Here is a bit of reference on the subject. It says in rare instances, but it is more common than you think. Typically it occurs on cylinders more toward the rear. I doubt that the head is cracked, at least I have yet to see one on the several cylinder heads that I have replaced. I have even replaced several heads on 07 H3's as well.
#PIP3935D: Misfire At Idle Due To Leaking Intake Valves - Supplement To 07-06-01-011 - keywords 2.8L 3.5L compression controls cylinder driveability DTC engine excessive head high L52 LK5 leakage leakdown low - (Mar 18, 2008)
Subject: Misfire at Idle due to Leaking Intake Valves - Supplement to 07-06-01-011
Models: 2004-2005 Chevrolet Colorado, GMC Canyon
2006 Chevrolet Colorado, GMC Canyon
and Last 6 of VIN Less than 287698
2006 Hummer H3
and Last 6 of VIN Less than 287591
This PI was superseded to update model years and engines, and to revise the repair recommendations. Please discard PIP3935C.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
On rare occasions, a SES light and P0300 may be experienced due to a single cylinder misfire at idle that may or may not be felt. This may occur before the VIN breakpoints above due to non-uniform intake valve seat wear, which may prevent the intake valve from sealing properly. Vehicles built after these VIN breakpoints contain harder intake valve seats to address this concern. If the engine misfires occur off of idle, this PI does not apply. Bulletin 07-06-01-011 contains a labor operation and related tips for performing a cylinder leakage test to diagnose this concern but it does not indicate what repairs should be performed if a leaking intake valve is found.
If the SI diagnostics do not isolate a cause for this concern, refer to the latest version of 07-06-01-011 and the "Cylinder Leakage Test" procedure in SI to perform a cylinder leakage test. If a leaking intake valve is found, replace the cylinder head by following the latest version of 06-06-01-017, which contains the necessary replacement procedure and part numbers. If the vehicle is a 2006 model, install the latest PCM calibrations to complete the repair. The cylinder head received from GMSPO contains harder intake valve seats to address this concern.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
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Monday, December 7th, 2009 AT 9:25 PM