P0303

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May 14, 2015
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#1
Make
Chevrolet
Model
Colorado WT
Year
2008
Miles
126000
Engine
2.9L
Gentlemen I need troubleshooting/diagnostic advice from people with substantially more engine management skills than I have. I have been chasing a random,intermittent P0303 for a couple weeks now that happens under acceleration or engine load. It may occur at idle and steady cruising but I do not notice it, the CEL comes on during acceleration. I drive this truck 50-200 miles a day and the reason I say random/intermittent is that some days the engine runs perfect, some days it misses all day from the first start in the morning and other days it misses at random times.
What I have done so far:
Replaced plugs with o.e. Delco. Still P0303
Swapped #2 & #3 coil. Still P0303
Swapped #1 & #3 plug & coils. Still P0303
Checked compression 200-225 all cylinders
Checked fuel pressure 60 psi
Checked for vacuum leaks with carb cleaner and propane none found. Is it possible there is an intermittent vac leak affecting cyl #3 that was not evident during testing that could cause a misfire?
Swapped #2 & #3 injector
Tried different brands and grades of gas
Wiggle check harness, could not create miss but again I do not notice it unless engine is under load. Harness is unmolested and properly installed as it came from the factory.
Temperature both vehicle & ambient seem to be irrelevant.
Truck is bone stock other than tires, belt, brakes, oil, and air filter.
This is a lot to read but I wanted you to have as much info as I could give. Thanks in advance.
 
Joined
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#2
From what you say, it sounds like you have not swapped out the spark plug wires yet. Does it have wires or is this a COP type?
 

nickb2

Wrench. Diagnostic Tech.
Joined
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#3
Try using a combustion chamber cleaner. This often helps with intermittent misfires. Also, if the engine oil is not a good one or improper grade, that also can wreak havoc on the VVT system.

You are following proper misfire diagnosis protocol. Most misfires will occur under load, unless a bad coil or plug. Normally, vacuum will be low under load. Somewhere around 5. Idling, somewhere around 20-22.

This has me thinking exhaust or intake restriction or a mechanical problem. A weak valve spring or a gummed up cylinder maybe.

The cam sensor is the one that actually reports which cylinder is misfiring. I am just throwing ideas out there. You could have a defective tone ring for the cam sensor. Sometimes, vibration form the drive train could cause a sensor to fluctuate also. Are you noticing any abnormal sounds coming from engine under load? An exhaust leak at the # 3 cyl could also cause a misfire. What are fuel trims at time of misfire.?
 
Joined
Mar 1, 2002
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#5
Any chance rain or carwash is a factor? Found this TSB... sorry if it is hard to read...
Condition
Some customers may comment that the malfunction indicator lamp (MIL) is illuminated.


Some customers may also comment that the engine runs rough.


The technician may find DTCs P0300, P0301, P0302, P0303, P0304, P0305, or P0306 set as current or in history, depending on which engine the vehicle is equipped with.


Upon further examination, the technician may observe evidence of water intrusion into the spark plug recess area.

Cause
This condition may be caused by an engine being exposed to a large quantity of water, which may result in some of the water seeping down past the vented threads of an ignition coil bolt, then flowing through the vent opening that is located in the bottom of the related ignition coil bolt hole, and collecting in the spark plug recess area.

Correction
Important: DO NOT reprogram or replace the engine control module (ECM) for this condition. If evidence of water intrusion in the spark plug recess area/s is not observed, then refer to the appropriate DTC procedures in SI.

  1. Observe the Misfire Current Counters parameter with a scan tool, in order to determine the affected cylinder(s).
  2. Remove the air cleaner resonator and outlet duct. Refer to Air Cleaner Resonator and Outlet Duct Replacement in SI.
  3. Disconnect the engine wiring harness electrical connectors from the affected ignition coil(s). Refer to Ignition Coil Replacement in SI.
  4. Remove the ignition coil bolt(s) and ignition coil(s) from the camshaft cover. Refer to Ignition Coil Replacement in SI.

  5. Note: If water is observed in ANY of the spark plug recess areas, then it will be necessary to apply Loctite® 565 PST Thread Sealant* to ALL of the ignition coil bolt hole vents and bolt threads.
  6. Inspect for water intrusion in the spark plug recess areas.

  7. Warning: Wear safety glasses when using compressed air, as flying dirt particles may cause eye injury.
  8. If any water is present, then remove the water from the spark plug recess areas with low pressure air.
  9. Remove the affected spark plug(s) from the cylinder head. Refer to Spark Plug Replacement in SI.
  10. Replace ONLY the affected spark plug(s) with new spark plug(s) as needed into the cylinder head. Refer to Spark Plug Replacement in SI.


  11. Fill the bottom of the ignition coil bolt hole vent with Loctite® 565 PST Thread Sealant* as indicated in the illustration, and apply it to the ignition coil bolt threads also.
  12. Replace ONLY the affected ignition coil(s) with new ignition coil(s) as needed. Refer to Ignition Coil Replacement in SI.
  13. Connect the engine wiring harness electrical connectors to the ignition coil(s). Refer to Ignition Coil Replacement in SI.
  14. Replace the air cleaner resonator and outlet duct. Refer to Air Cleaner Resonator and Outlet Duct Replacement in SI.
  15. Clear the DTCs with a scan tool.
 
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#6
Thank you for the replies.
Jackc it is coil on plug as nickb2 stated.
Nickb2, I will try a combustion cleaner nothing to lose. No abnormal engine noise, oil I use is Castrol 5-30. When you say defective tone ring can that cause a misfire? or just give a false misfire code. Exhaust is quiet but ll check around manifold for leaks. I can probably borrow a scanner that gives live data but does not record, might be hard to observe fuel trims but I will try and post results.
Mobiledan I have owned this truck about a year and never washed the engine or drove through puddles deep enough to flood top of engine so probably can rule out that tsb besides it was drry when I first changed the plus and swapped parts around. What baffles me is the sudden onset of the misfire and the inconsistent randomness of it happening.
Thank you all