DTC P0300 Random Misfire - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > Engine Tunes/Mods

Engine Tunes/Mods Performance Engine Tuning/Engine Modifications

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  #1  
Old 02-28-2010, 02:38 PM
tmcat tmcat is offline
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2003 GMC Envoy SLE
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DTC P0300 Random Misfire

I have an '02 Envoy, 4.2L with 99,000 miles. I now have a Random misfire problem. I have performed the TB cleaning, and fuel injector replacment.

What is the most common problem for this? The misfire occurs randomly during idle, and quite significantly under a part throtle load. Once it downshifts, the misfire disappears.

Could this be an EGR valve?
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  #2  
Old 02-28-2010, 03:14 PM
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wardak33 wardak33 is offline
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We don't have EGR valves on our engines. My advice to you is to change your spark plugs with ac delco iridiums (41-103). The manual states that spark plugs should be changed at 100k miles, and you're close to that mark. That should fix your problem, but you should also inspect air/fuel filters, fluid changes, etc. Good luck!
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  #3  
Old 02-28-2010, 03:27 PM
tmcat tmcat is offline
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Thanks for the reply.

I changed the plugs at about 77k. I did used AC Delco Iridium plugs.

You're right on the EGR. I was checking the manual, and it does not show an EGR valve.

Would the O2 sensor (pre Cat) be at fault? I believe the voltage is supposed to be between 0.1 and 0.9 volts.
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  #4  
Old 02-28-2010, 03:35 PM
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wardak33 wardak33 is offline
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Yes, an oxygen sensor can cause misfires. I don't know the specific voltage range values, but I think you are correct.
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  #5  
Old 03-01-2010, 12:53 AM
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Chickenhawk Chickenhawk is offline
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The front O2 sensor output should vary rapidly between 200 mV and 900 mV. It is read by a scan tool. (Don't try measuring output with a multimeter; it won't work.)

Are you getting a trouble code for a random misfire? If so, you should pull all your plugs again, make sure they are 41-103 AC Delcos and that all the coil packs are attached properly. (They are very often not attached properly after a plug change.)

You can also look at the cam position sensor actuator as it may be missing a screen or need cleaning.

If this doesn't help, you should get a spark plug tester and test each coil pack for good spark.


If you are NOT getting a trouble code, I would have your fuel pressure tested for possible fuel pump failure.
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Old 03-01-2010, 10:24 AM
tmcat tmcat is offline
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My DTC is P0300 (Random Misfire).

The MIL flashes on when the engine starts to misfire under a load.

I changed the plugs to AC Delco 41-103 several months ago. I had a temperature problem, but this has been resolved with a new thermostat.

I will check the cam position sensor (CMP) to see if it is dirty. But the car runs fine except for an occasionaly "hiccup" at idle and when it's under a load.

My scanner shows the O2 sensor output changing a lot and quite rapidly. Is this normal?

Any other ideas??
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  #7  
Old 03-01-2010, 05:23 PM
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Chickenhawk Chickenhawk is offline
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Coil packs.
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  #8  
Old 03-01-2010, 06:18 PM
cbwilsha cbwilsha is offline
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A code P0300 may mean that one or more of the following has happened:

* Faulty spark plugs or wires
* Faulty coil (pack)
* Faulty oxygen sensor(s)
* Faulty fuel injector(s)
* Burned exhaust valve
* Faulty catalytic converter(s)

* Faulty camshaft position sensor
* Defective computer

C.B.
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  #9  
Old 03-01-2010, 11:23 PM
tmcat tmcat is offline
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Thanks, CBWILSHA, for the information.

Wouldn't the code be more specific if the problem were a specific spark plug wire, plug or coil. In this GM computer code, it should show a specific cylinder where the misfire is taking place. I would think the "generic" code (P0300) would be something that is "common" with all cylinders.

Just trying to understand why the Random Misfire code is appearing.
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  #10  
Old 03-01-2010, 11:41 PM
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wardak33 wardak33 is offline
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the code stands for Random/Multiple Misfire, so it may be more than one cylinder thats misfiring. Test each coil pack for a good sharp spark, and check the conditions of the spark plug while you're at it.

Have you checked the oxygen sensor values yet?
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