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Tim's Auto Repair
Tim's Auto Repair, mechanic
Category: Chevy
Satisfied Customers: 13071
Experience:  Have owned a repair shop for 25 yrs.
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I have a P0300 Random - Mult. Misfire detected 99 chevy astro van replaced coil. spark p

Customer Question

I have a P0300 Random - Mult. Misfire detected 99 chevy astro van replaced coil. spark plugs, dist. cap & rotor.
code returns after 5 miles on the freeway
Submitted: 4 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 4 years ago.

hi

tell me how it runs out on the road......do you feel the missfire?please explain

thanks

Customer: replied 4 years ago.

It runs very well on surface streets and on the freeway. even better sence I have been replacing parts. I cannot feel the missfire at Idle or during acceleration. I would never know there was a misfire of not for the check engine light. This car passed smog 4 months ago.

Expert:  Tim's Auto Repair replied 4 years ago.

ok

i would think you may need to check the fuel pressure..these must have no less than 60psi at all times..a weak fuel pump can cause this code to show

thanks

Please ACCEPT my answer so I can get credit for my work.i don't receive commission unless you do ..I'm not always going to be giving you good news,so please don't let this stand in the way of you accepting my answer.it does not cost you more money.we will still be able to communicate.. Bonuses and positive feedback are appreciated!if you are not satisfied with my answer,please do not leave bad feed back,i will gladly opt out and let another expert handle the question.if i have sent any diagrams,please print them.they will only stay on here for an hour or so.PLEASE ASK IF YOU NEED MORE HELP

 

FUEL PRESSURE

Testing and Inspection

CIRCUIT DESCRIPTION
When you turn ON the ignition, the vehicle control module Vehicle Control Module (VCM) energizes the fuel pump relay which powers the fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM receives reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM also turns ON the fuel pump for 2 seconds when you turn OFF the ignition.

An electric fuel pump pumps the fuel through an in-line filter to the fuel meter body assembly. The pump attaches to the fuel level sender assembly inside of the fuel tank. The fuel pump provides fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel returns to the fuel tank by the fuel return pipe.

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Diagnostic Chart (Part 1 Of 5)

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Diagnostic Chart (Part 2 Of 5)

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Diagnostic Chart (Part 3 Of 5)

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Diagnostic Chart (Part 4 Of 5)

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Diagnostic Chart (Part 5 Of 5)

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

 

  1. When the ignition is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gauge should read 414-455 kPa (60-66 psi ). The spring pressure inside the fuel pressure regulator controls the fuel pressure.
  2. A fuel system that drops more than 34 kPa (5 psi ) in 10 minutes has a leak in one or more of the following areas:
    • The fuel pump check valve
    • The fuel pump flex pipe
    • The valve or valve seat within the fuel pressure regulator
    • The fuel injectors
    • A fuel pipe
  1. A fuel system that drops more than 14 kPa (2 psi ) in 10 minutes after being relieved to 69 kPa (10 psi ) indicates a leaking fuel pump check valve.
  2. Fuel pressure that drops-off during acceleration, cruise or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. You can diagnose a lean condition using a scan tool. If an extremely lean condition occurs, the heated oxygen sensors will stop toggling. The heated oxygen sensor output voltages will drop below 300 mV . The fuel injector pulse width will increase. IMPORTANT: Make sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications by the scan tool.
  3. When you open the throttle quickly, the manifold pressure increases (low vacuum). The fuel pressure regulator increases the fuel pressure as the engine vacuum drops in order to compensate for engine load.
  1. A rich condition may result from the fuel pressure being above 455 kPa (66 psi ). A rich condition may cause a DTC to set. Driveability conditions associated with rich conditions can include hard starting followed by black smoke and a strong sulfur smell in the exhaust.
  2. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.
  1. A lean condition may result from the fuel pressure being below 414 kPa (60 psi ). A lean condition may cause a DTC to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring. NOTE : Do not. allow the fuel pressure to exceed 517 kPa (75 psi ). Excessive pressure may damage the fuel pressure regulator.
  2. Restricting the fuel return pipe with the fuel pipe shut-off adapter causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the fuel system, the fuel pressure should rise above 455 kPa (66 psi ) as the valve on the fuel pipe shut-off adapter connected to the fuel return pipe becomes partially closed.
  1. If the injectors are not grounded, check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking. If checking the spark plug associated with a particular fuel injector for fouling or saturation does not determine that a particular fuel injector is leaking, use the following procedure:
  1. Remove the fuel meter body assembly. Refer to Fuel Meter Body Assembly Replacement
  2. Reinstall the engine compartment fuel feed and return pipes to the fuel meter body and the chassis fuel pipes. Refer to Fuel Hose/Pipes Replacement (Engine Compartment).
  3. Lift the fuel meter body assembly just enough to leave the fuel injector poppet nozzles in the ports.
  4. Pressurize the fuel system by using the scan tool fuel pump enable.
  5. Visually and physically inspect the fuel injector poppet nozzles for leaks.

 

Powertrain Controls Diagnosis
P0300

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 ECT is below -7°C (20°F) , misfire detection is delayed until ECT is more than 21°C (70°F) . If start-up ECT is more than -7°C (20°F) , misfire detection begins after a 5 second delay.

  • No active VS sensor DTCs
  • No active TP sensor DTCs
  • No active MAF sensor DTCs
  • No active CKP sensor DTCs
  • No active 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 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 condition could cause an intermittent misfire.

Check for the following conditions:

  • Check the IC control circuit for a 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 cross-firing is not occurring.
  • If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
  • Check for contaminated fuel 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.
  • Check HO2S for abnormal voltage readings.
  • Check for a vacuum leak as a possible cause of the engine misfire.
  • Sticking intake or exhaust valves can cause a misfire when the Engine is cold.

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.

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Diagnostic Chart (Part 1 Of 2)

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Diagnostic Chart (Part 2 Of 2)

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 cylinders consistently.
  3. When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.
Tim's Auto Repair, mechanic
Category: Chevy
Satisfied Customers: 13071
Experience: Have owned a repair shop for 25 yrs.
Tim's Auto Repair and 12 other Chevy Specialists are ready to help you
Customer: replied 4 years ago.
my printer ran out of ink. poor timing, but I accept your answer could you please reply to my e mail address while I buy more Ink XXXXX@XXXXXX.XXX
Expert:  Tim's Auto Repair replied 4 years ago.

hi

i can not receive your personal e-mail address...just let me know when you get more ink,,and i will resend

thanks

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