2000 GMC Jimmy

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  • 2000 GMC JIMMY

Engine Performance problem
2000 GMC Jimmy 6 cyl Four Wheel Drive Automatic 12399 miles

What is po300 ramdom/multiple cilinder miss fire detected?

Do you
have the same problem?
Thursday, July 22nd, 2010 AT 8:36 PM

1 Reply

  • 4,228 POSTS

I'll give you the information on P0300 from service info. The question is have you felt a misfire. This could happen under certain circumstances, like cold only, under load, at idle only, to name a few. It has to register enough misfire counts to set the code.

DTC P0300 Engine Misfire Detected
Refer to Engine Controls Schematics

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 after 3 consecutive drive 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.

" 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 Conditions and Poor Connections in Wiring Systems.

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Thursday, July 22nd, 2010 AT 11:36 PM

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