Backfiring on deceleration

[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:





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!!!

[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!!!!



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.

[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.

[Viper Specialty]

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

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!