On going misfire issue

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  #1  
Old 01-10-11, 07:08 PM
Tolyn Ironhand's Avatar
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On going misfire issue

Posted before but starting a new one since the last post is quite old.

2000 Chevy Silverado 4x4 1500 pickup. 5.3 vortec. 103K miles. Getting trouble code P0300 random misfire. Went to a friends of mine shop (finally ) and put it on the scan tool. Showed a lot of misfires on 5 and 8, and a a few on #6. Odd thing is if the RPM's are kept under 2k it would not misfire. Get it to about 2200 RPMs and would misfire like crazy. Push it to about 2500 RPMs and it would stop. Then miss again at 2800, stop at 3000 RPM's. This was tested at stand still and on the road.
Pulled plugs and were quite worn so replaced plugs and wires. No change. Tried some injector cleaner. No change. Checked around the coils with a grounded tool and could not find anything wrong.

Last odd thing: There is no driveablitity issues. Runs good, cant hear or feel any thing wrong.

Any ideas would be helpful.
 
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Old 01-10-11, 07:15 PM
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Did you replace the cap and rotor?

Thats what was the issue with my s10 with 140,000 miles. Same thing random misfires. Plugs, wires, cap, rotor.... Runs lake a grape ape. Now if I can get better then 15 miles to the gallon thats another story.

Mike NJ
 
  #3  
Old 01-10-11, 07:51 PM
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Pretty sure it doesn't have one. It has 8 separate coils, one for each cylinder.
 
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Old 01-10-11, 08:53 PM
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Hmmm, your right.. I had that problem with a grand prix. I sold it. The injectors were screwed and it would cost more to fix then the car was worth. I had three coils on that. each coil was like $200. I think it was a coil though because 3 and 5 misfired and I think it was the same coil pack.

Try sea foam.

Sea Foam | Motor Treatment

I dont put in the crank though.

Mike NJ



Mike NJ
 
  #5  
Old 01-11-11, 09:09 PM
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A no cost way to verify cause of your misfire..........since you know what cylinderws are misfiring, at 2200 pull the coil to plug wire off #5 spark plug, or an old trick was to put a piece of rubber vacum tubing between the coil wire and coil. Use a 12v test light to ground the spark by touching the tubing with the probe of the test light, of course with the other end of the light hooked to ground.
If your miss did not get any worse, you have a bad coil. You could also swap the # 5 coil with say # 3 and see if your misfire code changes to # 3. Either way I bet you have 2 or 3 bad coils since you have already eliminated the plugs and wires. If you are still not sure, buy one new coil, not from the dealer, and put it in place of one of the misfire spots and check the results. Then go buy two more.
 
  #6  
Old 01-21-11, 02:58 PM
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If you definately have no misfire but the scan tool is showing a misfire, replace the crank sensor. The crank sensor is used to determne misfires. When it starts to fail it can start recording misfires when there are none. After replaceing the sensor, you must use the scan tool to perform 'crank relearn'.
 
  #7  
Old 01-21-11, 03:12 PM
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Plugged CAT.. check for backpressure at higher rpms

Collapsed Catalytic Converter - Bob Is The Oil Guy
 
  #8  
Old 01-21-11, 04:08 PM
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Originally Posted by 2granddaughters View Post
Plugged CAT.. check for backpressure at higher rpms

Collapsed Catalytic Converter - Bob Is The Oil Guy
Thanks for your continuing help on this one! I doubt the cat is plugged because there are no drive-ability issues. Runs and drives great. Been driving the truck since I posted and it just threw the code again today (70 mph = 2100-2200RPMs )

We did do an injector cleaner product but it was not Seafoam. Some other kind that this shop has good luck with.

I will try the coil check 28yrs. Maybe when it gets a little warmer
 
  #9  
Old 01-30-11, 12:33 PM
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do a crankshaft variation relearn and it will be OK
 
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