2015 Tahoe P0300 Random Misfire - Rough Idle and Flashing CEL

Question

My Tahoe started shaking pretty bad at stoplights and the check engine light started flashing yesterday. It seems to smooth out a bit once I get up to highway speeds, but the idle is terrible and I can smell unburnt fuel near the tailpipe. I am worried about damaging the catalytic converters if I keep driving it like this.

Chris Donovan · Accepted Answer

The issue turned out to be a cracked porcelain insulator on the number 6 spark plug and a failing fuel injector on cylinder 1. Even though I just changed plugs, I must have over-torqued one. I replaced the faulty injector with a new Bosch unit (Part #12668390) and swapped the cracked plug. For the injectors on these LT engines, make sure you use a new seal kit and torque the fuel rail bolts to 10 lb-ft plus a 45-degree turn. The truck is now idling perfectly and all fuel trims are back within +/- 3%.

Lina Brooks · Answer

Since you have P0171 and P0174 lean codes alongside the P0300, you should check for a vacuum leak. These EcoTec3 engines are known for intake manifold gasket leaks or a cracked PCV hose which will lean out the mixture at idle.

wrenchqueen · Answer

Pull the valve covers and check for a collapsed lifter or a bent pushrod. These GM AFM/DFM engines have frequent lifter failures that can cause a permanent misfire on specific cylinders, though usually that throws a P0301-P0308 code specifically.

Tom Patel · Answer

Check your fuel trims using a live data scanner. If the LTFT is high at idle but drops when you rev it to 2500 RPM, it is definitely a vacuum leak; if the trims stay high or get worse, you might have a failing high-pressure fuel pump (HPFP).

2015 Tahoe P0300 Random Misfire - Rough Idle and Flashing CEL

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