarket cat, fuel trim numbers go to LT 21%, ST 0% at 3000 rpm at 2000 both numbers are at or near 0. Cannot perform inj balance test with my Solus scanner on this car. Checked MAF performance with dtc P0101 for this engine, not great but did show it's working. Looks like I have injector problems and not enough equipment at this point.

[ricmorin]

Considering the near zero readings at 2000, I doubt injectors causing this. I'd consider vac leak but fuel is being removed. I'm leaning on a MAF. My gas analysis skills are a little rusty since NH did away with pipe testing years ago and went to strictly OBD2. Usually caused by high combustion temps, right? How about ignition timing? You said EGR was good? Roughed up idle when applied?

My thought is if the MAF is skewed slighly rich, the computer will take away some fuel, thus making it lean. Lean running engine can definitely cause higher NOX. If not the MAF, even a slightly skewed O2 can have the same result. What are the HC and O2 gas readings? Are they low too? If so, that's a lean condition.

[steven kiser]

is the car reaching operating temperature. i'm seeing more and more issues with thermostat's not keeping the engine hot enough to keep the nox on line.

[ricmorin]

A cool running engine won't usually cause high NOX readings. Generally it would be caused by a hot running engine.