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Tune-up Service
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Any tune-up today should start with a battery of
performance checks to base line, or confirm the engine's overall condition. These
tests should include the following:
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Battery voltage (very important with all of today's onboard electronics).
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Charging voltage (condition of regulator)
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Power balance or dynamic compression (to identify any mechanical problems such
as leaky exhaust valves, worn rings, bad head gasket, bad cam, etc. that could
adversely affect compression and engine performance)
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Engine vacuum (to detect air leaks as well as exhaust restrictions)
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Operation of the fuel feedback control loop (to confirm that the system goes
into closed loop operation when the engine warms up)
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Scan for fault codes (to verify no fault codes are present, or to retrieve any
codes that may be present so they can be diagnosed and eliminated)
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Check exhaust emissions (this should be a must in any area that has an
emissions testing program to confirm the vehicle's ability to meet the
applicable clean air standards, and to detect gross fuel, ignition or emission
problems that require attention)
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Verify idle speed (should be checked even if computer controlled to detect
possible ISC motor problems); Idle mixture (older carbureted engines only, but
injector dwell can be checked on newer vehicles to confirm proper feedback
fuel control)
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Check ignition timing -- if possible (should be checked even if it is not
adjustable to detect possible computer or sensor problems)
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Operation of the EGR Valve.
The most overlooked component of today’s engines!
The Oxygen Sensor
Though some motorists don’t even know what an oxygen sensor is, let alone the engine
may have one or more of these services, the fact remains that sluggish 02 sensors cause a lot of
driveability problems. A recent EPA study found that 70% of all vehicles that
fail an I/M 240 emissions test need a new sensor.
An Oxygen sensor is a voltage generator. It is
constantly making a comparison between the Oxygen inside the exhaust manifold
and air outside the engine. If this comparison shows little or no Oxygen in the
exhaust manifold, a voltage is generated. The output of the sensor is usually
between 0 and 1.1 volts. All spark combustion engines need the proper air fuel
ratio to operate correctly. For gasoline this is 14.7 parts of air to one part
of fuel. When the engine has more fuel than needed, all available Oxygen is
consumed in the cylinder and gasses leaving through the exhaust contain almost
no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is
running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and
flows into the exhaust. In this case, the sensor voltage goes lower than 0.45
volts. Usually the output range seen is 0.2 to 0.9 volts.
The sensor does not begin to generate it's full
output until it reaches about 600 degrees F. Prior to this time the sensor is
not conductive. It is as if the circuit between the sensor and computer is not
complete. The mid point is about 0.45 volts. This is neither rich nor lean. A
fully warm O2 sensor will not spend any time at 0.45 volts. In many cars, the
computer sends out a bias voltage of 0.45 volts through the O2 sensor wire. If
the sensor is not warm, or if the circuit is not complete, the computer picks up
a steady 0.45 volts. Since the computer knows this is an "illegal" value, it
judges the sensor to not be ready. It remains in open loop operation, and uses
all sensors except the O2 to determine fuel delivery. Any time an engine is
operated in open loop, it runs somewhat rich and makes more exhaust emissions.
This translates into lost power, poor fuel economy and air pollution.
The O2 sensor is constantly in a state of
transition between high and low voltage. Manufacturers call this crossing of the
0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the
better the sensor and other parts of the computer control system are working. It
is important to remember that the O2 sensor is comparing the amount of Oxygen
inside and outside the engine. If the outside of the sensor should become
blocked, or coated with oil, sound insulation, undercoating or antifreeze,
(among other things), this comparison is not possible.
This makes the regular checking of the Oxygen
sensor output and cycle rate (number of cross counts) of great importance.
Keeping the sensor fresh may improve fuel economy as much as 10-15% (which can save
$100 each year in fuel costs on the
average). Keeping the sensor in good operating condition will also minimize
exhaust emissions, reduce the risk of costly damage to the catalytic converter,
and ensure peak engine performance (no surging or hesitating).
For these reasons, the O2 sensor should be
considered a "tune-up" replacement item just like spark plugs, especially on
older vehicles (those built before the mid-1990's).
Key Benefits
Increased fuel economy
- Other things that should be considered
when tuning up a vehicle.
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