GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 83)

7-10
UNDERSTANDING RESULTS
A gas detector measures the content of HC,
CO, CO2, O2 and NOx in the exhaust
stream. If the engine is running efficiently, the
fuel and oxygen will have all combined,
leaving little unused oxygen and negligible
amounts of hydrocarbons. There should be
almost no carbon monoxide and a lot of
carbon dioxide. Typical readings for a car
with a healthy engine might be:
Hydrocarbons (HC) 0-50 ppm
Carbon Monoxide (CO) 0.0 - 0.5%
Carbon Dioxide (CO2) 14.0-15.5%
Oxygen (O2) 1.0 - 2.0%
Oxides of Nitrogen (NOx) See Note
Note: NOx varies significantly with engine
loading, so typical readings are not shown.
These are only sample readings. State and
Federal emission figures should be used
when available, and they will differ by type
and age of vehicle, whether or not it was
manufactured with a catalytic converter, and
according to the geographic area in which
the vehicle is tested.
LEAN MIXTURES
When the mixture is lean, there is plenty of
oxygen to combine with the fuel, so there will
be very little CO. Since there is a surplus
of oxygen, O2 will be high. CO2 will be
low because combustion is inefficient. If the
mixture is lean enough to cause a misfire,
unburned fuel will pass through the engine
resulting in high HC.
A lean mix raises operating temperatures
which will result in high NOx and will increase
the chance of detonation.
RICH MIXTURES
If the air/fuel mixture is rich (too much gas,
not enough air), there will not be enough
oxygen to support complete combustion.
Carbon monoxide (CO) molecules form, in
place of some of the CO2, and some fuel
remains unburned (HC). The resultant
exhaust stream will contain too much CO
and HC, lower than normal CO2 and very
little oxygen, as it was all used up burning
all that fuel.
A rich air/fuel mixture may result in fouled
plugs and poor fuel economy, and can be
caused by dirty air filters, leaking fuel
injectors, or other failures that may drive the
mixture rich. In extreme cases, there may be
black smoke, a rough idle, and hesitation.
MULTIPLE GAS RESULTS
Recognizing what combined gas conditions
mean can be used to determine engine
problems. The following assume that the air
injection to the catalytic converter has been
disabled for testing.
High HC combined with High CO indicates
a rich misfire condition. Not enough air is
present to burn all of the fuel completely.
Possible causes are:
• Fuel delivery problems:
Carburetor
Injectors
High fuel pressure
• Problems in the air intake system
• PCV system
• Evaporative canister system
• Vacuum leaks
• Poor valve adjustment