BMW 3 (E46). Manual — part 76
dimensional map similar to the one
shown is digitally stored in the ECM.
The initial ignition point is determined
by the crankshaft speed sensor during
cranking. Once the engine is running,
timing is continually adjusted based on
operating conditions.
Basic troubleshooting
principles
Poor driveability may have a variety of
causes. The fault may lie with the
ignition system, the fuel system, parts
of the emission control system, or a
combination of the three. Because of
these interrelated functions and their
effects on each other, it is often difficult
to know where to begin looking for
problems.
For this reason, effective
troubleshooting should always begin
with an interrogation of the On-Board
Diagnostics (OBD II) system. The OBD
II system detects engine management
malfunctions. When faults are
detected, the OBD II system stores a
Diagnostic Trouble Code (DTC) in the
Engine Control Module (ECM) along
with other pertinent fault information. In
addition, the Malfunction Indicator Light
(MIL) will come on if an emissions
related fault is detected. Additional
OBD II information, including a DTC
lookup table can be found in the back
of this manual under
OBD On Board
Diagnostics
.
Note:
On model year 1999 and 2000 cars, the
MIL is labeled Check Engine. On model
year 2001 cars, the MIL is labeled
Service Engine Soon.
An engine that starts and runs
indicates that the ignition system is
fundamentally working-delivering
voltage to the spark plugs. A
hard-starting or poor-running engine,
however, may indicate a problem with
how well the spark is delivered. Faulty
or worn ignition components, such as
old spark plugs, or an individual coil
problem may be the cause.
WARNING!
Inefficient combustion (rich air/fuel
mixture) can cause elevated
hydrocarbon exhaust emissions and
catalytic converter damage. For this
reason, if a severe misfire is
detected, the fuel injector will be
turned off to the specific cylinder
and the MIL illuminated.
One way to diagnose a faulty coil on a
car that is running is to use an
oscilloscope to analyze spark quality.
The illustrations show normal scope
trace of spark at idle.
Table b
lists common ignition coil
voltage faults and related causes.
Note:
Note the length of sparking period and
the up-down voltage attenuations. A
short sparking period and/or an
irregular/low voltage attenuation usually
indicates a faulty coil.
A normal oscilloscope pattern for a six
cylinder engine.
1 - Start of ignition voltage peak
2 - Level of ignition voltage
3 - Level of combustion voltage
4 - Period of combustion
5 - Combustion curve characteristics
6 - Start of decay process
7 - Termination oscillations
Note:
See
Ignition coil assembly, testing
and replacing
as described later.
Table b. Ignition trace diagnostics
Parameter
Ignition
voltage
low
Ignition
voltage
high
Spark plug
electrode
gap
too small
too big
Spark plug
electrode
condition
-
worn/burnt
Spark plug
electrode
temperature
too high
too low
Engine
compression
too low
too high
Spark plug
wire(s)
-
faulty
Fuel air
mixture
-
too lean
Misfire detection
Engine misfire, the result of inefficient
combustion in one or more cylinders,
may be caused by a variety of
malfunctions in various subsystems.
The OBD II system incorporated into
the MS 42.0 and MS 43.0 engine
management systems is designed to
detect and warn of misfire faults during
engine operation. Misfire fault codes
are listed in
Table c. Misfire fault
codes
. For a detailed list of fault codes
see
OBD On Board Diagnostics
.
Once engine misfire has been detected
and the fault codes downloaded from
the ECM, it is necessary to follow a
diagnostic path to correct the problem.
The ignition subset of the diagnostic
path is shown in
Table d. Ignition
system problems
Table c. Misfire fault codes
P code
BMW
fault
code
Fault code
Possible cause(s)
P0301
238
Misfire cylinder 1
Ignition fault: Defective or incorrect spark plug(s)
or connector(s); defective ignition coil(s)
Mechanical fault: Crankshaft pulse wheel loose
or damaged; damaged piston(s), valve(s) or
camshaft(s); oil pressure low
Catalyst fault: Excessive exhaust backpressure
due to restricted catalyst
Fuel supply fault: Contaminated fuel; fuel
delivery fault (blocked filter, low fuel in tank,
defective fuel pump); running loss system fault;
leaking or blocked fuel injector(s); fuel pressure
regulator fault; evaporative system fault
ECM/sensor input fault: Implausible sensor
signal; oxygen sensor(s) faulty; ECM faulty
P0302
239
Misfire cylinder 2
P0303
240
Misfire cylinder 3
P0304
241
Misfire cylinder 4
P0305
242
Misfire cylinder 5
P0306
243
Misfire cylinder 6
P1396
244
Crankshaft sensor
adaptation at limit
Misfire detection fault: Varying pulse wheel
tolerances or loose pulse wheel
Table d. Ignition system problems
Component
Condition
Test
Repair
Spark plug
Incorrect spark plug
Electrode gap too
small
Electrode(s) missing
Oil or fuel fouled
spark plug
Ceramic insulation
cracked
Inspect spark plugs.
Verify correct spark
plugs.
Replace plugs if
defective.
Check crankcase
ventilation and oil
level.
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