Gasoline Engine

Volkswagen Technician Helpline
Gasoline Engine Misfire Diagnosis
Multiple cylinder
misfires are caused
by things that affect all
cylinders such as poor
fuel quality or low
pressure. Check MVB 32
and monitor the fuel trim
readings and check
MVB 93 Field 3 for a
timing issue. The
specification is
0 +/- 5 degrees.
MVB 91 Fields 3 and
4 are camshaft
position specified and
actual. They should
match within 0.5
degrees. If the fuel
rail pressure drops
with KOEO, a fuel
injector is leaking.
Active
Misfire?
Y
Check
TSBs and
Tech Tips?
Multiple
ultiple
Cylinders?
Cyl der
Y
Yes
N
cyli
plu
o
N
Check
TSBs and
Tech Tips?
ir
Perform
Repair
Y
N
Perform
Repair
Y
N
Swap the ignition coil on the misfiring cylinder
to a different cylinder and move the spark plug to
p the
another
cylinder and recheck for misfires.
Cylnder
Multiple
Cylinders?
Y
Check the ambient
conditions that the
engine misfired,
Cold, operating
temp, under load
or at idle, etc.
N
ir
Replace Failed Component
ir
Y
Yes
Did Misfire Move
to a Different Cylinder?
Single cylinder memory misfire faults can be
caused by anything related to the misfiring
cylinder. The spark plug and coil should be
swapped to other cylinders to rule out those
components as a cause.
N
forfaults
other faults
related
the misfiring
Check forCheck
other
related
totothe
misfiring cylinder, such as circuit faults
cylinder, such as circuit
faults
and repair
those and
first. repair those first.
Verify
Check for
circuit
other faults
integrity
relatedfrom
to the
misfiring
cylinder, such
circuit
faults
the ignition
coil as
and
fuel
and repair those first.
injector to the ECM. Verify
injector and coil pulse from
the ECM. If OK, swap a known
good fuel injector.
Excessive carbon buildup should be checked.
Misfires after a hot soak can be caused buy a
leaking fuel injector. Watch the fuel rail
pressure with the key ON and the engine OFF
with the engine warm. If pressure drops,
remove the spark plugs and check for fuel in
a cylinder. If found, change that injector.
Test compression on the misfiring cylinder.
o
N
Is Compression
Below Specification?
Y
Perform a cylinder leakage test at 100 psi.
o
N
Is Leakage Excessive?
Y
Is Air Leaking from
Tailpipe or Intake
Manifold?
Y
N
If air is escaping from the crankcase, check
for worn piston rings, a damaged piston or
cylinder wall.
N
Misfires at operating
temp may be caused by
bad fuel previously in the
tank or an intermittent
related concern.
Ambient conditions
should be checked; i.e.
under load or other
possible related faults
(timing, fuel pressure,
lean faults).
Y
Check for worn camshaft lobes,
broken valve springs, or a
bent connecting rod.
Cold
Misfire?
The intake or exhaust valves are leaking.
Check for burnt valves, broken valve springs
worn valve guides or a damaged valve seat
in the cylinder head.
Y
Cold misfires are caused
by excessive carbon
buildup, poor fuel
quality, low fuel
pressure on startup
or failing fuel injectors.
Verify fuel quality using
TT 20-09-01 or
monitor ignition
angle reduction under
load around 3,000 RPM
and WOT.
A reading of more than
-4 degrees indicates
bad fuel. An in-tank fuel
pump that allows the fuel
to bleed back to the tank
when turned off
can also cause misfires