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Using Gas Analyzer Readings for Engine Diagnosis

General Background

Concentration of combustion products in the vehicle's exhaust, most of which pollute the air, give important diagnostic clues to the vehicle's engine efficiency. The component gases which contribute the most to air pollution are hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). Three of the five gases measured at the tailpipe are regulated pollutants - HC, CO and NOx. The remaining gases, oxygen (O₂) and carbon dioxide (CO₂), while non-regulated, play a significant role as diagnostic aids. Omitec's four gas analyzer measures HC, CO, CO₂ and O₂ concentrations. The five gas analyzer adds the measurement of NOx as well.

Omitec's exhaust gas analyzer is a highly versatile and accurate test instrument. In addition to testing carbon monoxide (CO), carbon dioxide (CO₂), oxygen (O₂), hydrocarbons (HC) and oxides of nitrogen (NOx) (for the 5-gas version) for repair requirements or after a tune-up, it can be used to assist in detecting and locating, fuel, exhaust, emission control and engine service problems.

Following are a few general facts and tips to keep in mind when using the gas analyzer:

1. High Carbon Monoxide (CO) readings usually indicate a fuel mixture richer than ideal (rich mixture - air fuel ratio below 14.7). In general CO is an indicator of combustion efficiency. The amount of CO in a vehicle’s exhaust is directly related to its air-fuel ratio. High CO levels result from inadequate O₂ supply needed for complete combustion. This is caused by a too rich mixture - too much fuel or not enough air (AFR readings below the optimal 14.7, Lambda below 1.0). Circumstances that can lead to high CO emissions:
• Low idle speed
• Improper float settings in carbureted vehicles
• Dirty or restricted air filters
• Excessively dirty or contaminated oil
• Saturated charcoal canister
• Non-functioning PCV valve system
• Improper operation of the fuel delivery system
• Improperly functioning thermactor system

NOTE 1.Catalytic converter intervention and CO concentrations
High CO readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but a CO reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. Low range CO readings are possible, and not uncommon, from a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter CO levels will be masked by the catalytic converter and the potential for an CO problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR/Lambda readings.




NOTE 2.: Great caution must always be exercised when dealing with CO. Concentrations of <1% can be lethal. Testing should always be done in a well ventilated area.



2. Normal CO readings. If the combustion process is succeeding at or near the stoichiometric point (AFR equals 14.7, Lambda equals 1.0), CO levels during an idle test will typically measure less than 2%.
3. Low COreadings. There is, effectively, no reading for CO that can be characterized as too low or "below optimal". CO concentrations will appear "normal" even in a lean burning environment, where AFR is above 14.7 (Lambda is above 1.0).
4. High hydrocarbon (HC) readings usually indicate excessive unburned fuel caused by a lack of ignition or by incomplete combustion. Concentrations are measured in parts per million (PPM). Common causes include a faulty ignition system, vacuum leaks, and fuel mixture problems. Circumstances that can lead to a high HC emissions are:
• Incomplete combustion due to fouled spark plugs.
• Improper timing or dwell
• Damaged ignition wires
• Poor compression
• Vacuum leak
• Ineffective or faulty air management system (ECM control of air/fuel ratios)

NOTE. Catalytic converter intervention and HC concentrations
High HC readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but an HC reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. HC readings at or near "normal" are possible, and not uncommon. From a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter HC levels will be masked by the catalytic converter and the potential for an HC problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR/Lambda readings.

