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JOHN aka Eauto
JOHN aka Eauto, The Car Guy
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Experience:  ase tech
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Chevrolet Astro random misfire in 2000 4.3 vortec cahnged

Resolved Question:

random misfire in 2000 4.3 vortec cahnged plugs, wire cap, rotor , checked ign module
Submitted: 3 years ago.
Category: Chevy
Expert:  JOHN aka Eauto replied 3 years ago.

here is a service bulletin

Subject: SES Light and P0300 When Towing , Cruising Uphill or on Hard Acceleration - 4.3 5.0 5.7 accelerate cruise cylinderhead RPO L30 L31 L35 LF6 LU3. DTC P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 PIP3081 - (07/23/2004)

Models: (96-02 Chevrolet Astro) and (96-02 GMC Safari) and (96-00 Old Body Style Chevrolet Suburban) and (96-00 Old Body Style Chevrolet Tahoe) and (96-02 Chevrolet Express) and (96-00 GMC Yukon) and (96-00 Chevrolet Old Body Style C/K Truck) and (96-00 GMC Old Body Style C/K Truck) and (96-02 GMC Savana) and (96-99 P32 Cab and Chassis) and (96-02 Chevrolet S-10) and (99-02 Chevrolet Silverado with 4.3L V-6 Engines (96-02 GMC Sonoma) and (96-02 GMC Sierra with 4.3L V-6 Engines).

The following diagnosis might be helpful if the vehicle exhibits the symptom described.

Condition/Concern:
The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3,4,5 or 6 will be the ones to experience this.

Recommendation/Instructions:
If the P0300 diagnostics did not isolate a concern, the following may help:
Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:

  • Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.
  • Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.

Reassemble the cylinder heads using new valve seals.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

NOTE: GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.

 

 

Customer: replied 3 years ago.
It misses evan at idle or at higher speeds, could the coil or pickup coil be a problem
Expert:  JOHN aka Eauto replied 3 years ago.
could be a stuck valve..or a bad injector...i would start with a compression check
Customer: replied 3 years ago.
I beleive the compression is good (so I am told) and the miss varies fron cyl to cyl
Expert:  JOHN aka Eauto replied 3 years ago.

any other codes?

 

Customer: replied 3 years ago.
no just randon misfire
Expert:  JOHN aka Eauto replied 3 years ago.

has fuel pressure been checked?



CIRCUIT DESCRIPTION
The Crankshaft Position (CKP) sensor is the primary input to determine if misfire is occurring. Engine misfire is detected by monitoring crankshaft speed variations between cylinders. If a crankshaft deceleration occurs during a combustion or power stroke, the control module will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is more than a maximum allowable speed, the misfire is detected. Misfire may occur in a specific cylinder or in all cylinders randomly.

When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP. The control module determines which cylinder has misfired based upon the Camshaft Position (CMP) sensor input. Misfire data is stored for each cylinder in separate accumulators. After 100 combustion events, the misfire totals are compared to a calibrated maximum number. If the misfire is excessive, this Diagnostic Trouble Code (DTC) will set.

CONDITIONS FOR RUNNING THE DTC

IMPORTANT: If start-up Engine Coolant Temperature (ECT) is less than -7°C (+20°F) , misfire detection is delayed until the ECT is more than 21°C (70°F) . If the start-up ECT is more than -7°C (+20°F) , misfire detection begins after a 5 second delay.

  • No active Vehicle Speed Sensor (VSS) DTCs.
  • No active Throttle Position (TP) sensor DTCs.
  • No active Mass Air Flow (MAF) sensor DTCs.
  • No active CKP sensor DTCs.
  • No active Camshaft Position (CMP) sensor DTCs.
  • The fuel level is more than 10 percent.
  • The engine speed is between 450-5000 RPM.
  • The system voltage is between 11-16 volts.
  • The throttle position is steady within 2 percent for 100 ms.

CONDITIONS FOR SETTING THE DTC
The VCM detects a deceleration in the crankshaft speed characteristic of either an emission misfire or a catalyst damaging misfire.

ACTION TAKEN WHEN THE DTC SETS
If the VCM determines that the engine misfire is significant enough to have a negative impact on emissions, the VCM turns ON the Malfunction Indicator Lamp (MIL) after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

CONDITIONS FOR CLEARING THE MIL/DTC

  • The control module turns OFF the MIL afterXXXXXtrips when the test has Run and Passed.
  • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
  • A scan tool can clear the DTCs.

DIAGNOSTIC AIDS

  • The misfire index counts the number of misfires. The scan tool can monitor the misfire index. There is a Current and a History misfire counter for each cylinder. Use the Current misfire counter in order to determine which cylinder is misfiring, or use the History misfire counter for misfires that are not currently present.
  • Many different conditions could cause an intermittent misfire. Inspect for the following conditions:
  • An intermittent short to ground at the IC control circuit.
  • Intermittent conditions related to the spark plug wires and the coil wire. Inspect the spark plug wires and the coil wire for the following conditions:
  • The spark plug wires are securely attached to the spark plugs and the distributor cap.
  • The wire routing is correct in order to avoid crossfiring.
  • Worn plug wires. If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
  • Inspect for contaminated fuel or a low fuel level which may include the following conditions:
  • Poor fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
  • A low fuel level. If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and a possible misfire condition. Check the fuel trim numbers in the freeze frame to determine if this has occurred. It would be likely if the Short Term fuel number was more than +20.
  • A restricted fuel filter.
  • Inspect for sticking intake or exhaust valves on engines with a misfire when cold.
  • Inspect the HO2S for abnormal voltage readings.
  • Inspect for a vacuum leak as a possible cause of the engine misfire.

