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Emissions Analyzers: What is Really Going On Inside that Engine?

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An emissions analyzer is one of the most useful pieces of diagnostic equipment you can own. Scan tools are great for reading fault codes and sensor data, but a scan tool by itself can’t tell you what is actually going on inside the engine’s combustion chambers. A scan tool can read misfires and tell you which cylinder is misfiring, but it won’t tell you why a cylinder is misfiring. An emissions analyzer can. By revealing the exact composition of the exhaust gases that are coming out of the engine, you can see if the fuel mixture is running rich or lean, and if a misfire is ignition, compression or fuel related.

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Many technicians think an emissions analyzer is only useful for troubleshooting emission faults or for verifying a vehicle is in emissions compliance (not polluting). Truth is, an emissions analyzer is a great tool for diagnosing a wide variety of engine performance problems, even coolant leaks due to a blown head gasket. An emissions analyzer can also be used to check the operating efficiency of a catalytic converter, and to verify the operation of the upstream and downstream oxygen sensors — especially when the OBD II system has set codes for a lean fuel mixture, low oxygen sensor switching activity or a converter code. Such capabilities make an emissions analyzer a very useful tool for troubleshooting and repairing today’s vehicles.

Emission analyzers are available in a variety of configurations, including 4-gas and 5-gas units, and portable and stationary units. Four-gas units read hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2). Five-gas units read all of these plus nitric oxides (NOx).

How an Analyzer Works
When using an emissions analyzer, exhaust gas readings are obtained by inserting a small probe into the tailpipe while the engine is running. Exhaust is pulled into the analyzer by a small fan. Most units use an infrared light to check for HC, CO and CO2 in the exhaust. Depending on how the light is absorbed and diffused, the analyzer figures how much of each gas is present in the sample and displays a value for each gas. An electro-chemical sensor is used to measure the amount of O2 and NOx in the exhaust, and again a value is calculated for each and is displayed on the unit.

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Exhaust gas readings are continually updated about every five to 15 seconds depending on the response speed of the analyzer and the length of the hose (faster is better). Depending on the capabilities and features of the analyzer, it may also calculate and display the “Lambda” or air/fuel ratio of the engine and combustion efficiency.

To assure accurate readings, analyzers are calibrated using a standard calibration gas at the factory. This may have to be done periodically depending on the requirements of the unit. The equipment also requires a few minutes to warm up before it will generate accurate readings. Most units are also certified to meet California BAR 97 and European OIML Class O requirements (special standards for emissions test equipment accuracy).

Portable units usually weigh around 10 lbs. or less (some hand-held units only weigh about 5 lbs.), are 12-volt powered and come with a cigarette lighter power plug and cable, and internal battery pack. Stationary analyzers are usually 110-volt powered.

Many exhaust analyzers have a serial or USB port, or a wireless card that allows the analyzer to interface with a laptop or desktop PC and/or printer. Special software is usually included for printing reports, and storing and displaying data. Some units may also interface with a high-end professional-level scan tool or engine analyzer to display exhaust gas readings on that equipment.

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Exhaust analyzers do require some maintenance. The main filter and water filter have to be replaced every few months (depending on use), and the life of the O2/NO chemical sensor is typically 12 to 18 months.

Using an Analyzer
By comparing the various exhaust gas readings, you can get a pretty clear picture of what is actually going on inside an engine. This can be done in the service bay with a stationary analyzer, or on the road with a portable analyzer. One gas reading alone usually does not provide enough information for a complete diagnosis. But when all four or five gas readings are tabulated and compared, an experienced technician can usually see what’s happening and make a very accurate diagnosis.

There is a bit of a learning curve for first-time users of this type of equipment, but exhaust analysis is no harder to learn than a scan tool or a digital storage oscilloscope. Most exhaust analyzers come with a good instruction manual, and some equipment suppliers also provide training, either on-site or online.

