L98 exhaust / backfire question
Thread Starter
Team Owner
Joined: Nov 2000
Posts: 29,689
Likes: 9
From: Central Florida
Jax Events Coordinator
CI 3-4-5-6-7-8-9 Veteran
St. Jude Donor '03-'08
L98 exhaust / backfire question
When I bought my car it had a y pipe into a cat then another y pipe to the two mufflers.
I pulled the cat and installed two resonators to kill the drone.
It worked.
However, it also created a new issue. Now the car will backfire when decelerating in gear. Its fine when I am accelerating and driving, but if I step on the brake without engaging the clutch, the car will occasionally backfire. Its more prominent if the rpms are higher than about 2500.
I'm not talking about the normal burbling, I mean a gunshot loud backfire.
I took the car back to the shop that did the work and asked the to check for leaks. They put it up on the lift and went through the system with a stethoscope and couldn't find any leaks.
I am going to fix this but I am not sure of the root cause. I'm sure going back to the original setup would fix it immediately but I would prefer to maintain the dual setup. Is it backfiring because there is no longer any back pressure that was there with the cat in place, or would a simple x pipe fix the problem. Obviously an x pipe would be a much less expensive proposition as opposed to two cats.
I'm sure I am not the first person to turn an L98 into a true dual exhaust, what did you guys do to eliminate this issue?
I pulled the cat and installed two resonators to kill the drone.
It worked.
However, it also created a new issue. Now the car will backfire when decelerating in gear. Its fine when I am accelerating and driving, but if I step on the brake without engaging the clutch, the car will occasionally backfire. Its more prominent if the rpms are higher than about 2500.
I'm not talking about the normal burbling, I mean a gunshot loud backfire.
I took the car back to the shop that did the work and asked the to check for leaks. They put it up on the lift and went through the system with a stethoscope and couldn't find any leaks.
I am going to fix this but I am not sure of the root cause. I'm sure going back to the original setup would fix it immediately but I would prefer to maintain the dual setup. Is it backfiring because there is no longer any back pressure that was there with the cat in place, or would a simple x pipe fix the problem. Obviously an x pipe would be a much less expensive proposition as opposed to two cats.
I'm sure I am not the first person to turn an L98 into a true dual exhaust, what did you guys do to eliminate this issue?
Last edited by Joecooool; Jun 29, 2008 at 06:32 PM.
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Mine started popping occasionally after opening up my exhaust as well. (To be clear, backfiring often means back up thru the intake).
Based on posts like the one above, you have to assume it's not normal -- that there's another problem. In my case, I eventually found two issues that probably explain the problem. One was a plug wire that was shorting against the head as it rounded the back corner. The other was one or two failing injectors.
After I replaced ignition parts, it ran almost perfect for a few weeks. Then the injectors started acting up. I confirmed that diagnosis by measuring the resistance hot & cold of the injectors. Then I pulled the plenum/runners. That was a PIA!
I've never reassembled due to my desire to mod it. (So I haven't confirmed the second "cure".) Still, I have to think the failing injectors were leading to a lean miss where unburnt fumes would enter the exhaust and explode.... BOOM.
The reason why it's happened after the swap could be from change in mixture (O2 sensor readings) and/or your improved ability to hear pops with an open exhaust.
Check your wires carefully and your injector resistance.
Based on posts like the one above, you have to assume it's not normal -- that there's another problem. In my case, I eventually found two issues that probably explain the problem. One was a plug wire that was shorting against the head as it rounded the back corner. The other was one or two failing injectors.
After I replaced ignition parts, it ran almost perfect for a few weeks. Then the injectors started acting up. I confirmed that diagnosis by measuring the resistance hot & cold of the injectors. Then I pulled the plenum/runners. That was a PIA!
I've never reassembled due to my desire to mod it. (So I haven't confirmed the second "cure".) Still, I have to think the failing injectors were leading to a lean miss where unburnt fumes would enter the exhaust and explode.... BOOM.
The reason why it's happened after the swap could be from change in mixture (O2 sensor readings) and/or your improved ability to hear pops with an open exhaust.
Check your wires carefully and your injector resistance.
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
I too get these loud pops when I let off the gas. It happened before I got the new exhaust, and after. It'll scare the mess out of someone cruising next to you! 
I also figured out that I have failing injectors, so I'm willing to bet that after I replace them, the popping noise stops.
I also figured out that I have failing injectors, so I'm willing to bet that after I replace them, the popping noise stops.
Burning Brakes
Joined: Feb 2008
Posts: 988
Likes: 7
From: Sarasota Fl
With my Corsa cat-backs when I rev in neutral or park (automatic trans) I will get an audible POP when I suddenly lift off the gas. I was told by a forum member that commented on my YouTube video that it was excess gas combusting rapidly in the exhaust. I don't know if this is your specific problem, but checking injectors and the other suggestions could possibly cure it OR lead to you to find another unrelated or related problem. JMHO.Jonathan
Team Owner
Joined: Sep 2001
Posts: 62,160
Likes: 1,733
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Excess gas combustion in the exhaust is the likely culprit. I would check first the resistance of each injector, and then the spark to each. If one is not firing properly you will have this problem. Stock exhaust would muffle this problem.
But I suppose it could be a diverter valve issue. Depends on exactly what you did.
But I suppose it could be a diverter valve issue. Depends on exactly what you did.
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
If there was enough fumes to cause popping by adding extra air, I would think you were running rich or had one of the problems mentioned (spark/injector).
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Drifting
Joined: Feb 2003
Posts: 1,614
Likes: 163
From: Davis CA
It happens occasionally with my 86 4+3 when doing track days on road courses -- like yesterday at Infineon. Not really an explosion (destructive) but a definite pop. My engine runs cool (about 200 at the end of 20 minute session in Infineon's wonderful Bay Area weather), and I attribute the pop to that (mixture a little rich).
Whatever, I don't worry about it because it does not sound like it would cause any damage.
Whatever, I don't worry about it because it does not sound like it would cause any damage.
Drifting
Joined: Feb 2003
Posts: 1,614
Likes: 163
From: Davis CA
Extending EGR tube for tall valve covers
I wrote many months ago about using a corrugated copper/brass water heater "hose" to extend the EGR tube on early TPI C4's for those who switched to tall valve covers. Well, there is a limit to this fix. When prepping the car for a track weekend, I noticed metallic dust in the engine bay. I convinced myself that it might be from the more aggressive brake pads. It wasn't.
The (about) 3 inch copper/brass extension was becoming Swiss cheese. It reached full holiness after my first track session. I was able to replace the "tubing" with the same material in order to finish the day (thanks to my wonderful wife who delivered the $10 hardware store part to the track). So, if you take this route, it will work but it is a temporary fix. Good news is that I didn't miss a session.
The (about) 3 inch copper/brass extension was becoming Swiss cheese. It reached full holiness after my first track session. I was able to replace the "tubing" with the same material in order to finish the day (thanks to my wonderful wife who delivered the $10 hardware store part to the track). So, if you take this route, it will work but it is a temporary fix. Good news is that I didn't miss a session.
Safety Car
Joined: Jan 2006
Posts: 3,566
Likes: 4
From: Mt Laurel NJ
St. Jude Donor '06-'07
I too get these loud pops when I let off the gas. It happened before I got the new exhaust, and after. It'll scare the mess out of someone cruising next to you!
I also figured out that I have failing injectors, so I'm willing to bet that after I replace them, the popping noise stops.
I also figured out that I have failing injectors, so I'm willing to bet that after I replace them, the popping noise stops.
Randy
