Backfire on deceleration?

vegasanthony

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So ive been running my 2000 5.9 magnum efi swap for about 6mo and i recently started getting backfires on deceleration..
Ive been reading on a causes of this..
Exhaust leaks seem to be one reason
And running lean another..

So my question is would a Exhaust leak near the header make it backfire
So badly near the muffler or should i start diagnosing at the fuel system
Seems to be drivers side only
20211030_151235.jpg
 

Lefty71

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Not so much exhaust leaks, as it is exhaust valves not closing 100 percent, in my experience. Maybe just time for a clean valve job??
 

royslead

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Are you running O2 sensors in each side or maybe just one, off the bank with the header leak? If so, that leak MAY be the cause. the O 2 sensor would pick that up and adjust for a lean condition, calling for more fuel. That extra, unburied fuel then ignites in the exhaust.
 

Ron816

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Not so much exhaust leaks, as it is exhaust valves not closing 100 percent, in my experience. Maybe just time for a clean valve job??
Shes dumping fuel, I had the same problem blew out 2 brand new mufflers turned out needle valve was stuck
 

TrailBeast

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So ive been running my 2000 5.9 magnum efi swap for about 6mo and i recently started getting backfires on deceleration..
Ive been reading on a causes of this..
Exhaust leaks seem to be one reason
And running lean another..

So my question is would a Exhaust leak near the header make it backfire
So badly near the muffler or should i start diagnosing at the fuel system
Seems to be drivers side only
View attachment 1715830010

check out this video of my Dart when it had a tiny unnoticeable leak between the header tubes where they meet at the collector.
Couldn’t hear it, couldn’t see it but once I found and fixed it the problem was gone.
A section of hose to use as a stethoscope/sound sniffer can be a big help in finding small leaks.
So yes, an exhaust leak allowing fresh air into the exhaust system absolutely can cause explosions inside the pipes and muffler.

https://youtube.com/shorts/EMIvqW__Zqs?feature=share
 

inertia

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Have someone block the tailpipes with cloths while its running.( careful-HOT)
The exhaust leaks will become evident .
Repair the exhaust leaks, - the backfiring will be eliminated, cuz of lack of (explosive) Oxygen being sucked in .
 
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shittyslant6

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So ive been running my 2000 5.9 magnum efi swap for about 6mo and i recently started getting backfires on deceleration..
Ive been reading on a causes of this..
Exhaust leaks seem to be one reason
And running lean another..

So my question is would a Exhaust leak near the header make it backfire
So badly near the muffler or should i start diagnosing at the fuel system
Seems to be drivers side only
View attachment 1715830010
usually, if you have open headers or manifolds it will pop out of the exhaust after running at a constant rpm; say 4000 after a little bit. I have a '73 318 in my duster with a carb. However, these pops are pretty small and wouldn't blow out a muffler like that. https://photos.app.goo.gl/Fr6RZLNPnryJoTh16
I wound not call that normal especially for EFI and a new magnum like that. more of an old school thing.
 

Phreakish

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I had leaks at my header flange to the head, and at the collector junction - car sounded like a herd of hunters on opening day. New gaskets, tightened bolts, now I only get the occasional pop on a super long clutch-out coast.

Lots of other things can cause pops and backfires, but it's 100% imperative that there be no exhaust leaks. Not even a little bit. After that, it's fuel and timing.
 

clementine

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Had that happen on a startup of a junkyard motor I swapped into a jeep grand. forgot to hook up the coil wire, and when I did....bam!! fuel in exhaust...super rich. How stinky when running?
 

vegasanthony

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Are you running O2 sensors in each side or maybe just one, off the bank with the header leak? If so, that leak MAY be the cause. the O 2 sensor would pick that up and adjust for a lean condition, calling for more fuel. That extra, unburied fuel then ignites in the exhaust.
Yes on this

Im gonna plug the exhaust and see what that does
 

pishta

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We had an International 2 ton truck used to post guard. We would load up and get driven to our 7-8 different guard posts and change guard and move on. At night, the duty driver would let off the gas and compression brake about 500m before the site and then blip the throttle and it would sound like an 81mm mortar going off. That would wake anyone up! Funny how that only happened at night? :p
 

royslead

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We had an International 2 ton truck used to post guard. We would load up and get driven to our 7-8 different guard posts and change guard and move on. At night, the duty driver would let off the gas and compression brake about 500m before the site and then blip the throttle and it would sound like an 81mm mortar going off. That would wake anyone up! Funny how that only happened at night? :p

