Back to basics: What is a backfire, and what causes it?

An intake manifold back fire is when…? It is caused by…? It can be cured by…?

There are two kinds of backfires. In the old days of carbs, the air cleaner really did two jobs, It cleaned the air and it kept intake backfires from occurring. Gas fumes could come up from the carb and in sufficient concentration that they could ignite. You would hear something between a pop to an serious explosion.

Other other is the more usual exhaust backfire.

If you want a little help, some more information would be helpful. Tell us what make model and year of car as well as any modifications made to it that you are talking about.

I’m not referring to a particular car; but, an Otto cycle (four stroke) engine with fuel injection. No, not exhaust “backfire”. That’s why I asked about intake manifold backfire.

The most common intake backfire that I am familiar with occurs when the ignition timing is too advanced, causing the gas mixture to ignite before the intake valve is completely closed.

Throttle body or multiport?

The reason I ask is that with a throttle body it is possible for a volatile ratio to be in the intake manifold and be ignited via any spark, which might come from any cylinder’s valve being burned or sticking such that an ignition spark could, conceivably, fire back up into the manifold.

With a multiport injection system, there will either be an injector at each intake port, and all of its spray would be drawn into it’s designated cylinder, or a single injector feeding two intake ports…in that case the heat energy could come from the spark from the cylinder OTHER than the one that’s on its intake stroke.

In all cases a leaking injector combined with an ignition source could provide the proper conditions. An ignition source could be any spark, including one from a cylinder on it’s power cycle that has a path past a sticky, unclosed, or burned valve.

A backfire needs a volatile mix and a source of heat energy.

Your responses are forward to the questioner (in your parlance, the “OP”), of the question RANDOM CYLINDER MISFIRE.
You may also like this article by Raetech on achieving a 130% volumetric efficiency in an induction system:

I’d like to share this with you from “A lean condition during acceleration can cause the fuel/air mixture to burn so slowly that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire.”
Knowing why things happen is as important as what happens, I believe.