Backfiring on deceleration

[Cowger]

Hi everyone,

My 2005 roadster has just developed a new behavior: On moderate to heavy deceleration (using engine braking alone), it will do its normal "burble" for about 3 seconds and then POP a pretty significant backfire. Simultaneous with the backfire, the upshift light goes yellow and the speedo drops by about 50%, just for a moment (other gauges may also move). Strange. After the single backfire, it goes to no burble -- just quiet deceleration.

The "normal" behavior that I've always known with the car is that it burbles on deceleration for 10 seconds, then goes to just running smoothly / quietly as long as your foot is off the accelerator.

Any ideas? It's like the "burble" programming is messed up. The car is otherwise running fine (as far as I can tell).

Somewhat related to this, does anyone have an OBD2 plug-in that adapts to an iPad app that they can recommend? I'd like to see what the ECU has to say about all this...

Thanks!
Bryan

[bluesrt]

you need a burble diagnostic aid- , not throwing any codes?

[Slithr]

Bryan,

This is the scan tool I use via Bluetooth, but unfortunately had to buy a Nexus 7 tablet to use it. Maybe an iPhone/iPad variant is now available ... not sure.

http://www.scantool.net/obdlink-mx-bluetooth/?gclid=Cj0KEQjw1NufBRCx8ayaqY2t6KkBEiQA2nLWm2H6LUl gd1fhkTKJJ3Xm-GI8WUrFt7wPmPBWWaoPg4gaArCx8P8HAQ

[slowhatch]

I use Torque Pro app for android.

[Steve M]

The backfire by itself wouldn't be much cause for concern, but coupled with this:


Simultaneous with the backfire, the upshift light goes yellow and the speedo drops by about 50%, just for a moment (other gauges may also move).

...would be cause for concern in my book. That sounds like a misfire of some sort...I'd be checking plugs and wires first, scanning for and diagnostic trouble codes next. If those things check out, I'd be looking at things that control when the fuel and spark gets thrown in the mix, like a crank/cam position sensor.

[Cowger]

Thanks, guys. It doesn't seem like there are any solid solutions out there for a wireless OBDII plug-in and iDevice app, so I think I'll just go down to the auto parts store today and borrow theirs or just buy one. I'll let you know what I find...

[Cowger]

The backfire by itself wouldn't be much cause for concern, but coupled with this:



...would be cause for concern in my book. That sounds like a misfire of some sort...I'd be checking plugs and wires first, scanning for and diagnostic trouble codes next. If those things check out, I'd be looking at things that control when the fuel and spark gets thrown in the mix, like a crank/cam position sensor.

Exactly, Steve, it's very strange behavior, and the suddenness of its onset has me thinking that something has failed. I definitely would like to understand what changed.

So I borrowed a OBD2 code reader today, and found no codes in it. My buddy said "right, unless the check engine light is on, there won't be any codes". Is this true? Would a more sophisticated reader find something that a simple one does not?

[mad prof]

Do you have any air leak in the exhaust system? I had very similar problems and found that the exhaust pipe was not sealing properly with the cat. After taking care of this, no more backfires, just a pleasant rumble on deceleration.

[usmcly]

Check the gap on your sprak plugs, I had the similar issues. Pulled all my plugs out and they were gapped at .32 when i installed them and 8 out of 10 was .38-.40, I replaced the plugs with new ones and gapped at .32 and everything went back to normal and power came back.

[Steve M]

Exactly, Steve, it's very strange behavior, and the suddenness of its onset has me thinking that something has failed. I definitely would like to understand what changed.

So I borrowed a OBD2 code reader today, and found no codes in it. My buddy said "right, unless the check engine light is on, there won't be any codes". Is this true? Would a more sophisticated reader find something that a simple one does not?

I don't know if a dealer scan tool would get more info, but there are plenty of conditions that take multiple (2 usually) events before it sets a code.

