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Tim's Auto Repair
Tim's Auto Repair, mechanic
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Experience:  Have owned a repair shop for 25 yrs.
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96 chevy s-10 blazer has p0300 and p0301 codes

Resolved Question:

96 chevy s-10 blazer has p0300 and p0301 codes
Submitted: 6 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 6 years ago.


yes..the egr could cause this may need to remove and clean it with carb may not be shutting all the way off,causing a major vacuum leak from the exhaust....also,check the fuel pressure..these must have no less than 60psii at all times...a weak fuel pump can also cause this problem


Customer: replied 6 years ago.
i removed the egr (lunear style) and i can push the pintle open easily is that normal and i checked fuel pressure and it was about56 psi should i replace egr or look into thefuel pressure
Expert:  Tim's Auto Repair replied 6 years ago.

the fuel pressure is weak..these must have no less than 60psi at all times..the egr is may need to replace the fuel pump



Testing and Inspection


Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart

Circuit Description

When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after you turn the ON the ignition or if the engine stops.

An electric fuel pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed in order to provide fuel at a pressure above the regulated pressure needed by the injectors. The pressure regulator keeps the fuel available to the injector at a regulated pressure. Unused fuel is returned to the fuel tank by a separate line.

Diagnostic Aids

Tools Required


  • J 34730-E Fuel Pressure Gauge.
  • J 34730-1A Fuel Pressure Gauge with J 34730-250 Fuel Pressure Adapter Kit.
  • d 42873-1 3/8 Fuel Feed Pipe Shutoff Valve and the J 42873-2 5/16 Fuel Return Pipe Shutoff Valve.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition leaving the engine OFF, the fuel pressure should be 415-455 kPa (60-66 psi). This pressure is controlled by a spring pressure within the regulator assembly.
  1. The fuel pressure that continues to fall is caused by one of the following items:
    • The in-tank modular fuel sender is not holding pressure.
    • The fuel pressure regulator valve is leaking.
    • A Central SFI injector is leaking.
    • Fuel line is leaking.
  1. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure; however, the pressure at idle should be less indicating the pressure regulator control.
  1. If the fuel pressure is less than 415 kPa (60 psi), it falls into the following 3 areas:
    • The pressure is regulated but less than 415 kPa (60 psi).The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC.
    • A restricted flow causing pressure drop-Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, it the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.
    • A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

Notice: Do not allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel pressure regulator.

  1. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the battery voltage applied to the fuel pump test terminal, the pressure should rise above 455 kPa (66 psi), as the valve in the return line is partially closed.
Customer: replied 6 years ago.
i crimped the return line and reset the engine light the fuel pressure is at 75 psi and it set the codes p0300 andp0301 again. what could be the problem
Expert:  Tim's Auto Repair replied 6 years ago.
can you feel the miss fire now?
Customer: replied 6 years ago.
yes i can feel the misfire and the engine light flashes
Expert:  Tim's Auto Repair replied 6 years ago.


a flacking engine light will usually happen if the catalyst converter is may want to have a muffler shop check this




DTC P0300 Engine Misfire Detected

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 is below minus 7°C (20°F), misfire detection is delayed until ECT is more than 21°C (70°F). If start-up ECT 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.
  • 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.

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 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.




Customer: replied 6 years ago.
Relist: Incomplete answer.
the flashing engine light means severe misfire which can cause cat converter damage i just cant figure out why i have a misfire i replaced #1 fuel injector plug wires spark plugs and compression on #1 was 180 psi
Customer: replied 6 years ago.
i changed the fuel pump and i ran good for a little while and then the engine lite came back on with same codes
Expert:  Tim's Auto Repair replied 6 years ago.
do you still have good fuel less than 60psi?
Customer: replied 6 years ago.
on gauge reading is about 57psi
Expert:  Tim's Auto Repair replied 6 years ago.


will the pressure go up when you give it throttle?if so,what does the pressure go to?

Customer: replied 6 years ago.
i gave it throttle and pressur went up to 62psi briefly and droped back down to 57 psi
Expert:  Tim's Auto Repair replied 6 years ago.

did it drop while you were holding the throttle steady?


Customer: replied 6 years ago.
Expert:  Tim's Auto Repair replied 6 years ago.
ok..when you give it throttle...and hold it to about 3500 rpms,,the pressure will rise,then drop while you are still giving it throttle?right?
Customer: replied 6 years ago.



Expert:  Tim's Auto Repair replied 6 years ago.


you still have a fuel problem..these shoudnt drop below 60psi at all is common to get a bad new pump for these

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Customer: replied 6 years ago.
i cleared codes and pinced off the fuel return line this got fuel presure to about 80psi i test drove and under global abd 2 i got code po306 under manufactor code i got po300 with the fuel pressure at 80 that should eleminate the fuel if codes are back im thinkn maybe i have a defective ignition part what do u think
Customer: replied 6 years ago.
i figured out the problem it had an intermittent vaccum leak. the o ring gasket by the injector harness plugs in was intermittently sealing i grabed the plug and wiggled it and got a vaccum leak. thanks for your time