what causes backfire

[glennpenn]

What causes an engine to backfire?

Glenn
911S - 1976

[ossiblue]

Originally Posted by glennpenn

What causes an engine to backfire?

Glenn
911S - 1976

Generally, backfiring happens when combustion takes place outside of the combustion chamber--either on the intake side or the exhaust side.

In a fuel injected 911, there are several possible reasons, usually determined by where the backfire is located and under what circumstances it occurs.

Backfiring though the intake can be from timing issues, ignition issues, or from too lean fuel mixtures, among other reasons.

Backfiring from the exhaust can be from overly rich fuel mixture, leaks in the exhaust system, or ignition problems, among others.

If you want help on your specific vehicle, let us know year and model as well as where and when the backfire occurs, plus any history as to when it began and any work done to correct the issue.

[tharbert]

Backfiring into the intake makes a popping sound. Exhaust backfires can simply sound like subdued firecraskers in your exhaust. If enough fuel collects in the exhaust, you can get that infamous ka-pow gunshot sound. I've seen mufflers laid open from backfires.

[CCM911]

Mine started backfiring on the second to third shift under hard acceleration after I replaced the cat with a straight through pipe. I am assuming that this is due to excessive fuel in the exhaust.

Been doing this for over ten years with no damage to my engine.

Your results may differ.

[Festive_Zombie]

Additonal intake backfiring can be cause by vaccum leaks.

[JQ911]

I experienced a backfire since I installed Fabspeed Supercups (1in 1 out). It normally occurs if I downshift and just cruise along followed by a burble sound upon deceleration, normal? Never had the backfire on my stock muffler until the install.

[Jdub]

Recheck your CO2. Be absolutely sure you sealed all of your exhaust connections with new gaskets and that there are no leaks. Remove your oil cap at idle and note if the idle falls (it should).

[JQ911]

Originally Posted by Jdub

Recheck your CO2. Be absolutely sure you sealed all of your exhaust connections with new gaskets and that there are no leaks. Remove your oil cap at idle and note if the idle falls (it should).

Yeah, I noticed a slight drop in my idle when I checked the dipstick a few days ago...Will check the gaskets.

[gjmascoli]

I recently blew up my airbox on my '76 2.7L. The car had not run for about 13 years and I was so excited to get her running and get a few miles on her to see what needed to be worked on next, that I put off the installation of the pop-valve. So, with the pop-valve sitting on the back seat on a cold morning a few weeks ago....turn the ignition key and an instantaneous BOOM!

Now I'm planning out my first engine drop! Test stand and adapter ring ready to go.