[Cowger]

Good suggestions, guys, thanks! I'll check plugs and the exhaust fittings and we'll go from there...

Bryan

[Mr White]

Exactly, Steve, it's very strange behavior, and the suddenness of its onset has me thinking that something has failed. I definitely would like to understand what changed.

So I borrowed a OBD2 code reader today, and found no codes in it. My buddy said "right, unless the check engine light is on, there won't be any codes". Is this true? Would a more sophisticated reader find something that a simple one does not?

That's how code readers work, they just tell you why the dash light is on. They're cheap, usually run around 50 bucks, and generally aren't really helpful except in those cases where the car runs fine and you can't figure out why the light is on.

A scan tool, will give you live data while the car runs. They vary in price, some are slightly better than the 50 dollar scan tool and others are ridiculous. These are useful for diagnosis, and they are sometimes hard to distinguish because advertisements will muddy up the line between code reader and scanner. We could get into a good discussion about scan tools and whatnot.

So the backfire thing... unburnt fuel is making it out of your cylinders. There could be multiple reasons, weak spark, bad injector, something is telling your computer more air is going in than actually is (which could be many parts), and <insert name of hated political figure here>.

This is going to sound like an odd one, but I had another Chrysler vehicle that the gauges and lights would go nuts. It wasn't all the time, but it would be really random. They were symptoms of the battery's end of life. The car started fine, every time, which was odd. It's worth taking it to a place that can test it. It could also account for a weak spark on deceleration.

If that checks out, then the real fun starts.

[AZTVR]

Hi everyone,

My 2005 roadster has just developed a new behavior: On moderate to heavy deceleration (using engine braking alone), it will do its normal "burble" for about 3 seconds and then POP a pretty significant backfire. Simultaneous with the backfire, the upshift light goes yellow and the speedo drops by about 50%, just for a moment (other gauges may also move). Strange. After the single backfire, it goes to no burble -- just quiet deceleration.


I am no expert in troubleshooting such problems. However, like some of those above, I will give an educated guess until an expert comes along.

Interesting that the two electrical symptoms that you mention are related to sensors on the transmission. Does that have any significance? If it does, then a point of commonality with those symptoms and a misfire or momentarily excess fuel issue might be the ground to the engine. If it were loose such that it was intermittently not touching in a specific scenario; maybe it could cause all of the symptoms. It seems like such an intermittent would cause more random effects than you are experiencing; but, at least such a loose or corroded/bad connection is not unusual.

[Cowger]

Well, I pulled 4 of the plugs and found them to all be well within spec (which for my car is 0.033 - 0.038"). This didn't necessarily surprise me, since I had replaced them just 10k miles ago.

And if there's an exhaust leak, I can't find one...

I continue to be interested in how the "burble" algorithm works, so if anyone knows, please pipe up. I'm assuming it's controlled by the ECU and looks something like:

If (throttle_position = 0 && [insert other conditions]) then inject a small amount of fuel into [X] cylinders for 10 seconds...

Er something like that. I'm of course just guessing, but I'm hoping that if someone actually knows how this works, it might provide some insight into what all sensors / inputs are involved in that programming....

[Steve M]

I'm guessing the burble you are referring to is DFCO - deceleration fuel cutoff. On the Gen 4 cars, it enables around 1500 RPMs and above as you coast down...the Gen 3 cars are probably similar in how it operates. I'm thinking it also pulls timing, but not sure on that...either way, when enabled, the fuel gets cut (I don't think it does completely though), and whatever is left ignites in the exhaust.

DFCO occurs between RPM set points under deceleration and no throttle input.

