How do I diagnose a misfire that only happens under load?
Short answer: Misfires under load often come from weak ignition coils, worn spark plugs, low fuel pressure or volume, or lean conditions; use misfire counters, fuel trims and a road test under load to find the pattern.
Ignition components that work at idle can fail under load, when cylinder pressure is higher and more voltage is needed to fire the plug. Weak coils, worn plugs with wide gaps, cracked plug insulators and boot carbon tracking are common causes. Fuel delivery problems, such as a weak pump or clogged filter, and lean conditions from restricted injectors can also appear only under load.
Use the scan tool's misfire counters during a road test under the conditions that cause the problem. Note whether one cylinder or several are affected. A single cylinder points to that cylinder's coil, plug or injector; random misfires point to fuel, air or a shared ignition issue.
Swap the coil to another cylinder to see if the misfire follows, and inspect the plugs. Check fuel pressure under load, and review fuel trims. Have a helper drive while you monitor data, and choose a route where load testing can be done safely and legally.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.