What Causes Backfire Through Intake – Combustion Engine 101

Backfire Through Intake

Backfiring is associated with a loud popping sound from the engine/exhaust on running engines. In the past, on vehicles models from 1990’s or older backfiring was associated fuel to be discussed in the next section.

In these older models, backfiring causes the system to spit flames through the exhaust. The reason being the ratio of air to fuel slants in favor of fuel – less air and more unburnt fuel.

On newer models, backfiring does occur owing to leakage in the intake and not the fuel. In these models, the intake ratio of air to fuel slants in the favor of air – giving the engine too little fuel with a large amount of air.

Whatever the reason, when a vehicle backfires, a spark ignites within the block and alongside the loud popping sound – the exhaust could also spit flames. The event replicates drag racers accelerating off the starting blocks on a racetrack.

NOTE: When backfiring occurs sporadically, some component is damaged and requires replacement. Excessive backfiring is undesirable and may result in engine damage.

Backfiring In Older And New Model Cars

On Older Models 

Older Car Models

In models manufactured from 1990 and before it is quite easy to trigger the backfire. To initiate backfiring in these vehicles - do the following:

Step 1 - Start The Vehicle

Run the engine and bring it to a steady rev. To keep the experiment safe ensure you are doing this in an open space, any observers should stand at a safe distance (30 ft), and ensure there are no oil leaks or flammable compounds around the vehicle or scene.

Step 2 - Turn Off The Engine With Afoot Placed On The Accelerator/Gas Pedal

This gets the engine ready for some backfiring (alters the air to fuel ratio for the next ignition) – keep the pressure light to minimize the fuel intake.

Step 3 – Wait A Few Seconds And Restart The Engine

Keep your foot on the accelerator/gas pedal when restarting and once the engine is running, press the gas pedal down as hard as you can. This should prompt engine to backfire – with flames and all.

On Newer Models

New Car Models

It is much harder to force backfiring in modern vehicles. Some modern sports cars automatically backfire – this is engineered to lend the car an even sportier image.

If you want to see if your engine backfires automatically accelerate to 60 MP/h and then begin to decelerate rapidly – if it is engineered to backfire it will.

In practice, backfiring engines is about bravado/style – lending the driver and vehicle an alluring persona.

What Are The Main Causes Of Backfiring Through Intake?

A car

Modern vehicles are much harder to backfire given the air to fuel ratio is controlled by a sealed intake valve. In modern engine, V6 and V8 engines the intake valve is part of the manifold and is located on the side of the manifold.

The intake delivers a precise mixture of air and fuel to the cylinders – the valve opens periodically to drag in air then close to mix with fuel. When the valve deliver less fuel and more air the engine backfires. Why does this happen?

1. Faulty Fuel System

Fix Car

The fuel system comprises three essential components. The oxygen sensor, airflow sensor and vacuum. The three work in tandem to deliver an accurate air-fuel balance and the damage of either one could prompt backfiring.

2. Leaking Air Injection System

This will result in pressure changes and failure to draw an adequate amount of air for use in the engine. An obstruction in the air injection system could as well mimic this scenario.

3. Malfunctioning Fuel Pump

Another component that could also result in an imbalance in air-fuel ratios.

4. Other Causes

Spark Plugs

Poor timing of the intake valve – in running engines everything is synchronized. The valve must open in time; the spark plugs must fire after the valve is closed, etc. Anything that could cause this timing to falter would affect the combustion and lead to backfiring.

How To Deal With Engine Backfires

Step 1 – Check The Dashboard Monitors

Car Dashboard

If your vehicle backfires and the dashboard sensors show no warning signal on the computerized dashboard monitor, check through the subsequent steps.

If any warning lights are on read this guide for more information on the service codes – complicated computer monitor messages can be annoying/difficult to understand. The guide will help you narrow down what you need to have repaired.

Step 2 – Check Fuel System Pressure

You want to be certain the air to fuel ratio is accurate (14:1 air/fuel). The most common cause of this problem is a weak/malfunctioning fuel pump incapable of delivering the right amount of fuel for the injection pump to spray. In vehicles with fuel filters installed, the filter is clogged with debris.

Step 3 – Check/Service/Replace Mass Airflow Sensor

Mass Airflow Sensor

Advanced computerized monitoring systems in modern vehicles allow you to check this sensor and confirm its optimal operation. It can malfunction, in an operation termed coking, resulting in uneven/inaccurate air/fuel ratios.

You can remove it and clean it using though often just replacing it is better.

Step 4 – Check The Air Intake Tube/Boot

This is the channel used to carry air from the intake sensor to the throttle actuator. Any damage to this component such as a puncture will indicate on the oxygen sensor. Inspect the component and replace if needed.

Step 5 – Inspect The Vacuum For Leakage

The intake system must be completely free from leakage and sealed. The vacuum hose also combines as the power brake booster and if it fails or is broken, it can contribute to slight backfiring.

The additional air it allows in can be detected if the idling is abnormally high or low or if the brake pedal is harder to press than normal.

Step 6 – Malfunctioning/Worn Spark Plugs

Malfunctioning/Worn Spark Plugs

When starting an engine, an electronic driver signals the ignition coil to fire. The time/effort it takes for the spark plug to fire the coil is entirely mechanical design.

Any difficulty in starting the vehicle would suggest malfunction or worn plugs/damaged ignition coil. When these components malfunction, they tend to spark even when idling or even crossfire.

It is important to change plugs regularly – platinum plugs at 60,000 miles.

The above situation may continue undetected by the computer monitoring system.


Backfiring from the intake can be quite a serious problem if left unattended for too long. In modern vehicles backed by advanced computerized monitoring equipment, you can experience this one mechanical failure repeatedly without any warning lights.

The article briefly browses through some of the many possible reasons for this malfunction.

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