My car recently flunked its emission test due to high NOx. The fix that got it through the test was to retard the timing. My question is how this works. I don’t understand how timing works and I am wondering how this adjustment might affect the performance of the car. Was this the best repair for high NOx?

– Al Jacklin, San Francisco



Let’s look at oxides of nitrogen (NOx) first. NOx, a combination of nitrogen and oxygen, is created in large quantities when your engine’s combustion chamber temperature rises above 2,500 degrees Fahrenheit. NOx is a contributor to acid rain and combines with hydrocarbons to produce ozone, a major component of smog.

The engine-management system does its best to limit NOx production by carefully regulating the air-fuel mixture and ignition timing. A properly functioning catalytic converter reduces emissions further. Common causes of elevated NOx are a lean air-fuel mixture; advanced ignition timing; lower than specified octane fuel; a faulty EGR (exhaust gas recirculation) system, if used; carbon buildup in the combustion chambers; elevated engine temperature and a weak catalytic converter.

Ignition timing refers to the instant the air-fuel mixture is ignited, relative to the position of the crankshaft (and pistons, which drive the crankshaft via connecting rods). Base ignition timing might be adjusted on vehicles equipped with a distributor (largely phased out during the 1990s) and the timing advances additionally via mechanical or electronic means according to engine speed, load and temperature. Proper ignition timing – a combination of the base setting and advance system – is critical for optimum performance, fuel economy and low emissions. An engine with excessively advanced timing will run well, but is prone to ping (harmful abnormal combustion) and produce excessive NOx. Retarded timing reduces engine performance, fuel economy and emissions.

Why was your timing readjusted? The California emissions test requires the timing to be checked. Should it be off spec and the test fails, an adjustment is sold and performed, and the vehicle is retested. It appears the test failure was for excessive NOx only, so it appears the tech might have adjusted timing below specs (more than a couple of degrees is a test violation and ill advised from a performance standpoint) as a quick means to reduce NOx.

If you find your car’s engine performance and fuel economy to be OK, I wouldn’t worry about the adjustment. If not, the timing should be restored to specs and the true cause of the NOx failure investigated.

My hope is this was a borderline emissions failure and the timing was found to be a couple of degrees advanced – perhaps ignored during the initial inspection – then corrected. Proper conditioning of the vehicle, including fully heating up the catalytic converter, can be a large factor in passing an emissions test. A failed vehicle might pass the tailpipe test on the second try, without adjustments or repairs, simply due to the additional running time.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.


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