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Old 08-14-2012, 12:16 PM   #1
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What causes exhaust "popping" during deceleration?

I've noticed this for some time but now just really darn curious as to what it is.

When I decelerate (usually in 2 or 3 gear), once the RPMs cross below 2000, I hear a rapid popping sound from my exhaust - disappears around 1200 RPM.

I always thought it was the exhaust rattling against the body, but now I'm convinced it's not.

I'm noticing it more this summer since I have my top off.

It's a 22RE with LCE header and "Pro Flow" exhuast on it.


My friend who rode with me recently said it was backpressure with the free flowing exhaust - is that what it is? Is it bad?
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Old 08-14-2012, 12:58 PM   #2
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Backfire....

fuel in the exhaust system, advanced timing too much

http://en.wikipedia.org/wiki/Back-fire

mine does this...
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Old 08-14-2012, 12:59 PM   #3
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Causes

Exhaust system backfires occur in engines that have an emission system malfunction, like an air injection system diverter valve problem, an exhaust leak, or when the catalytic converter has been removed. In some high-performance vehicles, when a driver shifts up and lets off the accelerator, the engine has a moment of running rich. This causes an incomplete burn which causes the fumes to explode in the exhaust system along with an audible pop or bang sound. This is a result of working equipment, and is unlikely to cause damage.
A fuel-injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.
Common causes of backfires are:
Poor or unregulated engine timing is often a cause of intake backfires, but can also be responsible for exhaust backfires
Improper wiring in the ignition can also lead to timing issues and backfires
Low fuel pressure, clogged fuel filters, and weak fuel pumps could cause a severe lean air-to-fuel ratio during fuel injection
Missing or damaged catalytic converter can result in backfires out the tailpipe
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Old 08-14-2012, 01:18 PM   #4
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My stock 22re with 2 1/4 inch pipe after the stock manifold does exactly the same thing. I'm not running a catalytic converter either.
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Old 08-14-2012, 01:26 PM   #5
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its nothing... if it were while trying to accelerate i'd worry. Its a lack of back pressure with a free flowing exhaust. Mine does the same thing.
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Old 08-14-2012, 01:39 PM   #6
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It comes from a free flowing exhaust. if you still have your catalytic converter on, it greatly reduces it, but with no cat, it'll pop through the entire decel. You should hear my 67 mustang with a v8, long tubes, X pipe, no cats, spitnech pro streets and full 2.5" exhaust. it pops even in an automatic.
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Old 08-14-2012, 02:17 PM   #7
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Thanks guys for the quick replies!

I have a cat - a newer Magnaflow I installed last year. I think it's 2.25 all the way back.

If it's only happening on decel and not all the time, does that rule out an exhaust leak, and it's just unburnt fuel igniting in the pipes after the header?

So, sounds like no harm being done here?

Oh, and it only seems to happen when in gear - I can't replicate it when revving, decelerating in neutral.
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Old 08-14-2012, 02:28 PM   #8
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no harm done. it only happens in gear, on decel.
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Old 08-14-2012, 02:52 PM   #9
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My old truck with a side pipe would do it enough to shoot a flame out the pipe on heavy decel but I think it was running a little rich too
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Old 08-14-2012, 02:54 PM   #10
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Its because you drive a race car.

My Impreza does it quite a bit. When I get back on the gas it pops pretty loud and scares children and small animals.
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Old 08-14-2012, 02:55 PM   #11
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My guess is that you have a problem with the dashpot http://www.ncttora.com/fsm/1990-1995...e/throttle.pdf p.235. When you pop your foot off the accelerator and let the throttle valve slam closed, a big slug of fuel gets pushed through the cylinder without enough air to burn it. It finds enough air in the exhaust, where it explodes. That's the popping sound.

The dashpot is like a door closer; it keeps you from slamming the throttle plate closed, by closing it over a period of about one second. This allows enough air through to burn all the fuel left in the intake, producing power in the cylinder rather than exploding in the exhaust.

Obviously, exhaust systems aren't designed to have explosions inside them, so eventually you'll trash the muffler, the cat, or blow off a pipe. No harm done? Well, if replacing your exhaust system every year or so is okay with you, fine. But I wouldn't put up with it.
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Old 08-14-2012, 03:08 PM   #12
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With the LCE header and 2.25" to the back, 3way CAT(like you), I think it slightly lacked the OEM backpressure these want... but still hardly ever did that. It wouldn't do it every time I decelerate. More 'occasional'.... When I drove down Crenshaw from the top of the hill in Palos Verdes(quite a hill, around 800' elevation drop in 1/2 mile?)... I would be in 3rd doing 45 and could NOT GET A SINGLE POP, at all! But leaving my house(most common when it would happen) in 2nd gear coming down a pretty good hill at 25mph or so.... "purrrrrrrrrrrrrrrr-puppup-purrrrrrrrr-pup-purrrrrr pupup".

Since putting the stock exhaust back on(except 2.25" past the CAT)... it seems to do it less... but still does it on occasion..... .Almost always on the lil hill/turn while leaving my house.

Everything above my post has covered the common causes....

How's your mileage?

