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Michael Moore
Category: Chevy
Satisfied Customers: 2956
Experience:  ASE Master/GM WORLD CLASS/GM Master/GM ASEP/Dodge Gold/Mazda cert.
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t code po307 what does it mean

Resolved Question:

t code po307 what does it mean
Submitted: 8 years ago.
Category: Chevy
Expert:  Michael Moore replied 8 years ago.
DTC P0307 is a engine mis-fire on the number 7 cylinder. It can be caused by may differant reasons. A bad spark plug, plug wire, coil or injector can be just a few reasons for the mis-fire. Normally when the engine is mis-firing, the check engine light will flash.

What is the year, make and model, engine size and milage?
Michael Moore and 3 other Chevy Specialists are ready to help you
Customer: replied 8 years ago.
1999 chevy tahoe 5.7 104,000
Expert:  Michael Moore replied 8 years ago.
I would check the plugs, wires and cap and rotor. If you have not done a tune up, this may help. The only other concern may be a clogged injector. Try running some good fuel cleaner in the gas, this may help clear any clogged injectors. You can check the plug wires by running the engine at night with no lights on under the hood, the darker the better. Look for any arching spark plug wires, it will have a blue arch. If its intermitten, try spraying the plug wires with water, this can help find any arching plug wires. Inspect the spark plug, look on the white part for carbon tracking, it will look like the plug is cracked. If you find this carbon tracking, replace the plugs and wires, this will damage the wires also. I will give you the GM diagnosis chart for this code, they go by p0300 wich is a mis-fire code, just not the number 7 cylinder, but for all cylinders.
Expert:  Michael Moore replied 8 years ago.
DTC P0300 Engine Misfire Detected

Refer to Engine Control Schematics Power, Ground, and DLC .

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

If start-up ECT temperature is below minus 7°C (20° F), misfire detection is delayed until ECT temperature is more than 21°C (70°F). If start-up ECT temperature is more than minus 7°C (20°F), misfire detection begins after a 5 second delay.
No active vehicle speed DTCs
No active TP sensor DTCs
No active MAF sensor DTCs
No active camshaft sensor DTCs
No active crankshaft sensor DTCs
The fuel level is more than 10 percent

The system voltage is between 11-16 volts
The engine speed is between(NNN) NNN-NNNNRPM
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 type misfire or a catalyst damaging type 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 or 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.
Use a scan tool in order to 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 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.

Check for the following conditions:

Check the IC control circuit for an intermittent short to ground.
Check the spark plug wires and the coil wire for the following conditions:
- Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
- Check the wire routing in order to ensure that crossfiring is not occurring.
- If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
Check for contaminated or a low fuel level and the following conditions:
- Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
- If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and 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 above +20
- A restricted fuel filter can also cause a misfire.

Sticking intake or exhaust valves on engines with a misfire when cold.

Check HO2S for abnormal voltage readings.

Check 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 check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

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

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 cylinder(s) consistently.

  3. When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.







Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and Failure Records for reference. The control module's data is deleted once the Clear Info function is used.

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?


Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check


Install the scan tool.

Are there any other DTC's stored?


Go to the applicable DTC table

Go to Step 3


  1. Start the engine.
  2. Monitor the misfire counters in the misfire Data list with a load on the engine. Refer to Freeze Frame data.

Is the Misfire counter increasing?


Go to Step 5

Go to Step 4


  1. Turn OFF the engine.
  2. Using a spray bottle filled with water, wet down the secondary ignition system.
  3. Start the engine.
  4. Monitor the misfire counters in the misfire Data list with a load on the engine.

Does the scan tool indicate a misfire is present?


Go to Enhanced Ignition System Diagnosis

Go to Step 16


Is the misfire random on all cylinders?


Go to Step 10

Go to Step 6


Is the misfire consistent on specific cylinder(s)?


Go to Step 7



  1. Turn OFF the ignition.
  2. Connect the J 26792 Spark Tester to the ignition wire of the cylinder(s) indicating a misfire.
  3. Start and idle the engine.