An intermittent may be caused by any of the following conditions:

  • A poor connection
  • Rubbed through wire insulation
  • A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagrams.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagrams.

TEST DESCRIPTION
The numbers below refer to the Step numbers on the diagnostic table.

  1. The misfire is considered random on all cylinders if, while viewing the misfire fire counters in the misfire data list, the misfire seems to move to different cylinders.
  2. The misfire is considered consistent if the misfire is occurring on the same cylinders consistently.
  3. When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is considered no-spark.

 

 

Expert:  JOHN aka Eauto replied 3 years ago.

has fuel pressure been checked?



CIRCUIT DESCRIPTION
The Crankshaft Position (CKP) sensor is the primary input to determine if misfire is occurring. Engine misfire is detected by monitoring crankshaft speed variations between cylinders. If a crankshaft deceleration occurs during a combustion or power stroke, the control module will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is more than a maximum allowable speed, the misfire is detected. Misfire may occur in a specific cylinder or in all cylinders randomly.

When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP. The control module determines which cylinder has misfired based upon the Camshaft Position (CMP) sensor input. Misfire data is stored for each cylinder in separate accumulators. After 100 combustion events, the misfire totals are compared to a calibrated maximum number. If the misfire is excessive, this Diagnostic Trouble Code (DTC) will set.

CONDITIONS FOR RUNNING THE DTC

IMPORTANT: If start-up Engine Coolant Temperature (ECT) is less than -7°C (+20°F) , misfire detection is delayed until the ECT is more than 21°C (70°F) . If the start-up ECT is more than -7°C (+20°F) , misfire detection begins after a 5 second delay.

  • No active Vehicle Speed Sensor (VSS) DTCs.
  • No active Throttle Position (TP) sensor DTCs.
  • No active Mass Air Flow (MAF) sensor DTCs.
  • No active CKP sensor DTCs.
  • No active Camshaft Position (CMP) sensor DTCs.
  • The fuel level is more than 10 percent.
  • The engine speed is between 450-5000 RPM.
  • The system voltage is between 11-16 volts.
  • The throttle position is steady within 2 percent for 100 ms.

CONDITIONS FOR SETTING THE DTC
The VCM detects a deceleration in the crankshaft speed characteristic of either an emission misfire or a catalyst damaging misfire.

ACTION TAKEN WHEN THE DTC SETS
If the VCM determines that the engine misfire is significant enough to have a negative impact on emissions, the VCM turns ON the Malfunction Indicator Lamp (MIL) after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

CONDITIONS FOR CLEARING THE MIL/DTC

  • The control module turns OFF the MIL after X XXXXXXXXXXX XXXXX trips when the test has Run and Passed.
  • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
  • A scan tool can clear the DTCs.

DIAGNOSTIC AIDS

  • The misfire index counts the number of misfires. The scan tool can monitor the misfire index. There is a Current and a History misfire counter for each cylinder. Use the Current misfire counter in order to determine which cylinder is misfiring, or use the History misfire counter for misfires that are not currently present.
  • Many different conditions could cause an intermittent misfire. Inspect for the following conditions:
  • An intermittent short to ground at the IC control circuit.
  • Intermittent conditions related to the spark plug wires and the coil wire. Inspect the spark plug wires and the coil wire for the following conditions:
  • The spark plug wires are securely attached to the spark plugs and the distributor cap.
  • The wire routing is correct in order to avoid crossfiring.
  • Worn plug wires. If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
  • Inspect for contaminated fuel or a low fuel level which may include the following conditions:
  • Poor fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
  • A low fuel level. If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and a possible misfire condition. Check the fuel trim numbers in the freeze frame to determine if this has occurred. It would be likely if the Short Term fuel number was more than +20.
  • A restricted fuel filter.
  • Inspect for sticking intake or exhaust valves on engines with a misfire when cold.
  • Inspect the HO2S for abnormal voltage readings.
  • Inspect for a vacuum leak as a possible cause of the engine misfire.

An intermittent may be caused by any of the following conditions:

  • A poor connection
  • Rubbed through wire insulation
  • A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagrams.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagrams.

TEST DESCRIPTION
The numbers below refer to the Step numbers on the diagnostic table.

  1. The misfire is considered random on all cylinders if, while viewing the misfire fire counters in the misfire data list, the misfire seems to move to different cylinders.
  2. The misfire is considered consistent if the misfire is occurring on the same cylinders consistently.
  3. When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is considered no-spark.

 

 

JOHN aka Eauto, The Car Guy
Category: Chevy
Satisfied Customers: 12942
Experience: ase tech
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