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Backfires get their etymology from firearms parlance when explosive force is released through the breech instead of the muzzle of an unreliable firearm. The concept is roughly similar in vehicle engine systems.

In vehicles, backfires are usually of two kinds – first is where gases are expelled rapidly from within the intake manifold and the carburettor, and the other is when explosive expulsion takes place through the exhaust mechanism. To differentiate between the two in car mechanics terminology, the former is popularly referred to as ‘spitting’ or ‘coughing’ and the latter as ‘backfiring’.

Understanding the Backfiring Mechanism

Backfires are always accompanied by a loud, popping sound and many a times with a visible flame, referred to as the after-fire. This after-fire is a by-product of backfiring through the exhaust, something that was a common sight in older car models.

Backfiring through the exhaust is a result of problems arising out of rich fuel mixture and out-of-turn sparking of spark plugs.  The ratio of air to fuel in the mixture gets skewed in favour of fuel i.e. there is more fuel and less air. Together with the unburned fuel in the exhaust system and an open exhaust, the sparking results in the explosion associated with backfires.

Relatively newer models do not typically backfire; the ones that do are engineered to do so, like drag racers that you see on Top Fuel. On the other hand, spitting does happen in newer models and is initiated as a result of the air-fuel mixture being too lean, coupled with dysfunctional engine timing. In other words, the intake ratio skews in favour of air, meaning the engine then has too little fuel and a lot more air.

As a result, the un-burnt fuel is ignited inside the intake manifold itself. This often happens before the engine is warm. The accelerator pump in the carburettor does not supply enough fuel before the main jet kicks in. When intake manifolds were made out of aluminium, they were able to better handle this problem.  

Triggering Backfiring

In older models, triggering backfires is not all that challenging. By older models, the indication is towards models that had been manufactured before the turn of the millennium. Using the above reasoning behind the causing of backfires can teach you how to trigger backfiring. You are strongly recommended to take due precautions before trying this, such as ensuring that observers are positioned more than 30 feet away. Also ensure that there are no oil leaks or any flammable substance in close vicinity.

Old Models

If your older model hasn’t been tuned up in a while, backfiring should be straightforward enough to achieve. Again, we cannot emphasise enough the need to take adequate precautions.

  • Commence the process by starting your engine and letting it settle into a steady rev.
  • Next, turn off the engine with your foot on the gas pedal. Keep the pressure on the gas pedal low so as to not burn more fuel than necessary. What this helps to achieve is an alteration of the air-fuel ratio prior to the next ignition of the engine.
  • Remember that backfiring is caused by skewed ratios in the air-fuel mixture.
  • For the next ignition, maintain a light pressure on the gas pedal while restarting the engine.
  • Once the engine is up and running, you need to floor the gas pedal. In older models, this should result in backfire pyrotechnics.

Do keep in mind that repeated backfiring can cause lasting damage to your vehicle. Once again, take adequate precautions before attempting this.

New Models

Modern cars now have engine control modules that will adjust the ignition timing of the car automatically, making spitting and backfiring very difficult to simulate. In rare circumstances, the crankshaft position sensor can go rogue and cause backfiring. That being said, modern cars can backfire, but only if designed to be able to do so. To see if your engine is designed as such, you can accelerate to about 60 to 70 miles an hour and then decelerate rapidly.

Please be mindful of other vehicles and bystanders around before attempting this.

What Causes Accidental Spitting?

While it is all about style and bravado when you trigger a backfire yourself, spitting and backfiring is often indicative of mechanical issues with your vehicle. Usually, dashboard sensors pop a message up on the computerised monitor. In the event that they don’t and you hear the loud bang of explosive propulsion, here is how you go about narrowing down the problem.

Fuel System Issues

A malfunctioning fuel system is usually at the heart of spitting problems. Every internal combustion engine is supposed to maintain a pre-fixed ratio of air to fuel for optimum fuel burning. The golden ratio here is 14 parts of air to 1 part of fuel.

For the engine to work, fuel is under-ejected through a small jet opening. This is done under high pressure to break the full into a misty spray – a bit like the deodorant nozzles. The process is called atomisation and is central to the functioning of the engine. If the fuel pump is unable to supply the requisite volume of fuel for the injectors to produce the spray pattern, the 14:1 ratio is likely to get skewed with there being too little fuel to power the engine.

Another pain point in the fuel system could be the fuel filter. A clogged fuel filter can impede the volume of fuel that passes through it and consequently impact the fuel pressure. This is one of the primary reasons why it is advisable to replace the fuel filter at some point around the 30,000-mile mark.

Mass Air Flow Sensor

Mass air flow sensor reads the amount of air entering the engine’s intake system. Due to something called coking, the mass air flow sensor can malfunction as a consequence of the hot wire getting contaminated. This creates an issue because the sensor detects lesser air entering the engine than it actually is. It further leads to lesser fuel because the quantum of fuel injected is reliant on the reading of the mass air flow sensor. While one way to correct this is to take the sensor out and clean it up, a better idea would be to replace it altogether.

Air Intake Hose

Air intake system is located directly behind the front grille. What this system does is that it draws air through a long plastic tube. This air then flows into the air filter housing and is sent to the intake manifold.  A puncture or tear in the air intake hose will allow unmetered air into the engine. Yes, you guessed it right – this alters the air/fuel ratio too. The leaner mixture is more volatile than a 14:1 ratio and leads to premature ignition. Inspecting the hose for damage is the only way to fix this issue.

Vacuum Leaks

Vacuum hoses serve a critical role in the functioning of intake manifolds as the latter need to be free of any seepages or leaks to be in perfect working order. Damage to the vacuum hoses will also result in additional air being led into the intake manifold, thus upsetting the air/fuel mixture equilibrium.

Problems with vacuum leaks are not very difficult to realise – they often manifest in the form of high or low engine idling. A smoke machine or a carburettor cleaner can also help with singling out vacuum leaks as the cause of spitting.

Spark Plugs

Spark plugs work in a very ordered and time-driven fashion. For the engine to operate normally spark plugs will have to fire in a pre-ordained manner and failure to do so will result in the spark igniting the fuel with the intake valve still open. With the valves open, result of the ignition will be spitting and it will manifest through the intake manifold. This is often referred to as the ignition timing problem.

Spark plugs might seem an afterthought but they really are the bones for an engine to function normally. Sometimes what happens is that spark plugs might be installed incorrectly or the wires might have crossed. This creates a problem similar to the ignition timing problem. The air-fuel mixture is ignited with the intake valve not having shut. With no other place to go, the explosive propulsion comes through the intake manifold or the carburettor.

Plugs usually last for a while. However, a good rule of thumb for replacing platinum spark plugs is when your vehicle hits about 60,000 miles.

Concluding Remarks

While a backfire may be used in pop culture and by dragsters as a sign of pomp and bravado, for most regular vehicles it is an indicator of engine malfunctions. Anything that backfires can’t be good.

Most solutions are straightforward to execute and their prompt utilisation is important before your vehicle suffers irreversible damage. Despite modern computerised equipment in vehicles, certain issues can go undetected. In this article, we have sought to cover the likeliest scenarios so that you aren’t reliant solely on your dashboard monitor to single out the pain points.

Sources:

  1. Back-fire – Wikipedia
  2. Engine is backfiring Inspection Service – Your Mechanic

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