PS> Just wanted to add.... Backfire can manifest anywhere but combustion chamber.... Tbody, air intake tube, AFM.... I've had the dizzy off one time and POPPPPPP! Right at the AFM.... freaked me out! haha. But the type you're explaining sounds clearly like exhaust poppage. Z's have a really common backfire problem when their AFM's go south... Which is VERY common around 200+K miles, from what I've seen(my dad had 3 Z's... all different years/models. Even at 80K miles in his 79 2x2 he had to replace the AFM.)
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Old 08-14-2012, 06:57 PM   #13
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Its because you drive a race car.
Yeah...a race car...that's a SUV. And powered by a gutless 4 banger!!!

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When I decelerate (usually in 2 or 3 gear), once the RPMs cross below 2000, I hear a rapid popping sound from my exhaust - disappears around 1200 RPM.
Strange, that coincides closely with the decel fuel cut rpm. Hmmmm...

Blows a pretty big hole(pun intended) in the fuel in the exhaust theory huh?

Here's my theory...it has nothing to do with fuel exploding anywhere in the exhaust system(or elsewhere for that matter). And is more than likely due to the free flowing nature exhaust system, now that you've modified it. Which would be due to lower backpressure. But maybe not in the way you might imagine. Think of the exhaust gasses flowing out of the tailpipe in a series of pulses, kind of like waves/troughs in the sea. What happens is one pulse/wave catches up with another one. When they combine(or technically interfere with each other) like this they create a larger pulse/wave in the process. What you're hearing is the larger(and therefore louder) pulse/wave mixed in with those that haven't combined/interfered with each other in such a way. With more backpressure the pulses/waves travel slower, and/or more steadily/smoothly, having much less opportunity to catch up with each other. And the stock exhaust system is probably designed/tuned to prevent and/or minimize such sounds from being audible(if they were to happen at all). Just a theory...I could be wrong.

Here's a pic of the pulse/wave interference process I'm trying to explain.
Click the image to open in full size.


I used to think it had something to do with the dashpot too. I just can't find much logic in that being the case. The more I've learned about things auto mechanical.
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Old 08-14-2012, 07:20 PM   #14
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Here's my theory...it has nothing to do with fuel exploding anywhere in the exhaust system(or elsewhere for that matter). And is more than likely due to the free flowing nature exhaust system, now that you've modified it. Which would be due to lower backpressure. But maybe not in the way you might imagine. Think of the exhaust gasses flowing out of the tailpipe in a series of pulses, kind of like waves/troughs in the sea. What happens is one pulse/wave catches up with another one. When they combine(or technically interfere with each other) like this they create a larger pulse/wave in the process. What you're hearing is the larger(and therefore louder) pulse/wave mixed in with those that haven't combined/interfered with each other in such a way. With more backpressure the pulses/waves travel slower, and/or more steadily/smoothly, having much less opportunity to catch up with each other. And the stock exhaust system is probably designed/tuned to prevent and/or minimize such sounds from being audible(if they were to happen at all). Just a theory...I could be wrong.

I used to think it had something to do with the dashpot too. I just can't find much logic in that being the case. The more I've learned about things auto mechanical.
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I dont think its fuel related either. My 3.0 started making that sound on decel immediately after I installed a high flow cat and a knock off flow master. And that was 4 years ago, it hasnt changed no matter what ive done with the dashpot or any other component of the intake or fuel system. And she's a healthy 3vz.
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Old 08-15-2012, 08:34 AM   #15
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Thanks everyone - this is great information!!

I replaced my dashpot with OEM parts a few years back so I don't think it's that....

I like the noise - my wife doesn't - I think she feels like something is broken on the truck LOL.
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Old 08-16-2012, 08:30 PM   #16
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I'm running a glasspack with no cat and it does the exact same thing. I like the sound also, kinda sounds like poporn
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Old 08-17-2012, 02:52 AM   #17
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Listen to a semi downshift
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Old 08-17-2012, 08:55 AM   #18
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Listen to a semi downshift
Not related. See "Jake brake".
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Old 08-17-2012, 12:58 PM   #19
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i have a 22r running on thr rich side with an unknown header and a 2.5 inch glasspack and 6 ft of flex pipe to the rear and i get the poping as well. i think its just fuel burning, its the cause of extra flow yo
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Old 10-01-2012, 09:38 AM   #20
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I am not convinced that it is fuel burn off in the pipe either. My 3.0 runs lean, really lean, I fry plugs every 3K miles and have all kinds of pinging. I have replaced my O2 sensor several times but didn't know about the issues with the Bosch universals. Also my exhaust was previously modified, and the mounting flange was all wrong for it. I took some measurements off another rig and did some modifications of my own. I installed a shiny new Denso sensor, and time will tell. There is not much information on Lean trending engines, it's all about rich.

Anyhow on the subject, I had the same sound, and it was strong while i was lean. Now that I did my changes I don't notice it at all anymore. I have a close to stock exhaust system with only an empty CAT. And a slightly better than stock muffler. This has been the setup for years. The new modification just removed some welded on stupidity that lifted the sensor out of the stream by at least an additional 1/4". I no longer notice any kind of pinging, and I now even have some carbon deposits on my plugs! YEY!! So I would think if this was a rich issue, than it would be reverse! Dash-Pot makes no sense because the popping will ocure for as long as you coast in gear down a hill. The Dash-pot only does the trim out for a second, it could solve a single back fire on close, but would not be effecting the stream for several minutes of coasting.

I have heard that running with low back pressure "kills your low end torque" is this true? and why? I would think this would have more to do with the popping. The sound wave propagation theory is quite interesting though!

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Old 10-01-2012, 09:38 AM
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