Does your engine back fire and you are wondering why? When your engine
back-fires it can do so in one of two ways, first and most common way is when the
engine is running a small explosion of un-burnt fuel is processed into the
intake manifold creating the bang sound you hear. This occurrence can happen once or
a few times when the engine is under load, or it can happen continuously
signaling a mechanical failure which I will go over in the
The second kind of back fire will occur from the rear of the vehicles tail
pipe which is a rich fuel mixture problem that can happen when parts fail and in race cars using a
We will cover the primary occurrence of the engine under load first. This
problem must be corrected and can not be neglected because serious engine damage
can occur, it's an explosion of un-burn gas which can bend the
throttle actuator plate, push intake gaskets outward creating a leak and I have
even seen where the plastic intake manifold itself has blown apart, back in the
day they were aluminum and could withstand this kind of problem.
On the small side of things a simple vacuum hose might get pushed off which
is an easy fix, anyway let's get started.
Anytime you have an engine that is backfiring the first thing to look for is
a check engine light. If no warning lights are on continue down this guide. If
the check engine or service engine soon light in on scan the computer for
An intake explosion is caused by the fuel air mixture being to lean when the
engine is demanding power. Every internal combustion engine runs on a mixture of
14 to 1 which is fourteen parts air to one part fuel. This fuel must be
atomized correctly to complete the burn process.
The most common cause for this
is a weak fuel pump which cannot supply the volume of fuel needed for the
injectors to produce a proper spray pattern or deliver the amount of fuel needed
for proper acceleration.
Some cars have a fuel filter which can also cause a fuel pressure and volume
restriction. When they plug due to particles in the gasoline they can case a
lean condition, if you have not
changed the system filter lately locate and replace it.
Your vehicle's computer system monitors the air that enters the engine via
the mass air flow sensor. When this sensor malfunctions due to a condition
called coking where as the hot wire inside the sensor becomes contaminated it
will alter the reading to the computer which will make a lean mixture condition
that may or may not trigger a check engine light. The computer thinks there is less air entering the engine
than there actually is, less air means less fuel. The first step is to remove
the sensor and clean it using carburetor cleaner.
I have had better luck by replacing the sensor because once this condition
happens the hot wire can becomes unresponsive in which case the sensor needs to
The air intake boot or tube transfers air from the mass air flow sensor into
the throttle actuator and then into the engine. If there is a problem with these
parts such as a tear or rip it will allow un-metered air into the engine which
the computer will see via the oxygen sensor which in turn will deliver less fuel
creating a lean condition. Check this part for damage by removing it for
inspection and replace it if needed.
The engine's intake system depends on being a sealed unit free from leaks. If
a main vacuum hose such as the power brake booster feed line has broken or failing
off it will cause a lean backfire due to the additional air allowed into the
engine intake manifold. This problem will be accompanied by high or low engine idle and
a harder to push brake pedal than normal.
Repetitive Back fire
A repetitive backfire is generated when the engine is simply running at idle
or at cruising speed and can be inline with the engine RPM, a rhythmic sound
which is not as prominent as a lean under power back-fire. The problem list below
will give you an idea of what causes could be generating this condition.
The engine's ignition system is triggered by an electronic driver which
signals the ignition coil when to fire. By design the resistance it takes to
fire the coil via the spark plug is figured into the system. When this
resistance becomes excessive due to severely worn spark plugs or a bad ignition
coil it can cause these drivers to malfunction at which point they can crossfire
into an opposing cylinder either under power or at idle.
This condition may or may not be detected by the computer so the check engine
warning light may not come on. Spark plugs should be changed at regular intervals,
platinum plugs at about 60,000 miles which will correct this problem.
A camshaft is used to open the intake and exhaust valves of the combustion
chamber to allow intake air in, and the spent exhaust gases out. If the exhaust
valve lobe becomes worn and loses it's lift the fire from the exhaust gases are
still left in the cylinder which is then released into the intake manifold once the
intake valve opens. To check for this condition remove the valve cover.
After the valve cover has been removed and with the ignition disabled (coils
off) crank the engine over and observe the intake and exhaust valve operation, opening and
Also, if the intake or exhaust valves are not closing completely due to a
broken valve spring it will allow the combustion gases to enter the intake
manifold mush like a flat camshaft lobe. Use a flashlight in carefully check the
valve spring windings, these springs can break either at the top middle or bottom of
the spring which can be sometimes be difficult to see.
A bent push rod can also cause the valve not to work properly by restricting
the movement of the valve. While the valve cover is still removed, look at each
of the pushrods to see if there is an obvious bend to them, this can be done
also by detecting poor rocker arm movement while the engine is being cranked over.
Exhaust Tail Pipe Back Fire
An exhaust tail pipe backfire is caused by an excessive amount of un-burnt
fuel remaining in the exhaust system or fresh air being allowed to enter the
system which ignites a small amount of an un-burnt fuel.
If fresh air is allowed to leak into the exhaust system it will ignite the
un-burnt fuel inside the system creating a popping noise. It is difficult to
think of because most people think of a exhaust system to be under constant
pressure but this is not true, the system is fed with a series of pressure and
vacuum pulses which are created by the exhaust valve opening while the combustion
charge being expelled into the system, and then the valve closing, this is where the vacuum event
is created by the pulse velocity and the fresh air let into the system. This why when you
can get a
lean mixture trouble code when there is an exhaust system leak.
Inspect the system and repair any exhaust leaks present, this detection is
done by looking for a flat back shoot which is evidence of a leak.
On engines made from about 2001 and older were equipped with a air injection
system that was fed into the exhaust system when the engine was under load, this
was to burn unspent fuel which helped the emission system to work better. These
systems were fitting with a gulp valve that acted like a one way check valve to
allow air into the system when they detected exhaust system pressure which only
occurs under load. This gulp valve is fitted to the exhaust manifold via a large
pipe or an individual port tubing configuration with a rubber tube that
connected to an air pump.
When the valve goes bad air was released into the system at all times even
during de-acceleration which then created a backfire kind of popping due to the
fresh air igniting the unspent fuel. Most engines had one or two of these valve
one for each exhaust manifold.
To check these valves you must remove them and then try to blow through them
each way, only one direction should be allowed only. If air is allowed in both
directions the valve is bad and should be replaced.
When high performance turbo engines are under load they use an excessive
amount of fuel which then is transferred into the exhaust system once the
throttle is let up and the engine is de-accelerating. This is a normal event and
there is no problem with this condition.
On older engine's ignition timing is set at a specific degree in
relationship to the crankshaft, if this timing is misadjusted it can cause low
power, poor gas mileage, engine detonation (pinging) and backfiring through the
intake or exhaust system.
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Article first published 2016-04-20