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The crankshaft position sensor senses the misfires caused by speed fluctuation and report this to the computer at which time the PCM will lock the cylinder for identification in its random access memory
If your CHECK ENGINE light is on and you have found a P0300 diagnostic trouble code (DTC) by plugging a scan tool or code reader into your vehicles diagnostic connector, your engine has a "random misfire" problem.
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.
QUICK DIAGNOSTIC CHECKS
Start by checking engine intake vacuum. Connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading.
On most engines, intake vacuum should be steady between 16 and 22 inches. A lower reading usually indicates a vacuum leak, but it might also indicate an exhaust backpressure problem (such as a plugged catalytic converter), worn valve guides or weak valve springs. A vacuum reading that gradually drops while the engine is idling almost always points to an exhaust restriction. An oscillating vacuum reading usually indicates a leaky valve or worn valve guides.
If the engine is experiencing any of the following symptoms, a vacuum leak is probably causing the P0300 random misfire DTC:
Too fast an idle speed. The powertrain control module (PCM) will maintain normal idle speed and compensate for a small vacuum leak by closing down the throttle body air bypass. But if the leak is too large and the idle control system cannot compensate for the extra air, the engine may idle too fast. Common leak paths include the throttle body gaskets, intake manifold gasket, any of the engine vacuum fittings, hoses or vacuum-operated accessories (such as the power brake booster or EVAP canister purge valve). It is even possible that leaky O-rings around the fuel injectors may be allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft.
A rough idle or stalling. A large vacuum leak can lean the air/fuel mixture out to such an extent that an engine will not idle at all. An EGR valve that is stuck open at idle can have the same effect as a vacuum leak. So too can a loose positive crankcase ventillation (PCV) hose, a leaky PCV valve or the wrong PCV valve (one that flows too much air for the application). If somebody replaced the PCV valve recently, they may have installed the wrong PCV valve. The rough idle in all of these cases is caused by "lean misfire." The fuel mixture is too lean to ignite reliably so it often misfires and fails to ignite at all. Lean misfire will show up as elevated hydrocarbon (HC) readings in the exhaust, which may be enough to cause a vehicle to fail an emissions test.
Hesitation or misfiring when accelerating. This may be due to a vacuum leak, but it can also be caused by dirty fuel injectors, a weak fuel pump (low fuel pressure) or a faulty fuel pressure regulator. It can also be caused my ignition misfire due to worn or fouled spark plugs, bad spark plug wires, or weak ignition coil.
The important thing to keep in mind about vacuum leaks is that they have the most noticeable effect at idle. At part and full throttle, more air is entering the engine through the throttle opening so a vacuum leak has less effect on the air/fuel ratio.
Monday, December 10th, 2007 AT 3:49 PM
