Why does an engine backfire and what should I do?

Engine backfires are small, and abrupt, explosions of fuel vapour.

While these uncontrolled engine eruptions - sometimes sounding like a gunshot - may see bystanders jump and drivers cringe, backfires could well be a warning of more serious mechanical mayhem ahead.

These little detonations in petrol-powered motors are not quite so common today, thanks to more advanced fuel systems, no more carburettors and better electrical engineering under the bonnet.

Why does an engine backfire?

A backfire is essentially a mistimed explosion of fuel vapour.

It often occurs in an internal combustion engine’s exhaust system. Hot gases, or unused fuel vapours, will explode when mixed with fresh air leaked from a faulty exhaust system.

A damaged catalytic convertor could be at fault. An exhaust manifold pipe may be cracked.

Perhaps the vehicle’s air-fuel mix is too rich - too much petrol, not enough oxygen - and the combustion process hasn’t used all the fuel.

Or conversely it could be the mix is too lean, has burnt too slowly and, again, there’s excess gases when the exhaust system opens up.

In race cars there are often backfires - and flames from short and unrestricted exhausts - when a driver backs off the throttle and for milliseconds the engine is running too rich, unloading fuel into the exhaust system.

In sports, race or road cars, a backfire may result from a fault somewhere in the ignition system - inconsistent firing of the spark plugs, a damaged plug or a defect in the wiring harness.

Early ignition of the air-fuel mixture in the intake manifold can also cause an engine to backfire.

This again could be the result of a lean fuel mix, an errant spark plug, ignition timing out of tune or a leak in the intake system.

Dirty air filters or underperforming fuel pumps may mean a lean mix - too much air, not enough fuel - and cause intake system backfires.

What to do?

Aside from potential embarrassments in the traffic, backfires may herald the beginning of a bigger problem down the road.

Again, these little detonations are less common with today’s road cars but if a vehicle is backfiring, best to get to a trusted mechanic to chase down the problem.

Have a browse through AutoGuru to find someone to help.

If the issue is an air-fuel mix running too lean, there may be issues with scoring cylinder walls; if the mixture is too rich there’s dollars burnt and the engine will run hotter.

Either problem means less performance and more fuel used to compensate.

And faulty ignition or spark timing out of tune could lead to a very reluctant engine, not working to its optimum performance.

Not sure what’s causing the problem?

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