Failed emissions, NOX high, ONlY at 25mph

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Feb 19, 2007
Perris, CA
Took the wife's car (1999 Cavalier, 2.2 w/208k miles) for a smog check today and it failed. NOX was high (1159 measured, limit is 746) on the 25mph test. What's so confusing to me is that it was 40 (well under the limit of 459) on the 15mph test. I have no idea what would cause a difference of over 1100 ppm between tests. The other two things they measured, HC and CO, were virtually undetectable. HC was 1 at 15mph and 0 at 25mph. CO was 0.02 at 15mph and 0.03 at 25mph. Engine RPM was 1800 at 15mph and 1700 at 25mph. CO2 was 14.5 both times (but there is no limit) and O2 was 0.04 and 0.06. I bought a can of "guaranteed to pass" and put it in the gas tank immediately after failing the test. We have a 200+ mile trip tomorrow, so that will have a chance to work its way through. When I got home tonight, I put some seafoam through the brake booster, poured some down each of the spark plug holes and let it sit, and misted some water through the brake booster as well. Any ideas on what would cause these weird results, passing by a lot at one speed but failing by a lot at another?
Sounds like your cat is shot, they do wear out over time and 208k are a lot of miles for a cat.
Originally Posted By: spasm3
Check the egr system. Combustion temps that are higher form nox. More alcohol in the fuel may help pass.
I second the EGR,if there is one. Test it by applying vacuum (or suck on an attached vacuum line) while idling. The engine should die or come close to it. Sometimes carbon will build up in the valve and its plumbing thus reducing it effectiveness.I struggled with NOX a couple years ago. My engine had no EGR valve. To pass I poured a top engine cleaner down the throttle body, retarded the base timing by three degrees, ran a catylytic converter "cleaner" in the gas tank and replaced a leaky throttle body gasket which was leaning out the mixture. I sprayed carb cleaner at the base of the throttle body while idling and the engine would start to stumble. My HC and CO readings were also near zero, which may have indicated a slight lean condition. Enough to raise combustion temps but not enough to create a lean miss and raise HC. Also, many cars had a foil hose which supplies heated air from the exhaust manifold to the air snorkel, make sure the flapper door in the snorkel is not stuck open, supplying hot air into the air intake all the time. Make sure your cooling system is doing its job and retest on a nice cool day.Good luck and study hard for your retest.
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Another thought, with 208k miles, how old is the o2 sensor? They can get slow and lazy before actually failing and throwing codes! I'm talking about the upstream sensor. Its possible it may not react quick enough to prevent a lean mix raising nox.
If the front O2 is the OEM, it's probably time for a change. That's the one which is in the engine feedback loop and is subject to all the changes in mixture the ECU makes. They do get lazy, especially if the engine uses a little oil.
Skip the brake booster Seafoam. If anything, add it to a full tank of gas and run it empty. Better of with Techron, Regane, SI1, PI.... Lean burn... EGR or catalytic. But, does it have EGR? Should be able to pass emissions on E85. Run it empty, put 2-3 gallons of E85. Go for emissions test. Then go straight to gas station and fill up with gasoline. Premium high octane fuel and E85 have less BTU's so that might help with cooler exhaust temps and less NOx production. I'd replace the catalytic converter after all the cheaper options... Air filter, sparkplugs+wires, fuel filter, thermostat/coolant/cap, PCV, oil+filter, o2 sensors, injector cleaner, knock sensor, ... Look at your maintenance records and replace anything not recent. Trace all vacuum hoses and trim or replace any that are cracked/broken off, leaking...
Rockauto shows an EGR gasket but no valve. Wonder if it's one of those engines with the blockoff plate. I'd do a water mist in the intake with it idling at 2500 RPM to blow the carbon out, then do a highway drive. Did you idle a bunch before the test? That era GM had around a 185' thermostat but a 221' fan turn-on temp, if you can get it in the machine at 185 you may have better luck. And just go on a cooler day. The nonexistant HC numbers mean you're burning fuel just fine with a good ignition system.
I agree with some easier simple maintenance first, it could just be a quirk with your car for that particular test. You can also ask the tech to be sure to run the car first before starting the recording for actual test, sometimes they skip that step and they start testing from idle, or also they wait too long getting the computer to be ready before running the 2nd test, and the idle time raises your numbers. This doesn't mean your car is fine, because that by itself isn't enough to cause you to fail. But if you were borderline it maybe the last straw that pushes you over. My 98 Lexus GS300 had tested in 2010 and 2012 at 50% NOx limit at 15mph, while 25mph was low. But most recent test in 2014 it swapped, so it hit 50% of NOx limit at 25mph while 15mph was low. I remember for sure this time, this tech ran the car for a bit on the dyno before starting the test, but was futzing with the computer remote before the 2nd run. I think it's the idle time in between the tests that can possibly cause the numbers to read higher.
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Simple maintenance will have a greater effect on HC and CO. I doubt that tune up items are going to cause his combustion chambers to run excessively hot.Plugs, fuel filter, wires are not going to cause his NOX issue.
How good is the cooling system? Check your fan/coolant quality/thermostat for any problems. If your having a higher calculated load at 25mph & the thermostat is partially open, cylinder head temps could just raise high enough to fail the NOx limits
I would make sure the O2 sensor is jittering between .45 and .8 volts. A vacuum leak or a weak cooling system can do it too. Back in '03, I had to get an '88 carbed Grand Wagoneer past a dyno test with the same specs as CA. NOx was the problem and a 200$ EGR valve was the final solution.
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