A smooth idle does not rule out a serious running fault. During acceleration the engine takes in more air, cylinder pressure rises and the ignition and fuel systems have to work harder. A component that copes at idle may fail under load.
Ignition faults on petrol engines
Worn spark plugs, an excessive plug gap, a weak coil or damaged insulation can break down only when voltage demand increases. Swapping coils between cylinders can sometimes help, but it must be supported by fault counters and plug inspection; otherwise several unrelated faults can be confused.
Fuel delivery
An injector with restricted flow or a fuel-pressure system that cannot maintain demand may create a lean misfire during acceleration. Checking only idle pressure can miss it. Recorded live data during the event is more useful.
Boost and air leaks
On turbocharged engines, a split hose or leaking connection may open as boost rises. The car can idle normally because the system is not pressurised. Smoke testing and comparison of requested versus actual boost can locate the problem.
Mechanical causes
Low compression, valve sealing problems or incorrect cam timing may be most noticeable under load. Carbon deposits can also affect airflow or valve operation on some direct-injection engines.
Avoid repeated hard testing
A persistent misfire can overheat a catalytic converter and dilute engine oil with fuel. If the warning light flashes, reduce load and stop driving as soon as it is safe.
Capital Garage combines fault-code information with live data and practical tests. See our diagnostic service or book online.
Still not sure what is causing the problem?
Symptoms can overlap between several faults. A proper inspection and diagnostic process is usually cheaper than replacing parts based on guesswork.