I have a story that resulted in a muffler looking liAndre one in the op. Back in college, my buddy had an older car, and he would be cruising down the road, shut the ignition off, hit the throttle, and turn the ignition back on. Same thing, it sounded like a bomb. The final time he did it, we were cruising along a busy road, with an equally busy sidewalk, people jumped out of their skin, and I saw one boy hit the ground, ducking. That was the one that blew up the muffler. We laughed so hard, we didn't even realize the exhaust system was trashed, until a block away.
 

pishta

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I think this muffler was the size of a beer keg under this truck and it was old, probably already hollowed out but it was still in one piece. I think it was like this one. I'm sure the Navy just kept putting engines in this or kept rebuilding it.
1955-international-harvester-2-ton-r-150-flatbed-truck-for-sale-1.jpg
 

autopar3000

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I have the same problem with my 5.9 factory EFI Magnum. Someone told me it could be the fuel sync, but I can't see how the injectors spraying the intake valve a little too early or too late could cause an issue on the exhaust side.

@vegasanthony have you checked your fuel sync setting? I'm running OBD1 so it's not a simple thing for me. I used my best guess and seat of the pants to tune the sync.
 

royslead

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I have the same problem with my 5.9 factory EFI Magnum. Someone told me it could be the fuel sync, but I can't see how the injectors spraying the intake valve a little too early or too late could cause an issue on the exhaust side.

@vegasanthony have you checked your fuel sync setting? I'm running OBD1 so it's not a simple thing for me. I used my best guess and seat of the pants to tune the sync.

Not likely the fuel sync. I've corrected many of those, back in the day; none of them caused an issue like this. Impossible? Well there's always something that teaches us something new...On that note, how can you not get a hold of a scan tool for OBD-1 ? Chrysler used that same connector from the '80's, up until '95. I suppose, being in Ontario limits your availability, especially, if you're in a remote area. I would think maybe an older repair shop would have something. A lot of aftermarket scan tools had that capability, maybe you can find one, or even better - find an old Chrysler DRB II, or III. They are out there.
 
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autopar3000

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Not likely the fuel sync. I've corrected many of those, back in the day; none of them caused an issue like this. Impossible? Well there's always something that teaches us something new...On that note, how can you not get a hold of a scan tool for OBD-1 ? Chrysler used that same connector from the '80's, up until '95. I suppose, being in Ontario limits your availability, especially, if you're in a remote area. I would think maybe an older repair shop would have something. A lot of aftermarket scan tools had that capability, maybe you can find one, or even better - find an old Chrysler DRB II, or III. They are out there.

Thanks for confirming my thoughts on the fuel sync. In your experience, what symptoms would a bad sync display?

It's not impossible to find a scan tool. I haven't been to see my usual mechanic since I completed the swap, I'm sure he would have the adapter for his snap on tool. I've put a bit of effort into finding a DRB but many sellers over value them, in my opinion.

The car runs friggin awesome, aside from the backfiring on deceleration, so I'm not worried about it. I was also thinking that I don't get any backfiring below around 2400 RPM, it's only when I back off at highway speeds (A904 with 3.23 gear with 26" tire, so about 2600 rpm at 65 mph). So maybe the backfiring isn't entirely abnormal, considering the trucks that these engines originally came in had the 46rh / 46re with a 0.69 overdrive, so there wouldn't be a lot of decelerating above 2400 rpm. And with the stock Y pipe and big huge stock muffler it might not be so obvious as it is with my long tube headers, 2.5 inch dual exhaust, and performance mufflers.

Anyway, I'm putting a 46rh into the car next year, so we'll see what happens with the backfiring when the revs are way lower on deceleration. And I also picked up an entire OBD2 harness, intake with all sensors and injectors, and computer, so if I want the ability to put a tune into it for further performance mods I can swap to OBD2. But I just got it running a couple months ago, and again it's running pretty good on the old OBD1 setup now, so I'm not going to punish myself with a rewiring job just yet.
 

royslead

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If memory serves, a little bit of a hunting / non-smooth idle, perhaps a bit of acceleration lag. It would have to be REALLY off, I would think to cause any real drive ability issues (though it is possible to create a no start). If you get a chance to put a scanner to it, get it as close to 0 degrees, as possible, you may see it toggle back and forth, between +/-, and 1 and 0- pretty normal. If its all over the place, start looking for worn distributor parts, intermediate gear / shaft bushing, etc. in order to set and check it accurately, the computer has to be put into the proper mode, don't go by the sensor readings / normal screen pids, because the computer is altering that number constantly.
 
