|
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.
Interro's four gas analyzer measures HC, CO, CO2 and
O2 concentrations. The five gas analyzer adds the
measurement of NOx as well.
Interro'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
*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).
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 degrees 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
* 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 an
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.
|
