Emission Gases Explained Pt.1

Pt.2 / Pt.3 / Pt.4 / Pt.5

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

*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 an 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: Great caution must always be exercised when dealing with CO. Concentrations of less than one percent 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 CO readings. 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)

* 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).