I've wondered about this for a while. My understanding of chemistry (didn't do so well in class) would lead me to believe there's more air in the mixture than fuel, and when ignited, it causes the pop that comes out of the exhaust because it's more explosive, but there's gotta be some detailed explanation of what's going on. I can recognize missfires without a problem, but I never fully understand why they happen.. How does a missfire happen and what goes on during it?

 

lean misfire is when theres too much air compared to gas. what happens is this mixture ignites too early, it actually ignites when the piston is on its way up rather then when the piston is going down, so it hurts things as you could imagine.

the actual sound you get is not the backfire from your exhaust, that is from too much gas and not enough air, raw gasoline is dumped into the exhaust and ignitites inside the exhaust housing making a loud pop. the sound you will hear from a lean knock is litterally just a little ping sound. sounds like the sound you made with your tounge against the roof of your mouth imitating a clock as a little kid. you have to listen close for it. but it can be loud, if it is loud... consider that cylinder gone.

 

Thats pre-ignition not lean misfire

 

A lean misfire is where you have a leak in the exiting of the exhuast gases that allows fresh oxygen to mix with unburnt fuel and BOOM!

 

running lean will also create more heat. :\ Not good.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 3.504 &raquo;</TD></TR><TR><TD CLASS="quote">lean misfire is when theres too much air compared to gas. what happens is this mixture ignites too early, it actually ignites when the piston is on its way up rather then when the piston is going down, so it hurts things as you could imagine.</TD></TR></TABLE>
i thought that was detonation.
or predetonation, rather


Modified by LSvtec03 at 8:40 PM 10/7/2004

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by PlastikOwl &raquo;</TD></TR><TR><TD CLASS="quote">running lean will also create more heat. :\ Not good.</TD></TR></TABLE>

cant you start melting parts of the exhaust? like the cat or something?

 

Hang on a sec..... if carbon gets built up in the cylinders, compression ratio goes up, and if the fuel octane isn't high enough, the mixture can detonate just by the mere heat and compression of the mixture. That's detonation.

Now if a cylinder is running lean, the mixture also burns hotter, which can also detonate the mixture, right?

confusion

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 3.504 &raquo;</TD></TR><TR><TD CLASS="quote">
the actual sound you get is not the backfire from your exhaust, that is from too much gas and not enough air, raw gasoline is dumped into the exhaust and ignitites inside the exhaust housing making a loud pop.</TD></TR></TABLE>

I realize that unspent gases can cause pops in the exhaust, but my car doesn't run rich (tuned A/F). The pops occur when the throttle is closed (like if I rev up, or in between shifts), so I suppose I could see that happening if the injectors are still dumping fuel, but as seen by a wideband, I'm not running lean or rich. Also, what do you call the "pop" that results from unspent gases if it's not a "missfire" or a "backfire?"

There's nothing wrong with my car, it runs healthy. I was just curious about the pops because they've been there for as long as I can remember.

 

If your refering to the slight popping out of the exuast when you are in decel, but not fully off the throttle, that is because of a small lean condition (16's-ish A/F) that is being caused by the last column in Uber being a hair to lean.
I explained more in my PM, and I will get it cleared up when I drive down.

As far as what the sound is caused by? The way I see it, as a cylinder experiences a full mis-fire (0 combustion), it dramatically changes the exhaust pulse being ejected from that cylinder. Also, introducing both oxygen and unburned/atomized fuel in the exhaust is bound to cause some combustion.

Dave

 

I think my car has one of these problems like you guys just said above.
I sometimes hear a loud pop from my exhaust and I dont know if it is a misfired or backfire. It happens very often after I installed my greddy exhaust and skunk IM. Is it a serious problem ? Do I have to fix it and how? I want to run my car right in order to keep it a lil bit longer. What do you recommend?
Thanks for your reply.

 

NO internal combustion consumer based enigine will burn all its fuel, there is always some left over fuel that exits thru the exhaust.. hence when mixing with fresh OX it ignites.


And Preignition and detonation are two very different things... they sound simular but preignition is where a part within the combustion chamber is still very hot and ignites the charge as it is entering the combustion chamber on the compression stroke.. detonation is where the flame front moves across the piston/combustion chamber on an uneven front causing an oscillation in the piston

 

good tech info here.

 

A lean misfire is simply an a/f ratio that is so lean, that the spark plug didn't ignite it - hence it missed lighting off the charge.

This in turn sends a whole bunch of raw gas out the exhaust considerably increasing HC emmisions.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EE_Chris &raquo;</TD></TR><TR><TD CLASS="quote">A lean misfire is simply an a/f ratio that is so lean, that the spark plug didn't ignite it - hence it missed lighting off the charge.

This in turn sends a whole bunch of raw gas out the exhaust considerably increasing HC emmisions.</TD></TR></TABLE>

Good simple way to put it

 

In my case , the pops sound from the exhaust usually happen in between shifting+ when I rev up, there is alot of water coming out from the exhaust, more than usual. Is this a missfire or backfire ?
What do I have to do to stop it ?
Ive heard some people said that I might be running too rich or lean and I have to tune it in order to stop it. What do you think? thanks

 

I thought that the noice that is heard is the slamming together of 2 flamefronts in the combustion chamber. I guess some people might call that 'knock', which is also the term I would use for Patrick's scenario with the carbon buildup. The carbon causes hot spots that will ignite the fuel mixture independently of the normal spark.

Lean misifre, knock, ping, it's all the same to me.......bad things are happening.

 

Ok, I was wrong. Here is the answer:

"A condition caused by an air/fuel mixture that is too lean to sustain combustion. Lean misfire causes one or more cylinders to pass unburned fuel into the exhaust system causing a big increase in hydrocarbon (HC) emissions. Symptoms include a rough idle and hesitation or stumble on acceleration. Lean misfire is often caused by vacuum leaks or an EGR valve that's stuck open."

It apparently has nothing to do with preignition or multiple flame fronts, but a complete failure of the fuel mixture to ignite and the fuel/air just gets dumped into the exhaust system, unburned.

 

I'm punch drunk