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Tim's Auto Repair
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Experience:  Have owned a repair shop for 25 yrs.
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98 chevy astro: fuel injector..converter..wires, cap and rotor r

Resolved Question:

i have 98 chevy astro it has misfire, i changed the fuel injector but it still miss i can hear a rattle under the car in the catilitic converter can the conveter be bad and causing the misfire, plugs, wires, cap and rotor r about four month old
Submitted: 7 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 7 years ago.
please explain to me the miss...does the engine light come on?if so,can you give me the computer codes?
Customer: replied 7 years ago.

i have a p0300 code and it flashes as u drive. i check easch wire and it has good spark fuel presure is good


Expert:  Tim's Auto Repair replied 7 years ago.
how much fuel pressure do you have?
Customer: replied 7 years ago.
dont recall i will have to get back to u on that
Expert:  Tim's Auto Repair replied 7 years ago.


let me know


Customer: replied 7 years ago.

fuel presuure was 55 at idle and 65 psi at wot


Expert:  Tim's Auto Repair replied 7 years ago.


usually when the sel is indicates a bad catalyst converter,,,please print the test charts below


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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 VC 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.


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