[Steve M]

I'm not sure if this will be useful, but these are the tables that control DFCO on a Gen 4...I'm not sure if the PCM can (and does) track the current gear selected, but the tables are set up that way. It looks at pedal position, engine RPM and vehicle speed (VSS = vehicle speed sensor). Above the enable RPM and VSS, DFCO can come into play (provided the engine coolant temp is above the values shown)...below the disable RPM and VSS, DFCO is not in play. I'd assume the Gen 3 cars have similar parameters and set points.

http://i305.photobucket.com/albums/nn227/meentss02/HPTuners%20Viper/DFCOTables_zps9c4d2cb2.jpg

[JonB ~ PartsRack]

This sure sounds like a Cam Sensor to me......

Note to Steve M.: good stuff, I'm impressed!


Hi everyone,

My 2005 roadster has just developed a new behavior: On moderate to heavy deceleration (using engine braking alone), it will do its normal "burble" for about 3 seconds and then POP a pretty significant backfire. Simultaneous with the backfire, the upshift light goes yellow and the speedo drops by about 50%, just for a moment (other gauges may also move). Strange. After the single backfire, it goes to no burble -- just quiet deceleration.

The "normal" behavior that I've always known with the car is that it burbles on deceleration for 10 seconds, then goes to just running smoothly / quietly as long as your foot is off the accelerator.

Any ideas? It's like the "burble" programming is messed up. The car is otherwise running fine (as far as I can tell).

Somewhat related to this, does anyone have an OBD2 plug-in that adapts to an iPad app that they can recommend? I'd like to see what the ECU has to say about all this...

Thanks!
Bryan

[Cowger]

I'm not sure if this will be useful, but these are the tables that control DFCO on a Gen 4...I'm not sure if the PCM can (and does) track the current gear selected, but the tables are set up that way. It looks at pedal position, engine RPM and vehicle speed (VSS = vehicle speed sensor). Above the enable RPM and VSS, DFCO can come into play (provided the engine coolant temp is above the values shown)...below the disable RPM and VSS, DFCO is not in play. I'd assume the Gen 3 cars have similar parameters and set points


Interesting, Steve, thanks! I agree that Gen3 probably has something similar...

[Cowger]

This sure sounds like a Cam Sensor to me......


Okay, you got my attention... Do you suggest I just go ahead and replace mine? As soon as I click "Post" here, I'll be searching for what that involves...

[67PLY]

Slightly off topic, I thought I read something a long time ago that Ralph had mentioned the decel popping and such was that the PCM pulses the injectors to cool the catalytic converters. Off road PCM eliminates the decel pulsing of the injectors because the majority of vehicles using it have no cats.

[Steve M]

Slightly off topic, I thought I read something a long time ago that Ralph had mentioned the decel popping and such was that the PCM pulses the injectors to cool the catalytic converters. Off road PCM eliminates the decel pulsing of the injectors because the majority of vehicles using it have no cats.

I would not be surprised at all...DFCO is a fuel saving feature, but the Viper PCM also seems overly protective of the catalytic converters, and opts to douse them with additional fuel when it deems appropriate. I do enjoy listening to it grumble on its way to a stop.

[Cowger]

Slightly off topic, I thought I read something a long time ago that Ralph had mentioned the decel popping and such was that the PCM pulses the injectors to cool the catalytic converters. Off road PCM eliminates the decel pulsing of the injectors because the majority of vehicles using it have no cats.

Seems pretty on-topic, actually, and that's good to know. I had always assumed the "burble" / decel popping was programming just because it sounded cool, like what a big-lobed, cammed-out, 4 bbl engine sounds like on deceleration. Knowing that it's more than that helps in this problem solving riddle.

I checked out what it takes to replace a camshaft sensor, and it looks like it's no big deal. Just need to make sure that the engine is rotated correctly before installing it. I'll give that a shot, next, and report back...

[Alex Mills]

Thanks, guys. It doesn't seem like there are any solid solutions out there for a wireless OBDII plug-in and iDevice app, so I think I'll just go down to the auto parts store today and borrow theirs or just buy one. I'll let you know what I find...

