Emissions, clean air = no power?

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There seems to be a lot of misinformation about emissions and state inspection laws in regards to performance cars. First, let me say you will not read this and instantly become I/M 240 certified to repair emission problems but you may have a better understanding of what comes out your tailpipe and what you need to do in order to pass the test. I’ve found most people will try harder to get out of the test than try to pass it. If one understand the contents of the exhaust gases and what tuning is required to change them then one can be on his way to getting a clean bill of health at the test station.

In Illinois, we have an I/M240 inspection. If your state has a dyno this is most likely the test you get. 96 and newer cars get a OBD-II test, your car runs and your check engine light is not on-you pass (provided you haven’t disconnected the battery lately. 81 to 95 cars get the dyno, 68 to 80 and awd get an idle test. Idle test are fairly easy to pass so this article will focus on the dyno test and some misconceptions about it.

The I/M240 test consists of a city/highway drive cycle. Your car is accelerated to about 30mph then stopped and accelerated again to 30 then up to 59mph and stopped. The test last 4 minutes total. (Hence the 240 seconds) If at any time your car meets certain “clean” standards the test will abort early and you pass. During the test hydrocarbons (HC) and carbon monoxide (CO) are tested. Oxides of Nitrogen (nox) are measured but you will not fail for them. (California may be different). Now that we know how the test is done let’s look at some of the gasses and what they tell us.

HC-Hydrocarbons are raw gasoline. If your vehicle has a high reading for hydrocarbons raw gas is getting through the engine unburnt. This could be due to a misfire including a lean misfire or an extremely rich mixture. Anything that lets gas go through without getting burnt will cause high HC readings. (Also note that due to the way the station sucks the sample from near the exhaust pipe and not in the pipe, I have seen leaking filler necks fail for HC’s) A good running engine with a properly functioning catalytic converter should emit less than 50 ppm of HC from the tailpipe.

CO-Carbon Monoxide is partially burnt fuel. CO is formed when hydrocarbons are not completely combined with oxygen during the combustion process. High Co readings are always a rich indicator never a lean indicator. If you fail for CO you’re running to rich. Readings of below 1.2% after cat are normal.

NOx-Oxides of Nitrogen form when the combustion chamber temp exceeds approx 2500 degrees F. High NOx readings rarely accompany a rich mixture, NOx should be considered as an indicator of high combustion chamber temps, that commonly accompany high engine loads. Insufficient EGR flow and over advanced ignition timing are two common causes of high Nox emissions.

O2-Oxygen is always present in the exhaust stream. How much depends on many factors including catalyst efficiency or air injection systems. High concentration can be an indication of misfire, lean mixture or exhaust leaks. After cat readings without air injection are normally under 1%.

CO2-Carbon Dioxide is a measure of combustion and catalyst efficiency. CO2 exhaust concentrations increase with increased combustion and catalyst efficiency. A good running engine with a properly functioning catalytic converter produces CO2 concentrations of 15% or more.

So what does this all mean? Let’s look at some examples.

HC: 850 ppm
CO: .01%
CO2: 13.65%
O2: 2.37%

These readings will fail for HC in any state. The high HC indicates we have raw fuel coming out the tailpipe but the low CO reading indicates we are not running rich. Moving on to the CO2, our readings are somewhat low (remember 15% or more) so efficiency is down. The O2 reading is high meaning we also have oxygen getting through the engine unburnt. Combining fuel and oxygen both coming out unburnt and most likely you have a cylinder that has misfired. Going one step further, we can use a Lambda calculation (note 1) to determine the A/F ratio of the engine at 1.07 Lambda or about 15.7:1. A little on the lean side but not enough to cause a lean misfire so the most likely cause would be a secondary ignition failure.

HC: 330 ppm
CO: 8.49%
CO2: 9.93%
O2: .15%

In this example the vehicle failed for HC and CO. HC and CO readings tell us that it is running extremely rich. Lambda calculation at .77(11.3 A/F) The high CO was formed as there was not enough oxygen to completely burn the mixture and when we ran out of oxygen the rest came out as pure hydrocarbons. CO2 (efficiency) is low as a result as is the O2 reading. These reading are from a vehicle with a bad coolant temp sensor that was reading low temperatures, enabling warm up enrichment.

HC: 72 ppm
CO: .16%
CO2: 15.24%
O2: .86%

This example is from a passing vehicle, so everything is good right? Well, if we do a Lambda calculation we find that in reality we are running at 1.03 (15.1:1 A/F) slightly lean. I can also see that elevated HC and CO readings combined with some O2 left in the sample indicates that the catalytic converter is not using all available oxygen to burn the remaining HC and CO up. A good converter is 98% efficient at removing HC and CO so if you see any oxygen left it could have been used to burn the remaining HC and CO. While these readings are within acceptable limits, over time the elevated heat from the lean condition will degrade the catalyst to a point of not working.

If properly repaired (If the ECM calculations are off at part throttle emission they are also off under WOT conditions when we want the maximum power we can get.) first you would need to find the problem with the lean condition. MAF dirty, small vacuum leak, or maybe a degraded O2 sensor. Then replace the catalytic converter. I would then expect to see readings more like this

HC: 20 ppm or less
Co: .02% or less
CO2: 15.3% or more
O2: .02% or less

While this might seem nit picking, in the racing world, while your car maybe running good your competitor might have his running perfect. All professional race teams pay attention to every detail like this.

In relation to our performance cars, the emission test only simulates daily driving conditions. During the test they will not get into boost on a turbocharged car or enter P.E. mode on your N.A. car. This should allow us to pass the emission test and still have all the power we require at WOT.


(1) In the above examples I referred to Lambda calculation. This is a formula that given the readings from the four gases mentioned can give an accurate A/F ratio that the engine is operating at. Lambda calculations are accurate whether you have complete combustion or no combustion at all.

The formula is.

hydrocarbons in ppm will have to be converted to percentage by multiplying by .0001

Hcv = atomic ratio of hydrogen to carbon in the fuel. This ratio is approximately 1.8 for gasoline.

Ocv = atomic ratio of oxygen to carbon in the fuel. This should be approximately 0, except for oxygenated fuels, where it’s approximately .017.

K1 = conversion factor from flame ionization detection (FID) to nondispersive infrared analyzer (NDIR). For gasoline, K1 = 6.0.

Plugging in our numbers from the first example gives us


Article by LowePerf