1991 OPEL ASTRA Electronic Workshop Manual

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Diagnostics using a 4 Gas Analyser
Before attempting to perform any diagnostics using a 4 gas analyser, it is important to
understand the equipment and the results obtained.
We shall begin with explaining the different gases tested :
Hydrocarbons (HC)Hydrocarbons are unburned or partially burnt fuel particles. High levels of HC,
measured in PPM (parts per million) in the exhaust gases, is usually due to
malfunctions in the ignition or fuel systems.
Carbon Monoxide (CO)Carbon monoxide is formed when there is insufficient oxygen present during
combustion and is measured as a percentage of exhaust gases. (CO - one part carbon
to one part oxygen) The most common cause of high CO is incorrect adjustment of
fuel mixtures. Refer to the relevant manufacturers manuals for correct setting
procedures.
Carbon Dioxide CO2CO2 - one part carbon to two parts oxygen. The greater the percentage of CO2 in the
exhaust gases, the more efficient the running of the engine, ideal specification for a 4
cylinder engine is between 13 and 16 %. If the CO2 level dropped too low, this would
indicate an incorrect air fuel mixture either caused by malfunctions in the
management system or incorrect adjustments.
Oxygen (O2)Oxygen is measured as a percentage of the exhaust gases and should be between 1 -
1.5% indicating an efficient combustion and air fuel mixture. Whilst adjustments are
being performed, it is important to note the changes in O2, as a sharp change of
between 0.5 - 1.0 % and 1.0 - 0.5 % will indicate the cross-over point from a rich or
lean mixture respectively. O2 measurement is also useful in detecting leaks and
misfiring as well as the efficiency of combustion.
Oxides of Nitrogen (Nox)Engines use O2 in the combustion process and exhaust Nitrogen (N2). This exhausted
N2 is exposed to high temperatures during the combustion process and is converted to
Oxides of Nitrogen (NOx) when that temperature exceeds approx. 1370 degrees
Celsius. Although NOx compounds do not directly affect the efficiency of an engine,
they are responsible for smog when combined with HC. Part of the solution to
minimise NOx is the catalytic converter and the exhaust gas recirculation process.

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HC and CO Analysis
HydrocarbonsThe ignition of the air : fuel mixture in the combustion chamber does not result in all
the fuel being burned, hence the HC emmission. Should there be any malfunction in
the system the quantity of Hc will increase.
Normal HC readings range between 0 – 350 PPM, depending on the age and
condition of the vehicle. Low HC readings indicate that most of the fuel is being
burned, and high HC readings indicate a higher quantity of unburned fuel.
Carbon MonoxideAs the air : fuel mixture is ignited, the resulting combustion process produces some
CO as a normal condition. However, if the air : fuel mixture is incorrect, there will be
a larger amount of HC, which will increase the CO, resulting in air pollution and
reduced engine efficiency. Normal idle CO for vehicles with the Motronic 1.5.4systems is 1 – 1.5 %. Idle CO can vary depending on the management system and can
range from 0 to 3 %

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Carbon DioxideCarbon dioxide is a product of combustion. A normal functioning engine should
produce between 13 and 16 % of CO2. If combustion was incomplete or the air : fuel
ratio was incorrect, the quantity of CO2 produced will be minimised resulting in a less
efficient engine. Any dilution of the exhaust gases (secondary air pumps, etc.) will
result in a drop in CO2 content.
OxygenThe engine inducts approx. 20.7% oxygen, 78% Nitrogen and 0.5 – 2.0% other gases.
During combustion, most of the oxygen is consumed, and if the engine is runningcorrectly, 0.5 – 2.0% is exhausted. Any more than the 2% oxygen exhausted is
valuable diagnostic information to the technician.Low O2 levels indicate a rich mixture. (A rich mixture will consume more or all of
the available oxygen during combustion). 0% oxygen indicates that all oxygen was
consumed during combustion, resulting in unburned fuel.High O2 levels indicate an incomplete combustion process. (Not enough oxygen was
utilised during combustion, resulting in unburned fuel)

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ConditionPossible Cause
(Conventional and Electronic Management Systems)

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Multi PointSimultaneousInjectors receive pulses from E.C.U. twice per engine cycle.
( Corsa 13 NE, 16NE, Astra 14SE, 16SE, 18XE, 20SHE )
SequentialInjectors are pulsed by E.C.U. once per engine cycle as per spark
plug firing order. ( Astra 20XE )
GroupedInjectors pulsed in groups or banks, once or twice per cycle
depending on system design
Single PointThrottle BodySingle injector mounted on throttle body pulsed as per engine
requirement ( Astra 160 TBI )
Operation
Injectors on the Bosch Motronic engine management systems receive battery voltage and are
triggered by the E.C.U. depending on the signals received from the various sensors attached
to the engine. Fuel pressure is constant (M.P.F.I. – 250 to 300 Kpa).
Injector opening frequency (Hz) and duration (pulse width) determine the amount of fuel
being delivered to the combustion chamber. Variations in pulse width and frequency ensure
that adequate fuel is supplied for efficient engine operation.LONGER PULSE WIDTH= RICHER MIXTURESHORTER PULSE WIDTH= LEANER MIXTURE
Pulse rate and width will increase with increased engine rpm and load.
Duty Cycle
Duty cycle is the combination of pulse width and pulse rate, and is expressed as a percentage
of time “ON” and time “OFF” eg. If an injector was on for 10ms and closed for 90ms, the
duty cycle “ON” is 10% and the duty cycle “OFF” is 90%. The total on and off duty cycle
reading should be 100%.
CrankingInjector duration is longer during cranking (richer mixture) for easier startingand to promote warm-up.
IdlingInjector duration is longer at start-up if engine is cold, and graduallydecreases as engine warms up.
AccelerationInjector duration is longer at initial acceleration to prevent “flat spot”RunningInjection pulse is controlled by the E.C.U. depending on the signals receivedfrom the various engine sensors.

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Typical Simultaneous Injection Duration
Engine ColdMs
Cranking ------------------------------------------>4
Idle -------------------------------------------3.5 to 4
Engine Warm
Idle---------------------------------------------2.5
1000rpm2.3*
20002.1*
30002.0*
Snap Throttle>7Sequential injection systems will be less than twice the above values due to single pulsingper engine cycle.
NOTES.THE USE OF A 4 GAS ANALYSER CAN GREATLY AID DIAGNOSTICDIRECTION AND TIME.
Comparing mixture content to injector pulse width can identify components and systems to
be tested in a logical sequence.
Example :A rich mixture could indicate high fuel pressure, incorrect manifold pressure,
incorrect coolant temp, etc,etc.
D.N. 12/97* Gradually leans out,depending on load