BMW 1989-1995 525i Service Manual page 54

100-12
ENGINE-GENERAL
Preventive Maintenance
The condition of the fuel. ignition and emission control sys-
tem components has a direct effect on engine performa nce
and driveability. BMW specifies maintenance of certain parts
at regular intervals to keep the engine in proper tune. Extend-
ing maintenance intervals beyond the time or mileage recom-
mended by the manufacturer can adversely affect the way the
engine runs. When troubleshooting driveability problems. a
good starting point is to perform a major engine service, par-
ticularly if one is overdue. For maintenance schedu les. major
engine service and other driveability-related maintenance
procedures, see 020 Maintenance Program.
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Basic Engine Settings
Idle speed. idle mixture (%CO), and ignition timing are un-
adjustable on the DME systems covered by this manual. The
adaptive DME system is designed to automatically compen-
sate for changes in engine operating condit ions. although the
adaptive range is limited. Once these limits are exceeded,
driveability problems usually become noticeable .
NOTE-
If the DME adaptive limits are exceeded. the Check En-
gine light will come on. indicating an emission related
fault. For Check Engine tight diagnostics and on-board
diagnostic fault codes. see On-board Diagnostics giv-
en above.
Oxygen Sensor
A high oxyge n level in the engine exhaust indicates a lean
air-fuel mixture and a low oxygen level indicates a rich mix-
ture. The oxygen sensor, shown in Fig. 8, measures the oxy-
gen content in the exhaust gas and gene rates a variable
vollage signal. Using that feedback signal as an input. the
DME control module fine tunes the air-fuel mixture.
NOTE-
The oxygen sensor signal is ignored unlil /he engine
reaches
a
specified minimum temperetute. Therefore.
when troubleshooting cold engine driveability prob -
lems. the oxygen sensor can be ruled out as a possible
cause.
As the oxygen sensor ages. its ability to react quickly to
changing conditions deteriorates , and it may eventually cease
to produce any signal at all. To check the operation of the oxy-
gen sensor, see 130 Fuellnjection-DME.
Air Flow Measurement and Vacuum Leaks
The primary input used by the fuel injection system to deter-
mine how much fuel should be delivered is the signal from the
air flow meter (M20/M30 eng ines) or mass air flow sensor
(M50/M60 engines) in the intake air ductwork.
DRIVEABIL/7Y TROUBLESHOOTING
Fig. 8. Oxygen sensor measures oxygen content of exhaust gas as
indicator of air-fuel ratio and combustion efficiency.
Because proper fuel metering depends on accurate mea-
surement of the incoming air, any unmeasured air that enters
the engine downstream of the air mass or air flow meter will
cause a lean air-fuel mixture and possibly affect driveability.
There are many places for unmeasured air to enter the en-
gine. First, inspect all hoses , fittings, ducts , seals. and gas-
kets in the intake air tract for cracks or looseness. II may be
necessary to remove parts that cannot be fully checked in
their installed positions. Also make sure all of the intake man-
ifold and throttle body mounting nuts are tight.
In addition to air leaks. air restrictions can also cause drive-
ability problems. Remove the air filter and hold it up to a strong
light source. If the filter does not pass light, it is restricted and
should be replaced. Also, remove the intake air ductwork and
check for oily depos its at the throttle plate. Buildup in this area
can cause an erratic idle. Clean away any deposits using a
cloth welled with carburetor cleane r and reinstall the duct-
work.
CAUTION-
Do not spray carburetor cleaner directly into the
throttle area. Spraying cleaner may force it into
the throttle position switch or sensor and cause
damage.
Battery Voltage
All of the electrical components in the fuel injection and igni-
tion systems require a minimum voltage to operate properly.
When troubleshooting engine driveability problems, one of
the first checks should be to make sure the batt ery is fully
charged and capable of delivering all its power to the electrical
system.
To make a quick check of battery charge, measure the volt-
age across the battery terminals with all cables attached and
the ignition off. A fully charged battery will measure 12.6 volts
or slightly more. compared to 12.15 volts for a battery with a
25% charge .