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Chapter 4 –
Testing
U S I N G A N A L Y Z E R R E A D I N G S F O R D I A G N O S I S
This exhaust analyzer is a highly versatile test instrument. In addition to testing carbon monoxide (CO), carbon
dioxide (CO
2
), oxygen (O
2
), hydrocarbon (HC), and optionally oxides of nitrogen (NO
x
) for emission control
certification requirements or after a tune-up, it can be used to assist in detecting and locating ignition, fuel, exhaust,
emission control and engine service problems.
There are a few general facts to keep in mind when using the analyzer:
1.
High carbon monoxide (CO) readings usually indicate a fuel mixture richer than ideal. The amount of
CO in a vehicle's exhaust is directly related to its air-fuel ratio. High CO levels result from inadequate O
2
supply needed for more complete combustion. This is caused by a mixture that is too rich, - too much
fuel or not enough air.
2.
High hydrocarbon (HC) readings usually indicate excessive unburned fuel caused by lack of ignition or by
incomplete combustion. Common causes include a faulty ignition system, vacuum leaks, and fuel
mixture problems.
3.
High oxygen (O
2
) readings indicate too lean an air-fuel ratio.
4.
Low O
2
indicates a rich fuel mixture.
5.
High carbon dioxide (CO
2
) readings indicate a nearly ideal air-fuel ratio and efficient combustion.
6.
Low CO
2
readings indicate a fuel mixture either too rich or too lean, exhaust system leaks, or analyzer
sample dilution.
7.
The byproducts of combustion are dependent on the air-fuel ratio.
8.
O
2
combines with HC to form CO
2
and H
2
O.
9.
O
2
combines with CO to form CO
2
.
10.
CO is an indicator of richness.
11.
HC is an indicator of leanness and misfires.
12.
O
2
is a better indicator of leanness and misfires than HC.
13.
CO and O
2
are equal at the stoichiometric air-fuel ratio.
14.
O
2
and CO
2
are indicators of exhaust system integrity, sample hose and probe integrity, or both.
15.
CO
2
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.
16.
Air injection systems dilute the exhaust sample with O
2
.
17.
O
2
is essential for proper operation of the catalytic converter. Its concentrations are essentially
unchanged by the catalytic converter, providing a "window" through the catalytic converter to the engine.
O
2
levels are higher on vehicles with properly operating air injection systems.
18.
If CO goes up, O
2
goes down.
19.
If O
2
goes up, CO goes down.
20.
With the air injection system disabled and the CO above 1%, the catalytic converter is oxygen-starved.
Without O
2
, 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 needed.
