DTC P0300 Engine Misfire Detected. Cadillac Fleetwood 1996

For 1990-2009 cars only

Makes 🢒 Cadillac 🢒 1996 🢒 Fleetwood 🢒 DTC 🢒 DTC P0300 Engine Misfire Detected

Schematic Reference: Engine Controls Schematics

Circuit Description

A 4X reluctor wheel is located on the front of the crankshaft. The crankshaft position sensor will send a pulse every falling edge of the 4X reluctor wheel. A misfire causes a change in crankshaft speed. The PCM times the interval between each pulse and compares each new time interval with the previous one to determine when an excessive change in crankshaft speed has occurred. A certain amount of acceleration/deceleration is expected between each firing stroke, but if the crankshaft speed changes are greater than an expected amount, the PCM will interpret this as a misfire. The PCM continuously calculates crankshaft position from the low and high resolution signals. This information is used to determine which cylinder is misfiring so that the PCM can increment the appropriate misfire counter.

Conditions for Setting the DTC

•	DTCs P0100, P0102, P0103, P0107, P0108, P0117, P0118, P0122, P0123, P0335, P0336, P0500, P0502, P0601, P0742, not set.

•	Engine speed between 325 RPM and 3000 RPM.

•	Ignition voltage between 9.0 volts and 16.0 volts.

•	Engine coolant temperature between -7°C (19°F) and 130°C (266°F).

•	Throttle angle steady within 1%.

•	ABS and Traction Control not active.

•	Transmission not changing gears.

•	EGR and AIR diagnostic tests not in progress.

•	A/C clutch steady state.

•	Not in fuel shut-off or decel fuel cut-off.

•	ABS signals not exceeding rough road thresholds.

•	The PCM determines that an emission type Misfire is present.

•	The PCM determines that a catalyst damaging Misfire is present.

Action Taken When the DTC Sets

The PCM will illuminate the Malfunction Indicator Lamp (MIL) under the following conditions:

•

The PCM determines the percent of misfire over a 1000 revolution period is high enough to cause excessive tail pipe emissions. The PCM will illuminate the Malfunction Indicator lamp the next consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions (load, RPM, temperature, etc.) as the previous ignition cycle that the test ran and failed.

•

The second consecutive drive trip that this diagnostic fails, under the same conditions (load, RPM, temperature, etc.) as the previous ignition cycle that the test ran and failed, this information will be stored in Freeze Frame. Any data previously stored in Freeze Frame will be copied into Failure Records and Freeze Frame will be overwritten by this DTC. The only exception to this is if a Fuel Trim DTC was already recorded in Freeze Frame. In this case, the Fuel Trim data stays in Freeze Frame and the Misfire data will be updated in Failure Records.

Conditions for Clearing the MIL/DTC

•	The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail within the same conditions that the DTC last failed.

Important: If the last failure was during a non-typical driving condition, the MIL may remain ON longer than three drive trips. Review Freeze Frame/Failure Records for the last failure conditions.

•	A History DTC will clear after forty consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

•	A last test failed (Current DTC) will clear when the diagnostic runs and does not fail.

•	PCM battery voltage is interrupted.

•	Using a Scan tool.

Diagnostic Aids

•	If cylinder #1 is misfiring, inspect the EGR valve for hanging open. Remove the EGR valve and inspect for contaminates. Clean the EGR as necessary. Refer to Exhaust Gas Recirculation Valve Replacement .

•	The vehicle running out of fuel can cause sufficient misfire to set DTC P0300.

•	A misfire DTC could be caused by excessive vibration from sources other than the engine. The following are possible sources:

-	Tire/wheel out of round/balance

-	Variable thickness brake rotor/drum

-	Drive shaft not balanced

-	Certain rough road conditions

•	For intermittents, refer to Symptoms .

Test Description

Number(s) below refer to step numbers on the diagnostic table.

Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as water is applied.

If the Misfire Cur counters are incrementing and there is no apparent misfire, an erratic CKP sensor signal could be the cause. Perform the diagnostic table for DTC P0335 first if this condition is suspected.

If misfire is present and a fuel control problem is suspected, force the system into Open Loop using the scan tool and allow the engine to run for a few minutes. If this eliminates the misfire, refer to any fuel control related DTCs which are set. If no other DTCs are set, refer to the Engine Scan Tool Data List.

This distributor ignition system has lower KV requirements than typical systems. The J 26792 spark tester (ST-125) may not fire consistently when testing this system. This chart only asks if the system is capable of firing the spark tester.

If the plug is gas fouled, dry out the plug before reinstalling it in another cylinder.

If the customers complaint is The MIL is flashing, indicates that a Catalyst Misfire has occurred. The vehicle must be driven in the conditions to run the catalyst diagnostic. Refer to DTC P0420 Catalyst System Low Efficiency Bank 1 and/or DTC P0430 Catalyst System Low Efficiency Bank 2 for Conditions for Setting the DTC.

