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JOHN aka Eauto
JOHN aka Eauto, The Car Guy
Category: Chevy
Satisfied Customers: 12944
Experience:  ase tech
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2000 Chevy Astro: with the 4.3 vortec V6..runs fine..cruise..Goes away

Customer Question

I have an intermittent problem with my 2000 Chevy Astro with the 4.3 Vortec V6. Sometimes it runs fine, sometimes there is a bad stumble at cruise speed. Goes away under full throttle or going up hills. Resumes on downhill or on level ground. Code scanner reveals code P0300, Random Cylinder Ignition Misfire. Also tends to run very rich at random occasions. What is going on with this, and how do I fix it?
Submitted: 5 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 5 years ago.

hi

a good tune up may fix this problem.....new plugs,wires,rotor and distributer cap...do it all at the same time

thanks

Customer: replied 5 years ago.

I have already replaced the plugs, cap, rotor and plug wires (all at the same time) and the problems remain. Also replaced fuel filter at the same time.

 

Even if plugs, wires, cap and rotor were faulty, the symptoms would not go away while under full throttle or while driving up hill. So the suggestion of tune up with cap, rotor, wires and plugs doesn't help me.

Expert:  Tim's Auto Repair replied 5 years ago.

ok

let ne send you a chart to break down the problem...a egr valve could cause this at low rpm's

thanks

Powertrain Controls Diagnosis

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.

Diagnostic Chart (Part 1 Of 2)
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.
Account | Vehicle
Customer: replied 5 years ago.
Relist: I prefer a second opinion.
First mechanic didn't really address the problems I listed.
Expert:  Tim's Auto Repair replied 5 years ago.

ok

i will opt out

thanks

Customer: replied 5 years ago.

I have an intermittent problem with my 2000 Chevy Astro with the 4.3 Vortec V6. Sometimes it runs fine, sometimes there is a bad stumble at cruise speed. Goes away under full throttle or going up hills. Misfire and stumble resumes on downhill or on level ground. Code scanner reveals code P0300, Random Cylinder Ignition Misfire. Also tends to run very rich at random occasions. What is going on with this, and how do I fix it? What would cause the intermittent random misfire when the engine is under low load?

 

I have already replaced the distributor cap, rotor, plug wires and spark plugs, as well as the fuel filter but this didn't fix the problem. I need to know specifically what components to look at to try to fix this, as I don't have the money to start replacing components one at a time until the problem goes away.

Expert:  JOHN aka Eauto replied 5 years ago.
Have you removed the coil and phsically inspected it for cracks or damage?
Customer: replied 5 years ago.
I have not removed or inspected the coil yet, but would a coil problem go away while driving up a hill or under full throttle conditions? As long as the engine is under a heavy load, it seems to be fine with no misfire or stumble. I will remove and check the coil this afternoon to see if there are any cracks or damage, although I don't suspect I will find anything wrong with it.
Expert:  JOHN aka Eauto replied 5 years ago.
They are common to crack the uphill thing probably not , I would check fuel pressure with a gauge you may have a fuel delivery issue.
Customer: replied 5 years ago.

If I have a fuel delivery issue, why would the OBDII scanner indicate a Code P0300, random cylinder misfire and not indicate a fuel delivery trouble code?

 

I spent all afternoon checking out various sensors and components according to my Haynes service manual. I checked the coolant temperature sensor, intake air sensor, crankshaft position sensor, MAP sensor, MAF sensor, EGR valve, and camshaft position sensor. Also checked the coil for cracks and found nothing wrong with it. When the engine runs poorly, it acts like a vacuum leak but I can find no vacuum lines or hoses on this engine to check. Everything on this engine (4.3 Vortec V6 with CMFI injection system) seems to be either electrically or electronically operated.

 

Every thing checked out fine according to the manual except for some erratic readings from the camshaft position sensor. Could a camshaft position sensor cause a code P0300 Random Cylinder Ignition misfire, and be an intermittent problem? Could this cause the engine to run rich also?

 

A few months back I replaced the distributor cap, rotor, wires, spark plugs and also the fuel filter all at the same time but none of this helped the problem. I'm running out of things to check... What else could I check that could cause intermittent problems with random ignition misfire and random running excessively rich? Are there any TSB's or other known problems on the 2000 AstroVan for problems like this?

 

Billy

 

Expert:  JOHN aka Eauto replied 5 years ago.
yes the crank and the cam sensor read off each other
Customer: replied 5 years ago.

 

I plan to go later and buy a new camshaft position sensor and install it to see if that fixes the problem.

 

But if I replace the camshaft sensor and that does not fix the problem, what would you suggest would be the next thing to look at that would cause these problems?

 

Billy

 

Expert:  JOHN aka Eauto replied 5 years ago.
the m,ain causes of miss fires are primary ignition failure, bad compession, weak cylinders, or fuel injection issues
Customer: replied 5 years ago.

Ok. But given that the van seems to run much better under heavy load or under full throttle, would you say that we could rule out bad compression or weak cylinders or primary ignition failure?

 

Is there a special fuel pressure gauge that I need for checking fuel pressure on a GM?

Expert:  JOHN aka Eauto replied 5 years ago.
no a standard fuel pressure gauge will work it threads to the sharader valve on the fuel rail on the intake
JOHN aka Eauto, The Car Guy
Category: Chevy
Satisfied Customers: 12944
Experience: ase tech
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