What the Readings Mean
HC is an indicator of misfiring — usually ignition or compression related. HC concentrations are displayed in parts per million (ppm) or a percentage (%). Normally, HC readings should be very low (less than 50 ppm on most late-model engines). If the HC reading in the exhaust is high, it may indicate a fouled spark plug, bad plug wire, a bad coil on a coil-on-plug or distributorless ignition system, or a burned exhaust valve that is leaking compression and unburned fuel into the exhaust. One misfiring spark plug, for example, can raise HC readings to the 1,600 to 2,000 ppm range!

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CO is a rich indicator. CO is usually displayed in percent and should be very low (less than a few tenths of a percent) unless the fuel mixture is running very rich. If the fuel system is running lean, the level of CO produced remains relatively flat above 15:1 air/fuel mixtures, but below about 14.4:1, the level of CO rises dramatically as the fuel mixture becomes richer.

If high CO levels are noted, the engine is starved for oxygen. On an older carbureted engine, this could be due to a rich idle adjustment, a high float level (or heavy carburetor float that has sunk in the bowl), or a leaky needle valve or power valve in the carburetor. It could also be caused by a dirty air filter, clogged air bleeds in the carburetor, or a sticky or misadjusted choke. On a fuel injected engine, high CO readings may indicate a leaky cold start injector, leaky injectors or excessive fuel pressure due to a defective fuel pressure regulator. High CO can also be caused by a faulty oxygen sensor or coolant sensor.

CO2 is not a smog-causing pollutant, but is used to indicate combustion efficiency. CO2 is displayed in percent, and the readings are highest when the air/fuel mixture is at the ideal stoichiometric ratio. On either side of 14.7 to 1, the level of CO2 in the exhaust drops sharply. The higher the CO2 reading, therefore, the closer the air/fuel mixture is to the ideal ratio and the more completely the fuel is being burned. Good CO2 readings are typically in the 13 to 15% range.

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You can use the CO2 reading to determine if a problem is fuel, ignition or compression related by creating a momentary rich or lean condition and noting the change in the CO2 reading. If CO2 is low because the fuel mixture is rich, the level should drop even more when the mixture is made richer by feeding some propane into the engine. If the fuel mixture is rich and you create a lean condition by pulling off a vacuum hose, CO2 should rise until the engine starts to lean misfire. If artificially enriching or leaning the mixture fails to change CO2 at all, the problem is not fuel related, but is due to ignition misfire or a compression leak.

O2 is displayed as a percentage and is used as a lean indicator. Good O2 readings are typically in the 1 to 2% range. Oxygen levels in the exhaust change dramatically when the air/fuel mixture goes lean. Rich air/fuel mixtures produce very low oxygen readings and high CO readings. Lean air/fuel mixtures, on the other hand, show high O2 readings and low CO readings. Unusually high O2 readings in the exhaust often indicate a vacuum leak (vacuum hose, intake manifold gasket, PCV system, etc.) or an exhaust system leak.

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NOx is displayed in parts per million (ppm) and is checked to assure a vehicle is not emitting excessive levels of NOx. Nitrous oxide readings are low at idle, but increase dramatically when the engine is lugging under load or accelerating. NOx is usually read with the vehicle running on a dyno or road simulator, or during a road test with a portable unit. If NOx readings are high, it usually indicates a problem with the EGR system.

Other Uses
An emissions analyzer can also be used to sniff for hydrocarbons in the coolant by holding the probe close to an open radiator cap. You don’t want to suck any liquid into the probe, just some air. If the analyzer detects any HC, it means the head gasket is probably leaking. Other tests that can also be done with an emissions analyzer include checking the operation of the EGR valve, PCV valve, choke and idle mixture adjustments on older engines with carburetors, checking for fuel vapor leaks from the EVAP system, and more.

One of the best uses for an emissions analyzer, of course, is to troubleshoot emission problems on vehicles that have failed an emissions test. Most inspection forms show the exhaust readings and which gases were over the limit. You can then use your own analyzer readings to double-check the test results and/or to do your own analysis. The most important part is using the analyzer after repairs have been made to verify the exhaust is clean and meets the test requirements. This step alone will greatly improve the odds of the vehicle successfully passing a retest, and reduce comebacks because the repairs you made failed to reduce emissions. Cap Portable analyzers can be taken outside of the bay for “on the road” analysis. Photo courtesy of AutoLogic, www.autologicco.com.

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