Failed Emissions - NOx too high

[Bri-Guy-GA]

How frustrating. Just got back from my yearly inspection and it failed. I have a 95 GP 3.1.

The readings are:

25/25 test 50/15 test

reading allowed reading allowed

HC 22 117 22 120

CO 0.03 0.65 0.04 0.67

NOx 1260 821 1177 907

CO+CO2 15.7 6.0 min 15.8 6.0 min

 

I replaced the plugs and wires about 6 month ago and it has been running good. What could be the cause?

[terryk2003]

I had the same problem on a car years ago and IIRC it ended up being the catalytic converter.

[Schurkey]

High NOx is often due to a malfuntioning EGR system. Defective computer/wiring harness for EGR solenoids, defective solenoids, plugged exhaust passages.

 

HC is low, CO is low...converter is not my first guess.

Edited July 28, 2012 by Schurkey

[GtpKo]

I had this problem with my 00 GTP, I believe my high NOx readings were due to a hot engine(coolant leak + air trapped) along with a dirty EGR. After cleaning the EGR out with carb cleaner, coolant leak fix and proper system bleed, then taking it in cold, I passed with flying colors.

 

It's been recommended that if you have room to breathe on CO, then taking it in cold would bring up CO and NOx down. High NOx can be created when cumbustion temps go above 2500deg. So maybe even running lean-ish could do it too?

[Garrett Powered]

leaking crossover?

[RobertISaar]

High NOx is often due to a malfuntioning EGR system. Defective computer/wiring harness for EGR solenoids, defective solenoids, plugged exhaust passages.

 

HC is low, CO is low...converter is not my first guess.

 

aren't these tests done at/near idle though, where EGR isn't utilized?

 

So maybe even running lean-ish could do it too?

 

this is what i would look at first, since CO and HC will drop when going lean until you get to the point of a misfire, but NoX will rise at the same time.

[Schurkey]

...taking it in cold, I passed with flying colors.

 

It's been recommended that if you have room to breathe on CO, then taking it in cold would bring up CO and NOx down. High NOx can be created when cumbustion temps go above 2500deg. So maybe even running lean-ish could do it too?

I think you're right, running lean could drive the NOx upward. As long as it's not so lean that the engine lean-misfires, in which case NOx and CO would plummet, and HC would skyrocket.

 

aren't these tests done at/near idle though, where EGR isn't utilized?

Test procedure is very important.

 

If the test is to be valid, the shop will run the vehicle long enough to ensure it's fully warmed-up. Taking it in "cold" won't affect the outcome because it won't be cold when they perform the test. If I were writing the test protocol, I'd precondition the vehicle by running it until the (electric) cooling fan engaged, or the ECM signalled >190 degrees F. coolant temperature, or somesuch.. First Guess: Some states/provinces do that, some don't. Some test-station workers actually follow the test protocols, some don't.

 

I made the assumption that a "25/25" test along with a "50/15" test refers to MPH on dyno rollers; I don't know how the ECM controls the EGR solenoids based on MPH. Older vehicles used ported (timed) vacuum which applied--or didn't apply--vacuum based on throttle position vs. intake manifold vacuum. Newer EGR protocol has the ECM driving solenoids--and I don't know what the metering scheme is.

Edited July 29, 2012 by Schurkey

[Bri-Guy-GA]

I think you're right, running lean could drive the NOx upward. As long as it's not so lean that the engine lean-misfires, in which case NOx and CO would plummet, and HC would skyrocket.

 

 

Test procedure is very important.

 

If the test is to be valid, the shop will run the vehicle long enough to ensure it's fully warmed-up. Taking it in "cold" won't affect the outcome because it won't be cold when they perform the test. If I were writing the test protocol, I'd precondition the vehicle by running it until the (electric) cooling fan engaged, or the ECM signalled >190 degrees F. coolant temperature, or somesuch.. First Guess: Some states/provinces do that, some don't. Some test-station workers actually follow the test protocols, some don't.

 

I made the assumption that a "25/25" test along with a "50/15" test refers to MPH on dyno rollers; I don't know how the ECM controls the EGR solenoids based on MPH. Older vehicles used ported (timed) vacuum which applied--or didn't apply--vacuum based on throttle position vs. intake manifold vacuum. Newer EGR protocol has the ECM driving solenoids--and I don't know what the metering scheme is.

 

The numbers are MPH and time. 25 seconds at 25 MPH and 50 seconds at 15 MPH. I cleaned the EGR, but most likely it is bad. I had a computer light a few years back and it was the code for EGR. It comes back every so often and usually goes away after cleaning it out. Might be time to replace the thing. I hate to do it as I plan on selling the car in a few weeks. Maybe see if I can get a used on at the pull a part.

[RobertISaar]

I made the assumption that a "25/25" test along with a "50/15" test refers to MPH on dyno rollers; I don't know how the ECM controls the EGR solenoids based on MPH. Older vehicles used ported (timed) vacuum which applied--or didn't apply--vacuum based on throttle position vs. intake manifold vacuum. Newer EGR protocol has the ECM driving solenoids--and I don't know what the metering scheme is.

 

long story short, with the 3 tower digital EGR as found on a lot of 95-earlier cars, especially W-bodies, they're just 3 different sized orifices to make a combination of 8 possible EGR flowrates(including none) that are probably effected by exhaust system backpressure and intake vacuum. i know backpressure is attemped to be compensated for in the calibration, i assume intake vacuum is as well, since it's actually easier to try and compensate for.

 

The numbers are MPH and time. 25 seconds at 25 MPH and 50 seconds at 15 MPH. I cleaned the EGR, but most likely it is bad. I had a computer light a few years back and it was the code for EGR. It comes back every so often and usually goes away after cleaning it out. Might be time to replace the thing. I hate to do it as I plan on selling the car in a few weeks. Maybe see if I can get a used on at the pull a part.

 

and i'm pretty sure you just gave out the answer to this problem. be sure to clean out the port in the manifold as well, since that likes to plug with carbon too.