5. Oxygen (O₂) readings. Oxygen, measured as a percentage of the exhaust volume, reflects the amount of gas remaining in the exhaust sample after the combustion process has taken place. Ambient O₂ readings should be about 20%, reflecting the natural amount oxygen found in the air. The ideal range for vehicles without a secondary air injection system is less than 1.5%. If there is an air injection system, O₂ levels will typically fall n the range of 3% to 4%. Pinching off the air hose of a vehicle equipped with air injection should produce O₂ levels similar to those found for vehicles without air injection.
6. High oxygen (O₂) readings indicate too lean an air-fuel ratio (AFR higher than 14.7, Lambda greater than 1.0). Circumstances that can lead to high O₂ emissions are:
• Lean fuel mixture (AFR above 14.7)
• Vacuum leaks
• Ignition related problems causing misfires.
7. Low O₂ indicates a rich fuel mixture (AFR below 14.7, Lambda below 1.0).
8. High carbon dioxide (CO₂) readings indicate a nearly ideal air-fuel ratio and efficient combustion
9. Low carbon dioxide (CO₂) readings indicate a fuel mixture either too rich or too lean, exhaust system leaks, or sample dilution.
10. Oxides of Nitrogen readings. Oxides of nitrogen (NOx), including nitric oxide (NO) and nitrous oxide (NO₂), are formed if the combustion temperatures within the combustion chamber exceed some 2,500°F. This can occur when the engine is under load. When excessive temperature conditions exist, the greatest amount of NOx is typically produced at the stoichiometric point (AFR 14.7 or Lambda of 1.0) as the engine is under a light load. If the combustion process within an engine is burning fuel at or near stoichiometric point, NOx levels on acceleration will typically read significantly higher than those measured at cruise and during deceleration. Typically, the NOx readings at idle will be 0 PPM.
11. High NOx Readings. Circumstances that can lead to abnormally high NOx emissions are:
• Malfunctioning EGR valve
• Lean fuel mixture (AFR above 14.7, Lambda above 1.0)
• Improper spark advance
• Thermostatic air heater stuck in the heated air position
• Missing or damaged cold air duct
• Combustion chamber deposits
• Malfunctioning catalytic converter

NOTE. Catalytic converter intervention and NOx concentrations
High NOx readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but a NOx reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. NOx readings at or near "normal" are possible, and not uncommon. From a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter NOx levels will be masked by the catalytic converter and the potential for an NOx problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR/Lambda readings.

12. Low NOx readings. There is, effectively, no reading for NOx that can be characterized as too low or below optimal. NOx is naturally 0 ppm at idle. NOx concentrations may appear normal even in a rich burning environment where the AFR is well below 14.7 (Lambda below 1.0).

Facts to Remember

The byproducts of combustion are dependent on the air-fuel ratio.

13. O₂ combines with HC to form CO₂ and H₂O.
14. O₂ combines with CO to form CO₂.
15. CO is an indicator of air-fuel mixture richness.
16. HC is an indicator of fuel mixture leanness (or richness) and misfires.
17. CO and O₂ are equal at the stoichiometric air-fuel ratio.
18. O₂ and CO₂ are indicators of exhaust system integrity, sample hose and probe integrity, or both.
19. CO₂ is an indicator of combustion efficiency that peaks at or near the stoichiometric air-fuel ratios, and decreases with lean or rich air-fuel ratio.
20. Air injection systems dilute the exhaust sample with O₂.
21. O₂ is essential for proper operation of the catalytic converter. Its concentrations are essentially unchanged by the catalytic converter, providing a "window" through the converter to the engine. O₂ levels are higher on vehicles with properly operating air injection systems.
22. If CO goes up, O₂ goes down (inversely related)
23. If O₂ goes up, CO goes down (inversely related)
24. With the air injection system disabled and the CO above 1%, the catalytic converter is oxygen-starved. Without O₂, it does not fire, allowing exhaust concentrations to be more like readings taken ahead of the converter.

If readings are within the manufacturer’s or local/state/federal allowable limits, it can generally be assumed that the fuel, ignition, and emission control systems are functioning properly. If they exceed the limits, repairs or adjustments are probably called for.

The chart below lists some of the kinds of problems that could result in abnormal gas readings.

(Legend: L = Low, H = High , M = Moderate)

The following table lists some of the results possible when the AFR (air-fuel ratio) is sustained at conditions ranging from too lean to too rich.