What actually causes misfires on these engines
Coils are wear items. Every heat cycle and every vibration degrades the internal insulation until it breaks down under load — which is why a misfire often shows up first in wet weather, when moisture tracks across a coil that was marginal anyway.
Spark plugs have an interval, normally around 60,000 km, and a worn plug puts extra load on the coil until one of them gives.
Oil leaking from a valve cover gasket into the plug tube fouls both the plug and the coil boot. This is the cause that gets missed, and it's the reason coils keep failing.
Less common: low compression, a faulty injector, or wiring insulation that has chafed. All real, all worth ruling out before you buy parts.
The order we work in
Read the codes and note which cylinder, then swap that coil with one from another cylinder and re-test. If the misfire follows the coil, you have your answer for the price of a coil.
Inspect the plug: condition, colour and gap tell us whether it was fueling, oil or simply age.
Look down the plug tube for oil. If there's oil there, replacing the coil without fixing the gasket just schedules your next visit.
Only if the misfire stays put do we do a compression and leak-down test, which we include in the diagnosis rather than billing separately.
Indicative cost
Coils and plugs: $400–$950 depending on cylinder count and part level. Valve cover gaskets with plug tube seals: $600–$1,400.
On coils specifically: fit OEM or a brand we've seen hold up, and we'll tell you which. Cheap aftermarket coils are the single most common reason the same car comes back within three months.
Questions owners ask about this
- Can I keep driving with a misfire?
- Not for long. Unburnt fuel goes into the exhaust and destroys the catalytic converter, which turns a $500 repair into a several-thousand-dollar one. If it's misfiring hard enough to shake the car, stop.
- Do I really need all the coils replaced?
- On a high-mileage engine, usually yes — the one that failed is the same age and design as the ones that will follow it, and labour is mostly shared. On a lower-mileage car with one coil that failed early, replacing the plug and that coil is often the honest answer, and we'll say so.
- Why does it only misfire when it rains?
- Moisture tracks across a coil or plug boot whose insulation is already breaking down. The water isn't causing the fault — it's exposing one that's already there.