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Failed Smog Test

Why Did My Corvette Fail the Smog Test for High NOx?

I had a tuneup done recently as well as had the EGR replaced and the EGR valve solenoid replace. Got an oil change done too. My mechanic had the smog shop next to him run a pretest and all passed. He then had the real smog test done and everything passed except the 25 mph portion of the test. It failed for high NO. What is interesting is when I first failed the first test, the 25 mph failed at the 666 meas. The new test failed at 888 ppm meas... higher than the original test. Why did the pretest and actual test produce different results and what would be your next steps for me to have checked? I am not ready to retire my vette for 1,000 unless that is my last resource. Thanks.


Your Vette failed for high NOx but what are CO levels like?

So a very important question to ask here first, is your Corvette in proper fuel control? NOx is created when an engine's combustion chamber temperatures exceed 2500F. Adequate fuel is required to keep combustion chamber temperatures low. A lean fuel condition (identified by low CO - partially burned fuel) can cause high NO. But it's not only lean fuel that raises NOx emissions. Any engine fault which causes engine combustion temps to exceed 2500F will do the trick. An overheating engine? Definitely high NOx failure. Even tiny hot spots due to carbon buildup on the internal side of the engine head can create just enough heat to increase NOx. Some common high NOx faults include vacuum leaks, fuel control problems, engine overheating, faulty EGR system, and /or carbon build-up (high engine compression).

Where should I start to look for the high NO problem?

As mentioned we'll need to find out if your Corvette is in proper fuel control. In order to control NOx your Corvette should be running 14.7/1 air/fuel ratio. Low CO can cause high NOx. An engine producing low CO is not necessarily a good thing. It looks great on the smog inspection report, but it doesn't necessarily mean your engine is running efficiently. On a late model vehicles, the situation is a bit different. These vehicles were designed to produce extremely low emission levels. 0 CO, 0 HC and 5-50 NO are not uncommon. The same does not apply to older cars and trucks. A very small amount of CO emissions is ok and helps keep NO levels low. Very small amount meaning no more than .01 to .07 percent CO. Above this and more than likely you've got low NO but high HC; which is also a smog check failure.

The two reasons vehicles fail the smog inspection for high NO most often are due to defective EGR systems and/or defective fuel control. Since having replaced your vehicle's EGR valve and solenoid, we're going to assume this system is functioning properly and recommend moving on to diagnosing the vehicle's fuel control system... basically making sure that there is adequate fuel being delivered to the combustion chambers (i.e. fuel injectors are not sticking, emission sensors working), and most importantly that there are no vacuum leaks (unmetered air). Unmetered air is simply air (oxygen) entering the intake manifold which is not being calculated for by the emission computer (ECU), and the computer not increasing fuel delivery correctly. If any air enters the combustion chambers which is not measured by the Mass Air Flow sensor, your Chevy's engine control computer will not know to add fuel to the air, thus distrupting the air/fuel mixture (leaning it out) and causing high NO.

Is my Corvette in proper fuel control?

To find out if your Corvette is adding the correct amount of fuel to the combustion chambers, a thorough diagnosis of the fuel injection “feedback” system will be required. This includes ensuring the oxygen sensors are working properly, the computer system is receiving the signals from the oxygen sensors, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery.

Along with the oxygen sensor(s) several other emission sensors need to also be inspected during the fuel feedback test such as the mass air flow sensor, manifold absolute pressure (MAP) sensor, engine coolant temperature sensor, and throttle position sensor. These sensors also play an important role in assisting the ECU with proper fuel control. At the end of the feedback test it will be know whether your Chevy Corvette is failing for high NO because of an electronic or mechanical fault, and what needs to be done to fix the problem.

Any idea what repairing this high NOx fault will cost me?

The cost of repairs can only be determined after the problem(s) is found. It could be something as simple as a defective oxygen sensor (approx $50.00 aftermarket + labor), to as expensive as needed new catalytic converter(s) which can run into the hundreds. The first step is getting the diagnosis. A smog repair station will usually require 2 hours of diagnostic time to figure out your Chevy Corvette's smog check failure cause. The cost for diagnosis will be calculated using their labor rate times the diagnosis hours needed or spent. Typical labor rates are $65.00 to $75.00 an hour.

posted by SmogTips Support 07-18-2017 05:23 PM
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