I use a PLX Kiwi to make a WiFi hotspot that I connect my iPhone to: http://www.plxdevices.com/product_cat.php?id=GSST

The iOS app I use is DashCommand: http://www.palmerperformance.com/products/dashcommand/iphone/index.php

[Cowger]

I use a PLX Kiwi to make a WiFi hotspot that I connect my iPhone to: http://www.plxdevices.com/product_cat.php?id=GSST

The iOS app I use is DashCommand: http://www.palmerperformance.com/products/dashcommand/iphone/index.php

Thanks, Alex. I looked at DashCommand and was intrigued by all the functionality, but the reviews I saw weren't very good. How has it worked for you?

[Alex Mills]

Thanks, Alex. I looked at DashCommand and was intrigued by all the functionality, but the reviews I saw weren't very good. How has it worked for you?

It's worked fine for me, but I have mostly used it for error code reading and clearing. I don't connect it to an iPad for live stats while driving or anything for example.

[Cowger]

It's worked fine for me, but I have mostly used it for error code reading and clearing. I don't connect it to an iPad for live stats while driving or anything for example.

Got it, thanks Alex.

I ordered a new camshaft position sensor and, while I was at it, figured I'd throw in a new crankshaft sensor as well, since it was cheap and looks fairly easy to replace. Interestingly enough, I found out that Dodge has already discontinued the crankshaft position sensor, so I'm still working at finding some place online that has them...

I'll get those installed and let you know how it goes. The car is actually getting a little bit better on this issue, but still is clearly not normal, so we'll see what these new sensors do.

Bryan

[Cowger]

I replaced both sensors (camshaft and crankshaft) today... The good news: I didn't break the car. The bad news: No improvement on this deceleration backfire issue. I suppose it makes sense that these sensors weren't the culprit -- they are both so fundamental to the basic running of the engine, and in all other aspects, it runs fine.

Next I think I need to double-check for air leaks on the exhaust system, as user "mad prof" has suggested. That will involve a bit more disassembly of the car, but seems worth checking out. I need to figure out which side is backfiring, which at least narrows it down to one bank...

[Cowger]

After concluding that the backfiring is only happening on the driver's side, I went ahead and pulled that cat / muffler assembly out today, put it onto my bench, and then tried to figure out how I was going to check for leaks. I finally came upon the idea of sealing both ends of the assembly with band clamps and some heavy duty rubber gloves, and then gently injecting pressurized air into the O2 sensor bung while checking for leaks:

http://i1099.photobucket.com/albums/g399/cowger1/photo1_zps98b37b5a.jpg (http://s1099.photobucket.com/user/cowger1/media/photo1_zps98b37b5a.jpg.html)

http://i1099.photobucket.com/albums/g399/cowger1/photo2_zps22ed119c.jpg (http://s1099.photobucket.com/user/cowger1/media/photo2_zps22ed119c.jpg.html)

Worked pretty well! It was easy to find two leaks at the joint right in front of the muffler.

So, I'm off to the auto parts store to get some exhaust sealant, unless anyone has a better idea of how to fix these leaks. (I could completely weld the joint, but that doesn't seem prudent for future service needs...)

[J TNT]

Glad to hear you found it ! Sounds like others will Benifit from your diagnosis :)

[Cowger]

Glad to hear you found it ! Sounds like others will Benifit from your diagnosis :)

Thanks for the enthusiasm, Jay, and I, too, was optimistic that I had found the problem. Unfortunately.... it didn't change a thing. I'm confident now that there aren't any leaks in the driver-side exhaust system, yet the problem continues, completely unchanged.

One thing that has been nagging at my brain, even as I was going about this recent exhaust system work: How can an errant backfire cause the speedometer to drop like that? Or light up the upshift light? Doesn't make sense. The more I think about it, the more I think that the backfire is a symptom, not a cause.

