Step by step guide on how to repair an engine backfire problem. This guide pertains
to most non hybrid vehicles.
Difficulty Scale: 4 of 10
Tools and Supplies Needed
Fuel pressure gauge
Engine code scanner
Protective eyewear and clothing
Begin with the car on level ground, in park with the emergency brake set, engine
Step 1 - If the check engine or service engine
soon light is illuminated visit -
Scan for trouble codesStep 2 - The
ignition system is designed
to operate at a specific voltage and amperage output. If this voltage/amperage is
forced to spike due to faulty or inferior
tune up parts it could cause
the system to crossfire. A crossfire condition can allow the electrical discharge
for a particular spark plug circuit
to transfer to a alternative cylinder which can produce a backfire in either the
intake or exhaust system.
Step 3 - A fuel system is designed to produce a
consistent fuel pressure. If this fuel pressure drops, it can cause a lean mixture
which can cause the engine to backfire. Visit -
testStep 4 - Engine ignition timing is set at a specific
degree, in relation to the crankshaft of the engine. If this timing becomes misadjusted
it can cause low power, poor gas mileage, engine detonation (pinging) and backfiring.
Visit - Set engine ignition timingStep 5 - On distributor style ignition systems,
if moisture is present inside of the distributor cap, it can cause internal ignition
crossfire. This crossfire condition can allow the electrical discharge for a particular
spark plug circuit to transfer to an alternative cylinder which produced a backfire.
To check for this condition remove the distributor cap and inspect for moisture
or arcing. If moisture is present dry components thoroughly and reassemble.
Step 6 - The engine is designed to run on a specific
ratio of air to fuel (14.5 to 1). If this ratio increases by withholding fuel, it
can cause the engine to backfire through the intake system. The number one reason
for this condition is a plugged fuel filter. Visit -
replacementStep 7 - A failed air intake boot can cause a backfire
by altering the feedback voltage to the PCM. Visit -
boot replacementStep 8 - Some engine exhaust systems have an air
injection system designed to help dilute the exhaust gasses. This air is only added
in operation during power demands. If the air injection check valve or sometime
referred to as a gulp valve fails it can allow extra air into the exhaust system
causing backfiring. To check your systems valve, remove it to check air flow. Air
should only travel one direction. If air travels in both directions, the valve has
failed and needs replacement.
Step 9 - Small leaks in the exhaust system can
take in cool air during deceleration of the engine causing a popping or backfiring
in the exhaust system. Inspect the exhaust system for leaks, such as broken welds
or rust holes.
There are two kinds of backfires, one is produced from the engine air intake
and the other is produced from the tail pipe of the exhaust system. The cause for
a backfire condition can vary depending on the malfunction of a particular system.
Some backfires can be extreme enough to cause damage to related components. Anytime
a backfire is observed immediately diagnose and repair as needed. If the engine
is running rough it can produce a backfire. A backfire occurs when there is an imbalance
in the air to fuel ratio required for your vehicle to operate properly.
A common backfire situation occurs when there is a small leak in the air injection
system that feeds the exhaust system. This can cause unburned fuel to explode suddenly.
One of the most common causes is a stuck or faulty air intake or gulp valve near
the exhaust manifold. Backfiring can also occur with a sudden drop in fuel pressure.
This may be due to a faulty fuel pump or a plugged fuel filter. Correcting problems
in the fuel system usually resolves these issues.
If the fuel mixture is too lean (not enough fuel) you may have a backfire in
the intake manifold, or too rich (too much fuel) you may get a backfire out of the
exhaust system. It is important to observe the location of the backfire. The information
below is separated into two sections; the first section contains possible causes
for backfires in the engine intake. The second section contains possible causes
for backfires in the exhaust system. (Note: if the engine is misfiring and the engine
backfires while the engine is under load please visit, engine misfires.
Change the fuel filter at regular intervals.
Tune up and service the fuel injection system in accordance with regular
Written by Ken Lavacot Co-Founder and CEO of 2CarPros.com 35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.
COMMENTS TO THIS ARTICLE
Please use our question form if you have a specific question about your car as we are not able to give you a full answer on this page.
Article first published 2009-08-03 (Updated 2015-01-05)