Most usually a P0300 random misfire on multiple cylinders code is caused by a vacuum leak or low fuel pressure an volume ! Do a fuel pressure an volume test . Also I would do a compression test an a cylinder leak down test .How many miles does the vehicle have , a lot ?
Diagnostic Aids
If the Misfire Index indicates that one bank of the engine is experiencing misfire, the HO2S for that bank may be malfunctioning (i.e. Bank 1 Sensor 1 or Bank 2 Sensor 1). If the Misfire Index indicates that paired cylinders (i.e. cylinders 1-4, 6-3 and 5-2 for V6 applications and 1-6, 8-5, 4-7 and 3-2 for V8 applications) are experiencing misfire, inspect the CKP sensor reluctor ring for damage.
The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring.
Many different conditions could cause an intermittent misfire.
Check for the following conditions:
- Check the spark plug wires and the coil wire for the following conditions:
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Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
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Check the wire routing in order to ensure that crossfiring is not occurring.
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If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
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In order to test for this condition, spray the wires with water. With the engine running, watch for a spark to jump from the wires. If a spark is visible, replace the wires.
- Check for contaminated and a low fuel level and the following conditions:
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Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
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If the fuel level is low, contaminants in the bottom of the fuel tank could enter into the fuel metering system.
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For more information, refer to Fuel Supply System.
Step
Action
Value(s)
Yes
No
1
Important: Before clearing the DTCs, use the scan tool to record the Freeze Frame and the Failure Records for reference. This data will be lost when the Clear Info function is used.
Was the Powertrain On-Board Diagnostic (OBD) System Check performed?
--
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check
2
Review Failure Records data.
Are there any other DTCs stored?
--
Go to Applicable DTC Table
Go to Step 3
3
- Install a scan tool.
- With engine idling, monitor the MAP sensor voltage on the scan tool.
Is the MAP sensor voltage greater than the specified value?
4.0V
Go to DTC P0108 Manifold Absolute Pressure (MAP) Sensor Circuit High Voltage
Go to Step 4
4
Duplicate the conditions during which the misfire occurred.
Is the DTC active at the ignition cycle?
--
Go to Step 5
Go to Step 10
5
Using a scan tool, review the Misfire Index. Is the Misfire Index increasing for more than one cylinder?
--
Go to Diagnostic Aids
Go to Step 6
6
- Install the spark plug tester J 26792 on number 1 spark plug wire.
- Disconnect the fuel injector harness at the intake manifold.
- Crank the engine, spark should jump the tester gap.
- Repeat for each spark plug wire.
Was the spark adequate on all of the spark plug wires?
--
Go to Step 7
Go to Step 11
7
Inspect the spark plugs for wear, excessive air gaps, cracks, or fouling.
Are the spark plugs OK?
--
Go to Step 8
Go to Step 12
8
Perform a cyclinder compression test. Refer to Engine Machanical.
Was a problem found?
--
Go to Step 14
Go to Step 9
9
Refer to Fuel System Pressure Test .
Is the fuel system OK?
--
Go to Step 10
Go to Step 13
10
Misfire is intermittent and is currently not active.
Go to the Diagnostic Aids
--
--
--
11
Refer to Enhanced Ignition System Diagnosis .
--
--
--
12
Replace the spark plugs.
Is the action complete?
--
Go to Step 14
--
13
Make repairs as indicated by the Fuel System Diagnosis. Refer to Fuel System Pressure Test .
--
Go to Step 14
--
14
- Using the scan tool, select the DTC and the Clear Info.
- Start the engine.
- Idle at the normal operating temperature.
- Select the DTC and the Specific.
- Enter the DTC number which was set.
- Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.
Does the scan tool indicate that this diagnostic ran and passed?
--
Go to Step 15
Go to Step 2
15
Using the scan tool, select Capture Info and Review Info.
Are any DTCs displayed that have not been diagnosed?
--
Go to the applicable DTC table
System OK
This is GM factory service info