Does the tester indicate spark is consistent on the tested ignition wire?


Go to Step 8

Go to Enhanced Ignition System Diagnosis


  1. Remove the spark plug(s) from the cylinder(s) that indicate a misfire. Refer to Spark Plug Replacement in Engine Electrical.
  2. Perform the Spark Plug Visual Diagnosis. Refer to Spark Plug Visual Diagnosis in Engine Electrical.

Did you find a problem?


Go to Step 15

Go to Step 9


  1. Install the spark plug(s) in the cylinder(s) that were not indicating a misfire.
  2. Start the engine
  3. Monitor the misfire counters in the misfire Data list.

Did the misfire(s) change cylinder(s)?


Go to Step 13

Go to Step 11


Perform the fuel system diagnostic. Refer to Fuel System Diagnosis . Repair as necessary.

Did you find a problem?


Go to Step 17

Go to Step 14


Perform the Fuel Injector Balance test. Refer to Fuel Injector Balance Test with Tech 2 .

Did you find a problem?


Go to Step 12

Go to Step 15


  1. Connect the J 34730-375 Injector Test Lamp to the cylinder(s) that indicate a misfire.
  2. Crank the engine for the specified value.

Does the lamp flash on all tested circuits while cranking the engine?

10 sec

Go to Fuel Injector Solenoid Coil Test

Go to Fuel Injector Circuit Diagnosis


Replace the spark plugs. Refer to Spark Plug Replacement in Engine Electrical.

Is the action complete?


Go to Step 17



Physically and visually check for these conditions in the following order:

  1. Contaminated fuel.
  2. Fouled or damaged spark plugs
  3. Ignition coil and connections.
  4. Damaged distributor cap.
  5. Leaking EGR valve.
  6. Restricted exhaust system.
  7. VCM grounds.

Repair as necessary.

Did you find a problem?


Go to Step 17

Go to Step 15


  1. Check for engine mechanical problems. Refer to Base Engine Misfire Diagnosis in Engine Mechanical.
  2. Repair as necessary.

Did you find a problem?


Go to Step 17

Go to Diagnostic Aids


Perform the CKP System Variation Learn procedure. Refer to Crankshaft Position System Variation Learn .

Is the action complete?


Go to Step 17



  1. Using the scan tool, clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the engine reaches normal operating temperature.
  4. Select DTC and the Specific DTC function.
  5. Enter the DTC number which was set.
  6. Operate the vehicle, with the Conditions for Setting this DTC, until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic Passed?


Go to Step 18

Go to Step 2


Does the scan tool display any additional undiagnosed DTC's?


Go to the applicable DTC table

System O

Customer: replied 8 years ago.
replaced plugs, wires ,cap and rotor did not fix problem.
Expert:  Michael Moore replied 8 years ago.
More then likley you have a clogged injector. Try cleaning the injectors, if that does not work, you may have to replace them. There is a problem with these injectors on this engine. They have a updated part to replace them, not a cheap fix. You may want to have a injector drop test performed to make sure the injector is the problem. The test is done by firing each injector wile watching how much fuel pressure is lost during the test, each injector will be tested and compard to each other, if there is a problem with the number 7, it will not drop any pressure of have a lower drop then the others.

This is a GM bulletin for the injector concern.

#00-06-04-003B: Rough Idle After Start, And/Or A Service Engine Soon (SES) Light (Unstick And Clean Central Sequential Fuel Injection (CSFI) Poppet Valves or Convert to MFI) - (Jan 30, 2003)
Subject:Rough Idle After Start, And/Or A Service Engine Soon (SES) Light (Unstick And Clean Central Sequential Fuel Injection (CSFI) Poppet Valves or Convert to MFI)
Models:1995-2002 Chevrolet and GMC S/T Models
1996-2002 Chevrolet and GMC C/K, M/L, G, P Models
1996-2001 Oldsmobile Bravada
1999-2000 Cadillac Escalade
with 4.3L, 5.0L or 5.7L Engine (VINs W, X, M, R -- RPOs L35, LF6, L30, L31)
Except 2002 VIN X -- RPO LU3 Models

This bulletin is being revised to update the Correction and Warranty Information. Please discard Corporate Bulletin Number 00-06-04-003A (Section 06 -- Engine).


