P0300: Random Misfire Detected

1. Check if the vehicle has recently run out of fuel. If
so, reset the PCM to clear the DTCs and retest.

2. If code P0300 is detected again, check for other
relevant DTCs:
P0301-12 individual cylinder misfire codes
PIP (Crankshaft sensor) circuit failure P0320
CMP (Camshaft sensor) circuit failure P0340/45
Injector failure P0201 - 0206
Ignition coil primary circuit P0351-P0356
Rectify the problem and clear the above codes
plus any other fuel or ignition related codes
before analysing the P0300 code as follows:

3. Check the fuel pressure as follows:
Using the WDS datalogger, monitor the fuel pressure sensor signals.
Start the engine and run at idle speed. Record the
fuel pressure. Increase the engine speed to 2500
rpm and maintain for 1 minute. Record the fuel
pressure.
Nominal fuel pressure should be 40 psi above
inlet manifold pressure. The fuel system must be
capable of maintaining approximately 40 psi at
idle and at wider throttle openings.
If the fuel pressure is within specification, go to
the air induction system checks.
If the fuel pressure is outside the above specification, service the fuel system as necessary. Reset
the PCM and run a KOER test to ensure that the
problem is resolved.
1 Air Induction System Checks

1. Visually check the air induction system for leaks,
damage or other defects and service as necessary.

2. Using the datalogger, check and compare the two
MAF sensor signals with the engine running at
idle.
If the signals are clean and balanced, go to the
vacuum system checks.
If the signals are clean but not balanced, check
the air filter, filter mounting or for restrictions in
the air induction ducting. Service as necessary.
If the MAF signals show any electrical noise (voltage spikes above 0.05 volts) and the filter and
ducting are serviceable, change the affected MAF
sensor.
2 Vacuum System Checks

1. Visually inspect all vacuum lines for damage,
such as pinched lines, cracks, proper routing and
assembly. Service or repair the vacuum system as
necessary.
If no fault is identified, go to the evaporative
emissions system checks.
If a fault is identified, service the system as necessary, reset the PCM and run a KOER test to ensure
that the problem is resolved.
3 Check the Evaporative Emission System

1. Visually check the evaporative loss system for
obvious defects which could cause a vacuum
leak:- e.g.
Loose or defective fuel filler cap or seal
Loose connections at the inlet manifold.
Loose connections in the vapour recovery lines in
the engine bay.
Loose connections at the vapour management
valves.
If no defects are identified, go to step 2.
If any obvious defects are identified, service as
necessary and run the KOER test to ensure that
the problem is resolved.

2. Run the WDS Evaporative Emission System Leak
Test.

3. If no leak is detected, the misfire fault is not
caused by the evap loss system, go to the fuel system checks.

4. If any leak is detected, service the evaporative
emission system as necessary. Clear the P0300
code and run a brief road test to ensure that the
problem is resolved.
4 Fuel System Checks

1. Disconnect the affected PCM and install the
break-out box to the harness connector only.
Check the resistance of the fuel injectors and harness at the following test points:
Injector Pins Injector Pins
1 C0636-2 to Fuse F18 7 C2464-2 to Fuse F6
2 C0636-9 to Fuse F18 8 C2464-9 to Fuse F6
3 C0636-5 to Fuse F18 9 C2464-5 to Fuse F63
4 C0636-4 to Fuse F18 10 C2464-4 to Fuse F6
5 C0636-8 to Fuse F18 11 C2464-8 to Fuse F6
6 C0636-3 to Fuse F18 12 C2464-3 to Fuse F63
Each resistance should be in the range 11 - 18Ω
Low resistance - check the affected circuits for
shorts to power or ground.
High resistance - check the affected circuits for
high resistance connections or open circuits.

2. Remove the break-out box and reconnect the
PCM. Connect the WDS or scan tool. Run the
engine. Check the injector drive signal pulse
widths.
The injectors must show a stable pattern of injector pulses, short pulses (approx. 2ms) at idle and
steadily increasing pulse length as power demand
is increased.
If the injector driver signal goes continually low
or continually high, or is erratic, replace the
PCMs.
If all signals are consistent, with pulse lengths
increasing with engine speed (from approx. 2 ms
at idle), the electrical control circuits are good.

3. Flow test the injectors:
Use a Rotunda Injector Tester or equivalent to
flow test the injectors according to the manufacturers instructions.
If any injector flow rate is not within specification, replace the defective injector. Rerun the
KOER Test.
If the injector flow rates are all within specification, the problem is not fuel related. Go to the
vacuum system checks.
5 Check Base Engine

1. Remove the inlet manifold on the affected side of
the engine. Remove the spark plug cover.
Remove each ignition coil in turn and check for
evidence of high voltage discharge, burned connector, etc.
If any fault is detected, replace the coil. Continue
with the following physical checks to identify any
additional contributory causes for the misfire.
If no fault is detected, go to step 2.

2. Remove the spark plugs and check for fouling,
cracked insulators, incorrect gap, etc.
If any fault is detected, replace all spark plugs.
Continue with the following physical checks to
identify any additional contributory causes for the
misfire.
If no fault is detected, go to step 3.

3. Check the cylinder compression pressures using
the Workshop Manual procedure.
If any compression pressure is low, repair the
engine as necessary.

4. Remove the camshaft cover. Rotate the engine to
TDC No. 1 cylinder. Check for correct valve timing (workshop manual procedure)
If any fault is detected, perform the valve timing
adjustment. Continue with the following physical
checks to identify any additional contributory
causes for the misfire.
If no fault is detected, go to step 5.

5. Reassemble the engine using new gaskets, seals
and sealants.
Reset the PCM and run a KOER test to ensure that
the problem has been resolved.