Why does my engine misfire

Hi Paul, I own a 2004 Ford Explorer which has been running well until recently when the engine started misfiring and hesitating. My mechanic has replaced the spark plugs, fuel filter, air flow meter, EGR valve and sensor, oxygen sensors and three ignition coils which parts were aged and randomly reported by the computer diagnosis. There was a slight improvement in power, however the hesitation and poor performance persist. Kindly advise which box we haven’t ticked. George Kalule

Hello Kalule, you have pretty much changed almost every component or part that affects the engine emission system. However, you have not checked or replaced the PCV valve located at the rear top part of one of the engine banks.

The PCV is the venting or release point for excess engine blow by gases. An engine relies on these gases to help build compression. Excess gases are released through this valve to avoid build up or are contained to maintain a good balance.

Either way a blocked or leaky PCV affects compression and causes poor engine performance. The PCV valve can fail due to age or premature blockage from oil with sludge. To avoid this damage use motor oil designed with detergency and anti-sludge additives during service.