Volvo Symptom D12D US02 Service Manual page 76

Group 20
Ignition Difficulties
At very low outside temperatures, misfiring when the
accelerator pedal is depressed could be due to one of
the following:
•
The fuel quality is poor.
•
The engine has poor compression.
•
One of the temperature sensors, for example, ambient
air temperature or coolant temperature, gives incorrect
values, i.e. the sensor indicates a higher temperature
than actual temperature.
As long as the sensor values are within the normal
operating range, no fault codes for an unreasonable
sensor value are set.
Electrical faults
A brief fuel shut-off will occur if the conditions of the
control unit and input signals are not stable.
The fuel shut-off could be caused by one or more of the
following:
•
The power supply to the control unit is interrupted.
•
The control unit has a poor ground terminal.
•
Loose connections in wiring and/or connections.
•
Loose connections between the sensor and control
unit (e.g. corrosion, loose pin connections or damaged
wiring).
•
Reversed polarity signals from the flywheel or camshaft
sensors.
Defective electrical signals
A brief fuel shut-off can also occur if any of the following
faults occur:
Fault Tracing
Temporary faults can be difficult to trace since the fault
may not occur when the vehicle is checked.
Fault tracing should be performed based on the
experiences of the driver. The information that the driver
has provided together with the gathering of factual
data should form the basis of how the problem is to be
resolved.
Whenever possible, try to recreate the problem in an
environment and situation similar to that described by the
driver.
Begin by determining if anything has recently happened
to the vehicle that could point to the electrical system, but
which is not related to the engine control system.
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D
Damage to wiring.
•
The signal from the accelerator pedal sensor is
interrupted.
•
The intake manifold pressure sensor gives a defective
signal.
•
Interference in the signal from the flywheel sensor.
•
The speed limiter is engaged.
•
Alternating between a defective and fault free sensor
signal (with fault code).
Mechanical Faults
A brief fuel shut-off occurs if the engine has a mechanical
fault. No fault code is set.
The fuel shut-off could be due to any of the following
causes:
•
Damage to the flywheel sensor teeth could mean
that the sensor provides defective engine speed
information.
•
The flywheel or camshaft sensors may be installed
incorrectly. The distance between the sensor and the
flywheel or the cam shaft tooth wheel is not within
specification.
•
The sensor is damaged or the tip is covered with
debris.
Lack of fuel
Misfiring occurs in the injectors that are not being supplied
with fuel.
The lack of fuel could have any of the following causes:
•
Temporary low fuel pressure.
•
Air or combustion gases in the fuel system.
For specific symptoms, see:
•
"Engine Runs Unevenly at Idle Speed" page 73
•
"Engine Speed Surges at Low or High Idle" page 73
•
"Engine Runs Unevenly or Misfires" page 73
•
"Engine Speed Surges Under Load or in Normal
Operating Range" page 73
•
"Engine Speed Surges in Power Take-off or Cruise
Control" page 74
"Uneven Operation, Fault Tracing" page 71