Backfires on Deceleration
Thread Starter
Cruising
Joined: May 2007
Posts: 13
Likes: 0
From: Stroudsburg PA
Backfires on Deceleration
(first Post) Just joined the Corvette family last week, bought a 1991 with a 6 peed. Both mufflers had holes in them and the car would backfire as I would decelerate. Replaced the mufflers with a set from Mid America. Stainless Steel LT-1 look a likes. Car still pops and backfires as I back off the acelerator. Stops as soon as I put the clutch in or shift up into a higher gear. Replaced the plugs, Cap, Rotor, wires look good. Car seems to perform great under load, just the backfire issue. Any help would be appreciated.
Melting Slicks
Joined: Jul 2004
Posts: 2,018
Likes: 1
From: OR
Can't remember what the real name of these things are, but the slang name back in the day was gulp valves. They work with the air injection reaction system. Assuming yours hasn't been disconnected, air is injected into the exhaust manifolds at idle, cruise and acceleration. On deceleration, the air is supposed to be diverted from the exhaust usually to the intake. That's because unburned fuel is in the exhaust on deceleration. If air is injected at that point, it ignites the fuel in the exhaust resulting in an explosion. When one of these valves fails, air is still pumped into the exhaust on deceleration. That's my guess only because I've seen it a million times on different cars over the years. jmo
Racer
Joined: Aug 2005
Posts: 385
Likes: 0
From: Apopka Florida
My car does the same thing, and since your mufflers had holes I can almost garuantee you have the same problem I do.
I had one catalytic converter go bad and instead of replacing them I just hollowed them out and put on 02 sims to clear the codes. As soon as I hollowed them out they would cause backfire on decel. Chances are there is nothing wrong with the car. Jack that baby up and check for 02 sims, and/or check the cats to make sure they are still full of that grey silicone honeycomb material.
I had one catalytic converter go bad and instead of replacing them I just hollowed them out and put on 02 sims to clear the codes. As soon as I hollowed them out they would cause backfire on decel. Chances are there is nothing wrong with the car. Jack that baby up and check for 02 sims, and/or check the cats to make sure they are still full of that grey silicone honeycomb material.
Instructor
Joined: Oct 2006
Posts: 246
Likes: 2
From: Elk Plain WA
St. Jude Donor '11
I Agree
The backfire is fuel exploding in the exhaust. I have seen it on several cars. Unless you find it particularly annoying, I would say don't worry about it. I like the suggestion of disconnecting the belt to see if it goes away.
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Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 405
From: Lakeville MI
You might want to investigate the reasons for excess unburned fuel in the exhaust. My guess is one or more leaking fuel injectors, or an overly rich condition prior to deceleration.
The Decel fuel cut-off (DFCO function) cuts fuel during deceleration. The engine is still pumping air into the exhaust. If there is excess fuel present from a previous event or a leaky injector, conditions are ripe for combustion.
The Decel fuel cut-off (DFCO function) cuts fuel during deceleration. The engine is still pumping air into the exhaust. If there is excess fuel present from a previous event or a leaky injector, conditions are ripe for combustion.
Thread Starter
Cruising
Joined: May 2007
Posts: 13
Likes: 0
From: Stroudsburg PA
Backfire during Deceleration
I appreciate all the imput. I may have a small exhaust leak upstream of the cats. It's odd that sometimes the backfiring is worse than at other times. I will check into the exhaust and post the results. Thanks again,
