Loud periodic backfire on decel??
Thread Starter
Racer
Joined: Oct 2011
Posts: 440
Likes: 0
From: Mukwonago Wi
Loud periodic backfire on decel??
My mods are in my signature. After I am in the throttle pretty good and let the car engine brake, I get a very loud backfire. The car was tuned before I bought it. Could this be because the car is running rich?
Instructor
Joined: Mar 2006
Posts: 220
Likes: 0
From: ames ia
usually this is a combination of factors... However it primarily comes down to a couple things...
1) large cam with overlap
2) DFCO kicking in
Basically, when you get off the gas and start to coast, DFCO (Deceleration Fuel Cutoff) should kick in and cut the fuel. If this isn't quite dialed in correctly, with the overlap etc. you can get a little fuel due to the overlap down the exhaust. Since your exhaust and air is nice and hot from some aggressive driving, you basically get a small backfire, cackle, popping noise....
I'm assuming that's what you have without any extra info... Its not all that harmful actually, and a tuner can usually adjust your DFCO settings to get rid of it if it bothers you to much... Often times, because of the change in MAP readings due to the cam, it just isn't coming in quick enough....
1) large cam with overlap
2) DFCO kicking in
Basically, when you get off the gas and start to coast, DFCO (Deceleration Fuel Cutoff) should kick in and cut the fuel. If this isn't quite dialed in correctly, with the overlap etc. you can get a little fuel due to the overlap down the exhaust. Since your exhaust and air is nice and hot from some aggressive driving, you basically get a small backfire, cackle, popping noise....
I'm assuming that's what you have without any extra info... Its not all that harmful actually, and a tuner can usually adjust your DFCO settings to get rid of it if it bothers you to much... Often times, because of the change in MAP readings due to the cam, it just isn't coming in quick enough....
Thread Starter
Racer
Joined: Oct 2011
Posts: 440
Likes: 0
From: Mukwonago Wi
Great info on the DFCO. But this happens wll after I have backed out of the throttle. Usually when the car has settled back down to around 3,500 RPM or so.
usually this is a combination of factors... However it primarily comes down to a couple things...
1) large cam with overlap
2) DFCO kicking in
Basically, when you get off the gas and start to coast, DFCO (Deceleration Fuel Cutoff) should kick in and cut the fuel. If this isn't quite dialed in correctly, with the overlap etc. you can get a little fuel due to the overlap down the exhaust. Since your exhaust and air is nice and hot from some aggressive driving, you basically get a small backfire, cackle, popping noise....
I'm assuming that's what you have without any extra info... Its not all that harmful actually, and a tuner can usually adjust your DFCO settings to get rid of it if it bothers you to much... Often times, because of the change in MAP readings due to the cam, it just isn't coming in quick enough....
1) large cam with overlap
2) DFCO kicking in
Basically, when you get off the gas and start to coast, DFCO (Deceleration Fuel Cutoff) should kick in and cut the fuel. If this isn't quite dialed in correctly, with the overlap etc. you can get a little fuel due to the overlap down the exhaust. Since your exhaust and air is nice and hot from some aggressive driving, you basically get a small backfire, cackle, popping noise....
I'm assuming that's what you have without any extra info... Its not all that harmful actually, and a tuner can usually adjust your DFCO settings to get rid of it if it bothers you to much... Often times, because of the change in MAP readings due to the cam, it just isn't coming in quick enough....
Instructor
Joined: Mar 2006
Posts: 220
Likes: 0
From: ames ia
It would happen if for instance, you ran her WOT to say 6000 rpm, then let off the gas to coast down to a "reasonable speed", it would be fine until you got to around 3,500 rpm and then suddenly BANG it would backfire?
Follow up question, does it happen if you engage the clutch and let the RPM's come down to idle, and then slow down say.. in neutral or something... or is it ONLY under engine braking..
Last edited by GoatKart; Apr 21, 2013 at 08:37 PM.
Instructor
Joined: Mar 2006
Posts: 220
Likes: 0
From: ames ia
Can't say i have ever logged one to actually look and see, but that is certainly what the tune file would suggest as well, as the normal DFCO tables are in tact and able to be reached otherwise... I know modification of the clutch transition section also allows it to be active in those areas as well if a person ever wanted to..
Thread Starter
Racer
Joined: Oct 2011
Posts: 440
Likes: 0
From: Mukwonago Wi
Yes, this only happens after i have been coasting for a bit and engine breaking. Suddenly putting the car in neutral allowing the engine to come to idle with no load, it will not back fire. Only during engine braking and around 3500 RPM.
Also, I have noticed that the car has a "haze" behind it and a strong exshaust smell after a WOT situation.
Also, I have noticed that the car has a "haze" behind it and a strong exshaust smell after a WOT situation.
So to clarify...
It would happen if for instance, you ran her WOT to say 6000 rpm, then let off the gas to coast down to a "reasonable speed", it would be fine until you got to around 3,500 rpm and then suddenly BANG it would backfire?
Follow up question, does it happen if you engage the clutch and let the RPM's come down to idle, and then slow down say.. in neutral or something... or is it ONLY under engine braking..
It would happen if for instance, you ran her WOT to say 6000 rpm, then let off the gas to coast down to a "reasonable speed", it would be fine until you got to around 3,500 rpm and then suddenly BANG it would backfire?
Follow up question, does it happen if you engage the clutch and let the RPM's come down to idle, and then slow down say.. in neutral or something... or is it ONLY under engine braking..
Melting Slicks
Joined: Jun 2012
Posts: 3,163
Likes: 6
From: Oceanside California
I think its mostly to do with your exhaust. When I went with strait pipes I started getting popping noises on decel. With your LT's gutted cats, x-pipe, and bullets, there is little back pressure which allows air to gurgle around in the exhaust system when the engine is not under load enough to be constantly pushing the gases out.
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Melting Slicks
Joined: Nov 2008
Posts: 2,938
Likes: 65
From: Rochester Hills MI
because racecar. 
I like some backfiring in my exhaust on decel. Makes it sound mean. Doesn't affect the car unless its extremely excessive. Just happens when there is some extra fuel in your exhaust, which is almost always going to happen after a decent spirited pull.
Now if you are getting a loud pop as opposed to a gurgle, it could be due to a misfire.
I like some backfiring in my exhaust on decel. Makes it sound mean. Doesn't affect the car unless its extremely excessive. Just happens when there is some extra fuel in your exhaust, which is almost always going to happen after a decent spirited pull.
Now if you are getting a loud pop as opposed to a gurgle, it could be due to a misfire.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
It was my understanding it was only not active in a clutch transition situation or in neutral, but was still active in gear?
Can't say i have ever logged one to actually look and see, but that is certainly what the tune file would suggest as well, as the normal DFCO tables are in tact and able to be reached otherwise... I know modification of the clutch transition section also allows it to be active in those areas as well if a person ever wanted to..
Can't say i have ever logged one to actually look and see, but that is certainly what the tune file would suggest as well, as the normal DFCO tables are in tact and able to be reached otherwise... I know modification of the clutch transition section also allows it to be active in those areas as well if a person ever wanted to..