Step

Action

Value(s)

Yes

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Important: If any DTCs are set, (except DTCs P0131, P0132, P0133, P0151, P0152, P0153, P0171, P0172, P0174, P0175, P1133, P1135, P1136, P1153, P1155, P1156) refer to those DTCs before proceeding with this diagnostic.

Install the scan tool.
Start and idle engine.
Using a scan tool, monitor all the Misfire Cur. counters on the misfire data list (There are a total of 8 counters, One counter per cylinder).

Are any of the Misfire Cur. counters incrementing?

Go to Step 3

Go to Step 4

Important: If only one cylinder is misfiring, go to step 5.

Visually/physically inspect the following items:

•	Proper vent system operation, Refer to Service Category Distributor Vent System Check .

•	Vacuum hoses for splits, kinks and proper connections.

•	Throttle body, intake manifold and EGR valve for vacuum leaks.

•	Crankcase ventilation valve and/or system for vacuum leaks.

•	PCM grounds for being clean and secure.

•	Check that all injector harness connectors are connected to the proper injector/cylinder.

•	Check that the ignition wires are connected to the proper cylinders.

Did any of the above checks isolate a condition requiring a repair?

Go to Step 19

Go to Step 5

Turn the ignition ON, engine OFF, review Freeze Frame and/or Failure Records data for this DTC and note parameters.
Turn the ignition OFF for 15 seconds.
Start the engine and operate the vehicle within the conditions required for this diagnostic to run, and as close to the conditions recorded in Freeze Frame/Failure Records as possible. Special operating conditions that need to be met before the PCM will run this diagnostic, where applicable, are listed in Conditions for Setting the DTC.
Using the scan tool, select DTC, Specific, then enter the DTC number which was set.

Does the scan tool indicate that this diagnostic failed this ignition?

Go to Step 5

Go to Diagnostic Aids

Turn the ignition OFF.
Disconnect ignition coil wire from distributor and install spark tester J 26792 to ground.
Crank the engine and observe the spark tester.

With engine cranking, does spark jump the tester gap?

Go to Step 6

Go to Step 12

Reconnect coil wire to the distributor.
Disconnect the ignition wire(s) that corresponds to the Misfire Cur counter(s) that was incrementing.
Using spark tester J 26792 check for spark at the ignition wire(s) that indicated a misfire.

Does spark jump the tester gap?

Go to Step 7

Go to Step 15

Remove the spark plugs from the cylinder that indicated a misfire.

Does the spark plug appear to be OK or only gas fouled (not oil or coolant fouled)?

Go to Step 8

Go to Step 9

Swap the suspected spark plug with another cylinder that is operating properly.
Operate the vehicle under the same conditions that the misfire occurred.

Important: If the Injector Coil Test Procedure checks to be OK, refer to Engine Mechanical diagnosis.

Did the misfire move with the spark plug?

Go to Step 17

Go to Fuel Injector Solenoid Coil Test

Are the spark plugs oil or coolant fouled?

Refer to Engine Mechanical for diagnosis

Go to Step 10

Do the spark plugs show any signs of being cracked, worn, or improperly gapped?

Go to Step 11

Go to Step 18

Replace or re-gap spark plugs (If improper gap is found, be sure to re-gap spark plugs using a wire type gauge, J 41319.)

Is the action complete?

Go to Step 19

Using a DVM (J 39200), measure the resistance of the coil wire.

Is the coil wire resistance less than the specified value?

10,000 ohms

Go to Step 13

Go to Step 14

Replace ignition coil.

Is the action complete?

Go to Step 19

Replace coil wire.

Is the action complete?

Go to Step 19

Using a DVM (J 39200), measure resistance of ignition wire(s) that are indicating a misfire.

Do any ignition wires measure greater than the specified value?

10,000 ohms

Go to Step 16

Go to Step 17

Replace faulty ignition wire(s).

Is the action complete?

Go to Step 19

Replace faulty spark plug(s).

Is the action complete?

Go to Step 19

Inspect the distributor for the following:

•	Carbon tracking

•	Contamination

•

Moisture

•	Physical damage

Refer to Ignition System (6D).

Did any of the above checks isolate a condition requiring a repair?

Go to Step 19

Refer to Engine Mechanical for diagnosis.

Was the customer complaining that the MIL was flashing?

Go to DTC P0420 Catalyst System Low Efficiency Bank 1 or DTC P0430 Catalyst System Low Efficiency Bank 2

Go to Step 20

Using the scan tool, select DTC, Clear Info.
Start the engine and idle at normal operating temperature.
Select DTC, Specific, then enter the DTC number which was set.
Operate the vehicle within the conditions for setting this DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this test ran and passed?

Go to Step 21

Go to Step 2

Using the scan tool, select Capture Info, Review Info.

Are any DTCs displayed that have not been diagnosed?

Go to the applicable DTC table

System OK