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GM Tech Tip: Cold Start Misfires

An owner may complain of misfire or roughness during a cold start. Also, a P0300 code might be present, or there may be misfire codes limited to one or two cylinders. Looking at the misfire counter on a scan tool, you will see a high rate on typically one cylinder. The cause of the misfires might be a coolant leak that allows coolant to enter the cylinder at the point where the liner meets the deck face cast.

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Small surface pockmarks or pitting appearance on the deck surface is normal and engines should not be replaced. This appearance is normal and is not connected to the coolant passage.

MODELS
Buick Enclave 2009-2017 3.6 LLT
Buick LaCrosse 2009-2016 3.0 LF1+LFW+LLT
Cadillac ATS 2013-2018 LFX, LF4
Cadillac CTS 2010-2015 3.0 LF1+LFW, 3.6 LFT+LFX+LLT
Cadillac SRX 2010-2015 2.8 LAU, 3.0 LF1, 3.6 LFX+LLT
Cadillac XTS 2013-2018 3.6 LF3+LFX
Chevy Camaro 2010-2015 3.6 LFX+LLT
Chevy Captiva 2012-2014 3.0 LF1+LFW
Chevy Colorado 2015-2016 3.6 LFX
Chevy Equinox 2010-2017 3.0 LF1+LFW, 3.6 LFX+LLT
Chevy Impala 2012-2018 3.6 LFX
GMC Canyon 2015-2016 3.6 LFX
GMC Terrain 2010-2017 3.0 LF1+LFW, 3.6 LFX+LLT

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An owner may complain of misfire or roughness during a cold start. Also, a P0300 code might be present, or there may be misfire codes limited to one or two cylinders. Looking at the misfire counter on a scan tool, you will see a high rate on typically one cylinder. The cause of the misfires might be a coolant leak that allows coolant to enter the cylinder at the point where the liner meets the deck face cast.

The photo above is an example of porosity in the cylinder. Cylinder deck to cylinder liner interface location can cause a leak path into the cylinder when it is warm. Such porosity is rare. Technicians should inspect the subject cylinder and look for a break in the carbon ring at the top of the cylinder. Usually, if porosity is present, there will be a break in the carbon ring.
Coolant present in a cylinder cleans the area of entry and can be found by wiping the carbon ring dry with a clean soft towel, exposing the leak path. Only if porosity is found, engine replacement becomes necessary.

INSPECTION
To inspect for this problem, add coolant dye to the system. Next, run engine through warm up and pressurization of the cooling system. Allow the engine to cool or cold soak and inspect the suspect cylinder with a borescope for coolant dye.

In some cases, it might be necessary to remove the head for inspection. Do not confuse fuel residue as coolant. This is the reason for using the dye to confirm the presence of coolant.

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It is hard to see the actual source that could be a pinhole, but it usually streams down the liner so that you can see it with a borescope.
If this concern is present, do not replace the cylinder head because that will not repair this concern.

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