The P0300 code in your 2018 Toyota Tundra indicates random or multiple cylinder misfires, and the rough idle confirms the engine is struggling with consistent combustion. Your Tundra's 5.7L V8 (3UR-FE) is generally reliable, but misfires can occur from various causes. A random code suggests system-wide issues rather than single-cylinder problems.
V8 Misfire Considerations
With eight cylinders, a random misfire code means the issue affects multiple cylinders without pattern. This points toward common systems like fuel delivery, ignition system backbone, or air intake rather than individual components like one coil or injector.
Common Causes in the 5.7L V8
Ignition coil failures can occur, though all eight failing simultaneously is unlikely—more likely is secondary ignition system issues. Vacuum leaks create lean conditions affecting all cylinders. Fuel quality or delivery problems affect combustion. Carbon buildup on intake valves (less common with port injection). Crankshaft or camshaft position sensor issues.
Diagnostic Approach
Using Toyota Techstream, review misfire counters for all eight cylinders. If misfires are relatively even, focus on common systems. Check for vacuum leaks. Inspect all ignition coils for cracks or corrosion. Test fuel pressure and injector balance. Check crankshaft position sensor signal quality.
Secondary Ignition System Check
Inspect all eight coil boots for carbon tracking or damage. Check spark plugs for wear or deposits—though iridium plugs should last 100,000+ miles, oil contamination or other issues can affect them earlier. Verify coil connector integrity and wiring condition.