A random misfire code can be set on newer vehicles with OBD II onboard diagnostics when multiple misfires occur randomly in multiple cylinders. The cause is typically a vacuum leak in the intake manifold, throttle body or vacuum plumbing, a defective Exhaust Gas Recirculation (EGR) valve that is leaking exhasut into the intake manifold, or even bad gasoline. Less common causes include bad spark plug wires, worn or fouled spark plugs, a weak ignition coil, dirty fuel injectors, low fuel pressure, or weak valve springs. If a misfire is occuring in only one or two cylinders, you will usually find a misfire code for that specific cylinder rather than a random misfire code.
Start checking all the vacuum lines then EGR Valve, The EGR valve should remain CLOSED when the engine is COLD and IDLING. When the engine is warm and is accelerating or working hard under a load, the EGR valve should OPEN. With vacuum-operated EGR valves, intake vacuum pulls the valve open when conditions are right. SOme EGR valves also sense exhaust backpressure and do not open until backpressure reaches a certain level. On others, the EGR valve is electronic and is controlled by the PCM. No vacuum is used to operate the valve.
When the EGR valve opens, it allows a small amount of exhaust gas to be siphoned back into the intake manifold. This dilutes the air/fuel mixture, cools combustion and reduces the formation of oxides of nitrogen (NOx). If the EGR valve fails to close when EGR is not required, or if it leaks because of carbon buildup on the valve stem or valve seat, exhaust will be sucked into the intake manifold all the time creating a lean fuel mixture. This will cause lean misfire and set a random misfire code. Cleaning the EGR valve seat and stem may eliminate the sticking problem. If this fails, replace the EGR valve.
Also Random misfires can occur if the fuel injectors fail to delivery their normal dose of fuel to the engine. This creates a lean air/fuel mixture and increases the risk of lean misfire. The underlying cause may be dirty fuel injectors (a buildup of varnish deposits in the injector nozzles that may occur over time from using low detergent gasoline or frequent short trip stop-and-go driving), or low fuel pressure caused by a weak fuel pump, defective fuel pressure regulator or dirty fuel filter.
As you can see there s to many things to check, start with the easy-ones.
Wednesday, February 6th, 2008 AT 9:48 PM