2015 Tahoe P0300 Misfire at Idle - Stumbles but No Specific Cylinder Code

Question

My 2015 Tahoe is idling really rough and the check engine light started flashing yesterday when I was stopped at a red light. It seems to smooth out a bit once I get up to highway speeds, but at every stop sign it feels like the engine is about to jump out of the mounts. The scanner only shows a P0300 random misfire and not a single cylinder code like P0301 or P0308.

garagegrease · Accepted Answer

Found the issue was a cracked spark plug wire on cylinder 6 and a small vacuum leak at the EVAP purge solenoid. I replaced all 8 ignition wires with a new set of GM Genuine 19351583 wires and swapped the purge valve (Part #12690512). The torque spec for the solenoid bracket is 89 lb-in. Total cost was about $110 for parts and it idles perfectly now with zero misfires on the live data stream.

Lina Brooks · Answer

You should check your fuel trims. If the Long Term Fuel Trims are climbing above +15% at idle but drop back down when you rev it to 2500 RPM, you definitely have a vacuum leak or a failing PCV valve.

Carlos Diaz · Answer

Since it is a 2015, check the spark plug wires specifically for heat damage. Even if they look okay, the internal resistance can increase and cause a weak spark that the PCM can't pinpoint to one cylinder.

backyard_bart · Answer

Check your intake manifold bolts and gaskets. These Gen 5 engines are known for vacuum leaks around the manifold which will cause a P0300 at idle but disappear under load when the vacuum signal drops.

2015 Tahoe P0300 Misfire at Idle - Stumbles but No Specific Cylinder Code

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