Edmunds Answers



  • karjunkie 10/26/08 2:04 pm PST

    Here are the common reason for a hot engine and high NOx at inspection time:

    1.Aftermarket cold air intake. Number one reason why cars fail the NOx test! Put the stock one back on with a clean filter
    2.Timing is off. Retard your timing to 14 degrees BTDC (need timing gun, or shop can do it for 15 bucks or so), turn up idle to compensate (follow proper procedure in manual).
    3. Radiator coolant is spent. Flush and replace with new coolant. You may also want to replace the thermostat to a slightly colder one.
    4. Worn spark plugs. Buy a new set gap properly and install. Try spark plugs a heat range colder that what you have.
    5. Dirty PCV valve. Remove and clean with throttle and carb cleaner.

    OK, once you do all this, which is routine maintenance anyway, here's a trick. Fill only a quarter of a tank with premium gas and add 3 bottles of HEET fuel line anti-freeze. This stuff is pure alcohol so it will burn super clean. If you don't pass inspection after all this, I'd be pretty surprised!

  • zaken1 10/26/08 2:19 pm PST

    High NOx is caused by either insufficient EGR, or ignition timing that is too far advanced. Sometimes the EGR valve will hold vacuum, but will not flow exhaust gas into the intake like it should; because either the valve assembly, a connecting pipe, or a manifold passage is plugged with carbon. If the EGR system works properly, when you pull a vacuum on the valve while the engine is idling; it should cause the engine to stall or the idle to become noticeably rough. If there is little or no change in the idle quality when a vacuum is applied to the valve; then the system is clogged somewhere.

    The ignition timing setting is very critical. Sometimes the NOx will go out of spec if the timing is only 3 or 4 degrees off. So the timing must be set exactly to the factory spec. And every once in a while, we come across a car that runs stronger than most other cars of that model; and which fails NOx, when everything is set according to specs. In those situations, the only way to make it pass is to retard the timing beyond the factory spec. The further you retard it, the lower the NOx will go. But you may have to reset the idle speed, if you retard the timing a lot. I would suggest at least 5 degrees retard, and sometimes as much as 10 or 15 degrees. But don't retard it so far that the engine runs rough, after the idle speed is corrected (otherwise, the HC will go out of spec). And too much retard will increase the coolant temperature and may cause overheating in high speed driving.

    I hope this helps!!!

  • tony78 10/26/08 3:54 pm PST

    I like zaken's answer.

    karjunkie's answer sounds illegal.

  • tek1010 10/26/08 4:13 pm PST

    Hey carjunkie if you over inflate the tires it reduces rolling resistance too.
    Did you put on a catalitic converter that was designed for your specfic car?


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