If you have ever heard a vehicle’s exhaust making loud noises that sound like firecrackers crackling or even gunfire, it might be because the engine is backfiring. Read on to learn more about how this happens.
First, there is induction backfire that happens in the intake manifold, and there is backfire that happens in the exhaust system. Induction backfire is typically caused by ignition system anomalies related to one or more faulty spark plugs, cross-firing between distributor posts in the cap, and/or ignition timing issues. Valve timing issues can also contribute to induction backfire.
Induction backfire is typically caused by ignition system anomalies related to one or more faulty spark plugs, cross-firing between distributor posts in the cap, and/or ignition timing issues.
– Richard McCuistian, ASE Certified Master Automobile Technician
As for exhaust system backfiring, engine platforms with throttle body injection and carburetors are more likely to backfire out the exhaust than multipoint fuel injected platforms.
On engines where the fuel is delivered above the throttle plate, gasoline “sweat” tends to gather on the inside of the intake manifold. When the throttle is released, the butterflies under the carburetor venturi or throttle body injector(s) close, shutting off the incoming atmosphere, and the pressure plummets within the intake. When that happens, much of this gasoline sweat is vaporized due to the low pressure and naturally makes its way through the intake manifold and into the exhaust.
Now, let’s take a closer look at the most common reasons why a car might backfire.
On electronic port fuel injectionengines, (MPI/GDI), backfiring almost never happens on deceleration because the fuel is injected either right behind the intake valve (no gasoline sweat in the manifold) or right into the combustion chamber, as with GDI, and the injectors are shut off during deceleration when no gasoline is needed. This tends to prevent backfiring and also helps with fuel economy. If an MPI or GDI engine does backfire, it’s usually due to ignition or valve timing issues.
Thus, modern engines are able to burn most if not all of the fuel inside the combustion chamber since computers are able to calculate the ideal fuel-air mixture for each engine stroke. Hence, backfires because of bad engine timing are more common in older vehicles with carbureted engines. Coil-on-plug systems actually calculate ignition timing separately for each cylinder.
Another reason why engines can backfire is because of poor fuel-air ratio. While backfires caused by poor engine timing are more common in carbureted engines, modern engines are more prone to backfires caused by an incorrect fuel-air ratio. Modern engines rely on the powertrain control module (PCM) to determine the appropriate fuel-air mixture, which depends on several factors like the engine’s load, the altitude the vehicle is at, and temperature. However, the PCM can make mistakes when there are faulty sensors or air leaks.
If there is too much fuel injected into the engine, not all of the fuel will be combusted properly. When this unburnt fuel from the rich fuel-air mixture exits the combustion chamber and enters the exhaust, the presence of heat and reintroduction of oxygen can cause the excess fuel to ignite and cause a backfire.
If the engine uses a fuel mixture that’s too lean for a given load, then the fuel will burn much more slowly. A fuel mixture is lean when there isn’t enough fuel and too much air for a given situation. When this happens, the exhaust valve in the combustion chamber might open while the fuel-air mixture is still combusting, causing the engine to backfire.
The PCM uses sensors like the mass airflow sensor and oxygen sensors to help it decide the right fuel-air mixture. However, if one of these sensors is faulty, then it can cause the PCM to use an incorrect air-fuel ratio, which can cause the engine to backfire. Many other factors can cause the PCM to receive false information. These include a leaking exhaust, a leaking intake manifold, and dirty air filters. A faulty fuel injector and fuel pump can also cause backfires because they may affect the quantity of fuel entering the combustion chamber.
One reason why spark plugs misfire is a cracked distributor cap that’s causing a spark plug to fire at an inappropriate time. Distributor caps are responsible for dispersing power to individual spark plugs in vehicles that don’t have individual ignition coils on each spark plug. Sometimes, there can also be carbon build-up on spark plug wires, which can cause them to short circuit. Short circuits can also lead to spark plugs firing at the wrong time. When this happens, fuel in the combustion chamber can ignite while the exhaust valves are still open, causing a backfire.
Spark plugs can also short circuit just because the wires are crossing with one another. Sometimes, they can also be installed in the wrong order.
On vehicles with air injection systems, the air being produced by the air pump is sent upstream (into the exhaust manifold) when the engine is cold to deal with excess hydrocarbons from cold enrichment and to heat up the light-off catalyst (the brick nearest the engine).
After the engine and that upstream cat are hot, the front brick deals with NOx, separating the oxygen from the nitrogen. Those two elements are bound to one another during the intense heat of combustion.
But during hot highway cruise the air is pumped downstream to aid the second catalyst brick deal with hydrocarbons and CO by adding oxygen molecules. These are the basics of air injection.
That being said, early air injection systems would dump the air pump flow through a muffler/filter on deceleration to prevent backfiring. Because that hot gasoline steam in the exhaust system that came from the gasoline sweat on the inside surfaces of the manifold tends to ignite spontaneously with the injection of any fresh oxygen into the exhaust stream.
Exhaust leaks can cause backfiring on TBI and carb vehicles for the same reason, because the exhaust has a pulsing quality that can draw air in through cracks or leaks in the manifold or exhaust pipes. Backfiring doesn’t always happen with exhaust leaks, but it always can happen.
To illustrate this, while driving a vehicle with a carburetor, simply switching the engine off and right back on while coasting (manual transmission) can fill the exhaust system with gasoline vapor so that when the key is switched back on, a highly explosive backfire usually results that can literally split the muffler wide open. This can also happen if the ignition is lost momentarily for any other reason. But again, this is far more likely on carbureted and TBI engine platforms.
Backfires are caused by fuel burning outside of the cylinder, which means it’s wasted energy. As a result, your vehicle can feel sluggish since it’s not harnessing all the available energy. It can also consume excessive amounts of fuel. Backfires are also usually a symptom of an underlying issue with your vehicle, most likely with the engine. You should have it checked out by a mechanic as soon as possible to prevent engine damage, which may be costly and labor-intensive to repair.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.