So, with all that, I'm ready to throw in the towel and take it down to the dealer for their diagnosis. It's only a mildly annoying problem that I could probably learn to live with, but at this point, I just need to know what the heck is causing this!!! :furious:

[Westxsrt10]

Smaller leaks when the exhaust is cold is normal and helps drain water out of the system. When the exhaust heats up most leaks subside from expansion.
If you find a big differential between the left and right side then there may be an issue from excess leakage. Check for leaks near the head manifold gasket as that will pull air in the system and cause o2 errors.

[Cowger]

Okay, a quick update on this issue: So far, I've done the following:
- New camshaft position sensor
- New crankshaft position sensor
- New air filters (needed 'em, anyway)
- New O2 sensors (up and down)
- Sealed up the exhaust resonator-to-muffler slip joint
- Installed new exhaust manifold gaskets

All-in-all, no change whatsoever. Aside from the new air filters, everything else has been just blindly throwing money at the problem in a swap-til-you-drop guessing game...

So... I've been in touch with Dan @ VSP and he suspects the PCM, at least enough to try to rule that out as a next step. The PCM controls the speedometer and the upshift light (recall that those both go crazy during the single backfire event). So a PCM is on its way, will be here on Thursday, and I'm looking forward to posting another update soon after!

[jvm728]

Good luck my friend. Interested to see the results of how this shakes out

[Roddy]

That "backfire" (SOUND) as you call it is entirely normal. People often call it a burble or crackle on deceleration. I can tell you my built LSX G8 GT had it, my 370Z and many other cars. It will sound louder with a free flowing exhaust as compared to stock. One thing I have to point out, is that your car is not "backfiring" from your description.

I could make you a video of my Catless SRT10 Viper..... with the right gas pedal modulation just before deceleration it sounds like a shot gun firing. Also technically it would be called foreward firing through the exhaust, backfiring is in reverse flow through the intake. If your sound is from the exhaust that is not a backfire.. and I 99.9% doubt that your Viper is backfiring.

I think you have spent alot of money trying to fix something that doesnt need to be fixed. Can you post a video? I'm pretty sure you are just hearing the deceleration crackles/burbles that are heard from pressure/backpressure in an exhaust system... granted its alot louder when you have a V10, and an aftermarket exhaust that has one set of pipes per 5 cylinders, with no balance tube (As is the design on the Vipers).

[Viper Specialty]

That "backfire" (SOUND) as you call it is entirely normal. People often call it a burble or crackle on deceleration. I can tell you my built LSX G8 GT had it, my 370Z and many other cars. It will sound louder with a free flowing exhaust as compared to stock. One thing I have to point out, is that your car is not "backfiring" from your description.

I could make you a video of my Catless SRT10 Viper..... with the right gas pedal modulation just before deceleration it sounds like a shot gun firing. Also technically it would be called foreward firing through the exhaust, backfiring is in reverse flow through the intake. If your sound is from the exhaust that is not a backfire.. and I 99.9% doubt that your Viper is backfiring.

What you are thinking this is, is not what is going on, nor is your terminology concrete. Backfire can certainly refer to a firing event AFTER the exhaust valve, and that is indeed firing "backwards". This is a true back-fire, and it is being caused by a ceased spark event loading fuel and air into the exhaust, followed by a cylinder firing and igniting the mixture. From what I can tell by the description and talking to the customer, it is absolutely NOT normal in the case of this car, and is also accompanied by other issues, such as a dropped bus message to the cluster. It has a PCM issue more than likely, hence why we sent a loaner to diagnose.

[Cowger]

That "backfire" (SOUND) as you call it is entirely normal. People often call it a burble or crackle on deceleration. I can tell you my built LSX G8 GT had it, my 370Z and many other cars. It will sound louder with a free flowing exhaust as compared to stock. One thing I have to point out, is that your car is not "backfiring" from your description.

I could make you a video of my Catless SRT10 Viper..... with the right gas pedal modulation just before deceleration it sounds like a shot gun firing. Also technically it would be called foreward firing through the exhaust, backfiring is in reverse flow through the intake. If your sound is from the exhaust that is not a backfire.. and I 99.9% doubt that your Viper is backfiring.