Some customers may comment on rough idle after start-up, especially if the vehicle has sat overnight. These symptoms may be intermittent. The Service Engine Soon (SES) light may also be illuminated. Current misfire or history misfire codes may be detected with the Tech 2 scan tool.


A deposit build-up on the CSFI poppet valve ball and/or seat may cause the poppet ball to stick open or closed. In either case, the specific cylinder will be mis-fueled, resulting in a cylinder mis-fire condition.


A new injector unsticking and cleaning process has proven to be effective in restoring poppet valves to an "as new" condition. CSFI injector replacement should NOT be considered as a correction for this customer concern.

Check the vehicle history to determine if the vehicle is returning for a second fuel system repair for this condition. If the fuel system has been repaired for this condition previously, then inform the customer that an alternate fix is available. A new MFI fuel system has been developed that will back service the CSFI fuel system. The MFI fuel system eliminates the CSFI poppet valve ball and seat. The CSFI fuel system will need to be replaced as a unit. Refer to Central SFI to MFI Conversion instructions in this bulletin.

Notice: Individual CSFI injectors can not be replaced with MFI injectors. The entire CSFI fuel meter body will need to be replaced. The bracket used to retain the injectors in the fuel meter body is different between the CSFI and MFI unit. The unit may not seal if you mix injectors. Severe engine damage could result.

Cleaning the CSFI injectors is the preferred repair. The CSFI unit should not be replaced until cleaning has been attempted. If the vehicle should return for the same repair, then the CSFI unit can be replaced with an MFI unit. If the vehicle is out of warranty, the customer should be given the option of which fix to pursue.

Important: "GM of Canada" dealers require District Service Manager approval prior to replacing the CSFI unit with a MFI unit under warranty.

The first step of this process is to use the J 41413 Evap Pressure/Purge Station, in conjunction with the J 44466-10 pressure regulator/hose assembly, to provide the required high pressure (150 psi) source to perform the unsticking procedure. The J 39021 fuel injector tester and accessories provide the means to "energize" the injector. Some later model-year vehicles may utilize the Tech 2® to "energize" the injectors.

The second step of this process is to run the engine on a solution of 10% Top Engine Cleaner and 90% gasoline. This will effectively clean any deposits from the ball and seats of the poppets. It is very important that the engine fuel system is separated from the vehicle fuel system. Top Engine Cleaner may have detrimental effects on the fuel pump.

Cleaning Procedures

This procedure should be performed before any attempt to convert the fuel system to MFI.

S/T and C/K Truck

  1. Loosen the fuel filler cap to relieve vapor pressure in the fuel tank.
  2. Remove the Positive Crankcase Ventilation (PCV) clean air tube from the air inlet tube and set aside.
  3. Remove the bonnet and inlet tube from the throttle body.
  4. Remove the brake booster vacuum hose and connector from the intake manifold.
  5. Remove the electrical connector from the CSFI fuel metering body.
  6. Remove ignition wires 1, 3, 5, (7) from the distributor cap.

  7. Object Number: 334517  Size: SH

  1. Remove the fuel line bolt (4) at the rear of the intake manifold.
  2. Relieve the fuel system pressure at the service fitting on the fuel line. Use a shop towel to contain any fuel that may exit the service fitting.

Expert:  Michael Moore replied 8 years ago.
  1. Remove the nuts (1) and clamp (2) from the fuel pipe.
  2. Remove the fuel pipes from the metering body. Be careful not to disturb the O-rings, washer, and spacer in the metering body.

  3. Object Number: 648221  Size: SH

Customer: replied 8 years ago.
I will try that. thank you