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Swinger 340

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Just a thought, being the popping is only after a let off after high RPM and no exhaust leaks can be found, it's possible a shorten exhaust pipe is the cause, by cool air being suck in and reaching the heated air in the pipe. I have always seen a Cat. ahead of the muffler, never after.
 

Ricks70Duster340

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I think this muffler was the size of a beer keg under this truck and it was old, probably already hollowed out but it was still in one piece. I think it was like this one. I'm sure the Navy just kept putting engines in this or kept rebuilding it.
View attachment 1715831408
Some of those old trucks had mufflers that looked like trash cans!
 

AJ/FormS

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This is NOT a backfire, but rather this is an afterfire.
All afterfires are caused by the exact same thing; namely a combustible mixture being lit off.
Eliminate any one of;
the fuel,
the oxygen, or
the ignition source,
and the afterfires should cease.
On decel with a programmable EFI, the easiest one to eliminate is probably the fuel.
On a carbed car, eliminating the oxygen is probably the first go-to.

On both, as to the ignition source, you want to start the fire in the chamber early enough on decel, so that the fuel/air charge has time to completely be consumed before the exhaust valve opens up. If the mixture is too lean, it may happen that the oxygen molecules are too far apart, and not all the fuel burns. This is called a missfire. Then when the exhaust valve opens, the header yanks the goofy charge out, and the hot exhaust valve can spark it up. Or maybe it doesn't and the O2 reads lean, and dumps more fuel in, which then accumulates in the system. The system is now under pressure relative to atmosphere, and the header may no longer be yanking from the head, but may instead yank atmosphere up the back side. And there is your bomb, ready to go off, all it needs is a spark or a hotspot, or a bit of flame inside the muffler, and boom!

The fire thus created will want to go up towards the head, but will likely run out of oxygen..... but the condition under decel if still existing, will continue to pump unburned fuel towards the muffler...... feeding the bomb until it runs out of oxygen..
Then it starts over, until the decel is quit.

Or if the header is still working and your cam has a lot of overlap, the header could be yanking air from the plenum, out into the primary pipe, which then mixes with the unburned fuel and voila there is your bomb from another angle.
This air could be coming from; the PCV, the idle-air bypass, the throttle valve opening, the AIS motor, even a ruptured diaphragm in the brake booster, etc. It all depends mostly on your sync, the pressure in the intake, and programming.
Sometimes more is less; that is to say a tiny bit more fuel on decel eliminates the missfire, and thus cleans up the exhaust, and thus, less afterfires or the elimination of them.
One thing occasionally missed is an intake sucking air at a port. Because you say the afterfires are only happening on the one bank, this is a real possibility. Check your plugs on that bank, maybe they have a story to tell.
 

autopar3000

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@vegasanthony did you ever resolve this issue? I just fixed mine. TL/DR: it was the EFI tune.

I put in the 46rh so now my revs are super low on deceleration, but that didn't fix it. There was less popping because less engine deceleration, but it was still there.

The 46rh was using pressure switches to control the OD and lockup functions. It was working fine, but doesn't allow for kickdown out of 4th or lockup so I would have to flick the rocker switch if I wanted that. I could have installed vacuum switches to do it automatically when the vacuum drops, but why would I do that when I'm already running the factory PCM?

I know the transmission would need a vehicle speed sensor to make it work, but that would do away with my cable driven speedo since the Magnums were electronic speedos. I was prepared to give up my speedo but then I saw on FABO a Magnum EFI expert mention you can use a speed sensor from a late 80s Ramcharger and still use the cable speedo (shout out to @magnummopar).

Bottom line, I installed the speed sensor, wired it all up, and now I have zero after firing on deceleration (plus PCM controlled OD and LU, which was the whole point of the work). So I guess the PCM needs to know the car is moving before it will cut the fuel on engine deceleration.
 
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