Thanks, Roddy, but I have to disagree. I know what my car sounded like before and I know what it sounds like now, and they are very different. And as Dan just pointed out, the other funky stuff going on pretty-much states without a doubt that something is wrong with this car...

[Cowger]

Found the problem!!!!

http://i1099.photobucket.com/albums/g399/cowger1/oldpcm_zps66c9de8f.jpg (http://s1099.photobucket.com/user/cowger1/media/oldpcm_zps66c9de8f.jpg.html)

That's my old PCM, and for whatever reason, some type of internal defect apparently developed that was causing all the issues I was seeing. I got Dan's loaner PCM today, swapped it in, and a short test drive confirmed it... nothing but sweet burble now!!! Needless to say, it's just really good to finally identify the root cause. :cool:

[Steve M]

Man...that's just odd. How often does a PCM fail like that? I can't say that I've never seen it...I had a PCM fail in my 2002 Camaro; I spent months chasing my tail on a random issue where the car would die just after you started moving on a cold start, but wouldn't do it any other time. PCM was replaced under warranty, and never had the problem again. They claimed water had gotten into it, but I never got to inspect the thing myself.

Glad to see you figured it out, but sorry to see you had gone through so much other stuff to figure it out.

[J TNT]

Glad to hear it worked out ! I admire your perseverance ����

[Viper Specialty]

Man...that's just odd. How often does a PCM fail like that? I can't say that I've never seen it...I had a PCM fail in my 2002 Camaro; I spent months chasing my tail on a random issue where the car would die just after you started moving on a cold start, but wouldn't do it any other time. PCM was replaced under warranty, and never had the problem again. They claimed water had gotten into it, but I never got to inspect the thing myself.

Glad to see you figured it out, but sorry to see you had gone through so much other stuff to figure it out.


This is the first time I have ever seen this exact failure... but its in a long line of oddball makes-no-sense failures I have seen. Definitely a logic error of some type.

[AZTVR]

This is the first time I have ever seen this exact failure... but its in a long line of oddball makes-no-sense failures I have seen. Definitely a logic error of some type.

Who the hell came up with the idea of putting computers in cars ? It will never work. :witless:

[Viper Specialty]

Who the hell came up with the idea of putting computers in cars ? It will never work. :witless:

I plead the 5th. While some of you want to strap a carburetor on and call it a day... I sit here and find ways to add more wiring, sensors, and other computer related gizmos. :)

[98intrigue]

Interesting. I'm trying to diagnose a similar issue that has been occurring with my car since I bought it in July. Three Crank Position Sensors and a couple blown ASD fuses later, I finally ordered the diagnostic manuals to see if I can trace some wiring and start troubleshooting.

[Cowger]

I plead the 5th. While some of you want to strap a carburetor on and call it a day... I sit here and find ways to add more wiring, sensors, and other computer related gizmos. :)

And we are all lucky that folks like Dan, who truly understand the electronics in our cars, are here.

Dealer / supposedly factory-trained technician: "uh, we're not sure what's happening... maybe try installing new exhaust gaskets..."

Dan: "I suspect your PCM and a loaner is on the way so we can at least eliminate that as a possibility."

'nuff said! :)

[Viper Specialty]

Interesting. I'm trying to diagnose a similar issue that has been occurring with my car since I bought it in July. Three Crank Position Sensors and a couple blown ASD fuses later, I finally ordered the diagnostic manuals to see if I can trace some wiring and start troubleshooting.

Shoot me an e-mail with all of the details, and you can consider it solved :)

Most of my loaner ECUs are on their way back if needed, but based on your ASD issue, its unlikely the ECU... but need more details.

[intense]

Dan is a real credit and blessing to the Viper community. If only all vendors were this reputable.

[Brian E]

Dan is a real credit and blessing to the Viper community. If only all vendors were this reputable.

Well said!