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Misfire is a common drive-ability problem that may or may not be easy to diagnose, depending on the cause. A
misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine's
power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that
it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be
hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and
electric rear defroster, for example).
When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a
misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of
hydrocarbons (HC) in the exhaust.
What causes a cylinder to misfire? Basically, it's one of three things: loss of spark; the air/fuel mixture is too far
out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from
jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs,
bad plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would
affect all cylinders, not just a single cylinder.
"Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This
can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak
pump, restricted filter or leaky pressure regulator. Low fuel pressure would affect all cylinders rather than an
individual cylinder, as would most air leaks. A leaky EGR valve can also have the same effect as an air leak.
Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most
likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are
misfiring, it's likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant,
it's likely the head gasket is the culprit.
Intermittent misfires are the worst kind to diagnose because the misfire comes and goes depending on engine
load or operating conditions. They seem to occur for no apparent reason. The engine may only misfire and run
rough when cold but then smooth out as it warms up. Or, it may start and idle fine but then misfire or hesitate
when it comes under load. Also, it may run fine most of the time but suddenly misfire or cut out for no apparent
reason. Intermittent misfires can be a real challenge to diagnose, so let's start with a steady misfire in one
cylinder before moving on to intermittent misfires.
In the next article we will examine how to fix the problem that is causing the misfire problem.
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