Popping noise in the exhaust under heavy deacceleration
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Champion
Joined: May 2008
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From: Apex, NC
Popping noise in the exhaust under heavy deacceleration
For those who have wondered in the past what causes the popping noise in the exaust while engine braking or any heavy deacceleration (cause I know it's been brought up a couple times before), I found the answer while reading the FSM.
Engine speed
DECELERATION MODE
This is a Closed Loop mode. During deceleration,
the following inputs are received by the powertrain
control module (PCM):
Engine coolant temperature
Manifold absolute pressureThis is a Closed Loop mode. During deceleration,
the following inputs are received by the powertrain
control module (PCM):
Engine coolant temperature
Engine speed
Throttle position
Exhaust gas oxygen content (O2 sensor)
Vehicle speed sensor
Air Conditioning (A/C) control positions
Exhaust gas oxygen content (O2 sensor)
Vehicle speed sensor
Air Conditioning (A/C) control positions
A closed throttle input, along with an abrupt decrease
in manifold pressure, indicates to the PCM
that the vehicle is in a hard deceleration. The PCM
may reduce injector firing to just once per engine
revolution to lean the air-fuel mixture. This is sensed
through the O2S sensor during hard deceleration.
The PCM grounds the EGR and EVAP purge solenoids.
This stops EGR operation and EVAP canister
purge functions.
in manifold pressure, indicates to the PCM
that the vehicle is in a hard deceleration. The PCM
may reduce injector firing to just once per engine
revolution to lean the air-fuel mixture. This is sensed
through the O2S sensor during hard deceleration.
The PCM grounds the EGR and EVAP purge solenoids.
This stops EGR operation and EVAP canister
purge functions.
So lets break that down. On the 8 cylinder engine, there are 4 cylinders firing every revolution. So under this "deacceleration mode" the number of cylinders can be cut down by as much as 1/4. This means that when one cylinder fires, a high pressure pocket of exhaust gasses goes into the pipe, followed by 3 much lower pressure pockets of just air. Its the same theory as thunder! When lightening strikes, it creates a high pressure pocket which expands the air around the lightening, and when the lightening bolt subsides, a low pressure pocket is immediately created sucking the air back in... creating a boom. Same thing, the high pressure exhaust gasses escaping out the tip, followed by the low pressure air behind it creates that loud popping noise.
So my point is, it's normal, there is nothing wrong with the engine, you aren't driving a POS (I remember one of ya'll saying it makes your truck sound like POS :P), so just enjoy the rediculous sound!
Last edited by 95_318SLT; Nov 9, 2009 at 07:31 PM.
Veteran
Joined: Mar 2009
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From: Mandan, ND
How come it does is MUCH more when its cold... Mr. Bill Nye?
It is VERY appearant in my truck as I basically have straight pipes. I come down a large long hill every day and rather than cooking my breaks I come down it in 2nd which will hold me at about 30 mph.
In the morning (esp now that its cold out) the popping and crackling is almost constant. If I go home for lunch and come down that hill when my motor and exhaust is warm, there is almost NO back-rap and it is just a smooth grumble.
It is VERY appearant in my truck as I basically have straight pipes. I come down a large long hill every day and rather than cooking my breaks I come down it in 2nd which will hold me at about 30 mph.
In the morning (esp now that its cold out) the popping and crackling is almost constant. If I go home for lunch and come down that hill when my motor and exhaust is warm, there is almost NO back-rap and it is just a smooth grumble.
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Champion
Joined: May 2008
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From: Apex, NC
Lol, its very apparent in my truck too with the headers, no cat, and the flowmaster 40. I've also noticed that it does it more when cold, but still noticable for me when warm too. Here are my thoughts on why... that paragraph says "may reduce to just 1 injector" which to me means it fires as many as it has to in order to produce the mixture it wants with 1 being the minimum. Remember it's closed loop so the o2 sensor is working. While the engine is cold, it is in its open loop, so being forced into its closed loop before the ECT giving a normal temp reading and without the o2s being fully heated, I think it injects a rich mixture in the first cylinder of the cycle, so it doesn't need to use any of the other 4 for that revolution, and perhaps when it is warm, it uses 2 or 3 cylinders with a leaner mixture in each one.
All this may or may not be right... obviously I didn't program the PCM so it's just my opinion... but it's an educated one at that! BTW, that Bill Nye comment made me laugh :P
All this may or may not be right... obviously I didn't program the PCM so it's just my opinion... but it's an educated one at that! BTW, that Bill Nye comment made me laugh :P
Registered User
Joined: Dec 2009
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Thank you so much for clearing that up. Since i have owned the truck i couldnt figure out or Relax with that sound. Ive had it straight piped and now flowmaster 40 and that popping was getting on my nerves. My old 87 never popped like that. now i know thanks
Champion
Joined: Dec 2003
Posts: 4,119
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From: Sycamore, Illinois (displaced to Arkansas)
I wanna say I hear it sometimes on "ricers," and REALLY noticed it on this SRT-4 like 5 years ago (which would have made it just about brand new) that was in a WalMart parking lot.
