Minor backfiring when decelerating
Thread Starter
7th Gear
Joined: Aug 2022
Posts: 7
Likes: 1
Minor backfiring when decelerating
I recently purchased a 1992 C4. It is backfiring a bit (not bad) when rapidly decelerating, especially if I engine brake. The seller noted this when I bought it, and said his diagnosis was that the electric air pump was not shutting off correctly when the engine is warm. My understanding is that the air pump pumps air into the exhaust manifold through two ports (one on each side) when the engine is cool to assist the CATs and is supposed to shut off when it's warm. If it's not shutting off, then there is too much O2 in the exhaust manifold and it will ignite any excess fuel in there and backfire. So, his solution was to disconnect the air hoses into the exhaust manifold and plug the holes. That said, he also unplugged the electric air pump, so it's not even running now. Why didn't that fix it? The seller said that some air was likely still getting in there when warm, so plug the holes. Does that make sense to you? I'm cool leaving the air pump, even replacing it.
One more thing...the seller removed the mufflers and left the CATs. I have the muffler and can reinstall. Related?
One more thing...the seller removed the mufflers and left the CATs. I have the muffler and can reinstall. Related?
Burning Brakes
Joined: Jun 2022
Posts: 844
Likes: 212
From: Florida
From my experience in a 2002 Z28 I owned years ago, you are on the right track, but blaming the wrong thing.
I had the same issue with the Camaro, however, it wasn't the AIR pump. It was the check valves sitting on the manifolds. They are one way valves. If the show air both ways, then you get the backfiring.
Drive your car for a bit, pop the hood and put your hand on the pump. It should not be running.
If that is the case, then is go straight for the check valves.
Ric
Edit: You may also have a leak between the manifolds and cats.
I had the same issue with the Camaro, however, it wasn't the AIR pump. It was the check valves sitting on the manifolds. They are one way valves. If the show air both ways, then you get the backfiring.
Drive your car for a bit, pop the hood and put your hand on the pump. It should not be running.
If that is the case, then is go straight for the check valves.
Ric
Edit: You may also have a leak between the manifolds and cats.
Melting Slicks
Joined: Mar 2002
Posts: 2,874
Likes: 764
From: Sligo PA
I would think that if this is whats going on then the O2 sensors are way off too. Being that they would be seeing lots more air then they should it would add too much fuel. I know in OBDII systems this would throw a code and make the SES light come on. I'm not sure on a 92.
Thread Starter
7th Gear
Joined: Aug 2022
Posts: 7
Likes: 1
Thanks for the feedback.. I'll warm it up and check the pump, as you say, and replace the check valves if needed. Easy and inexpensive thing to try for sure. If that's not it, we can dig further.
I forgot the car came with a scanner! It threw two codes: 32 EGR FAILURE and 26 QDM FAULT. My guess is bad check valves could cause both those codes? Yeah?
I forgot the car came with a scanner! It threw two codes: 32 EGR FAILURE and 26 QDM FAULT. My guess is bad check valves could cause both those codes? Yeah?
Burning Brakes
Joined: Jun 2022
Posts: 844
Likes: 212
From: Florida
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Drifting
Joined: Jun 2020
Posts: 1,355
Likes: 396
From: Northwest Ohio
Wait hold on. I am guessing you have a 6 speed right? What happens if you decel with the AC on? You are talking about the pops and occasional rifle shots once everything is good and hot especially if you downshift decel?
If so there is likely nothing wrong! 92-93 batch fire the injectors, this allows a bit more fuel when the engine is unloaded/engine braking to make its way into the exhaust and “pop”. It may also be “tuned in” by GM in the OEM tune.
I have a 93 and it does this gloriously through my corsa. When I had the stock cat back it was much louder in the cabin (resonator resonating through the floor).
I would have to hear/feel it to know if you are experiencing something abnormal, so you could quite well have a problem. I changed the O2 sensors on mine for good measure and it made no difference…you might want to consider doing the same…also perhaps leaky injector(s).
Again SOME of this should be expected on a 6 speed 92/93.
If so there is likely nothing wrong! 92-93 batch fire the injectors, this allows a bit more fuel when the engine is unloaded/engine braking to make its way into the exhaust and “pop”. It may also be “tuned in” by GM in the OEM tune.
I have a 93 and it does this gloriously through my corsa. When I had the stock cat back it was much louder in the cabin (resonator resonating through the floor).
I would have to hear/feel it to know if you are experiencing something abnormal, so you could quite well have a problem. I changed the O2 sensors on mine for good measure and it made no difference…you might want to consider doing the same…also perhaps leaky injector(s).
Again SOME of this should be expected on a 6 speed 92/93.
Last edited by pedricd; Aug 28, 2022 at 10:27 AM.
Le Mans Master
Joined: Jun 2009
Posts: 6,528
Likes: 752
From: South Dakota
Maybe a few others with 92-93 6 speeds can comment if they have this issue while engine braking.
Drifting
Joined: Jun 2020
Posts: 1,355
Likes: 396
From: Northwest Ohio
There are threads where owners complained they made a bunch of mods, tuned the car etc… and it *no longer* did this, and they wanted it back lol!!!!
Reason that the 6 speed does this most is due to the propensity to engine brake vs the auto, plus the auto maintains a bit more load on the motor preventing it. That is why I mentioned AC, if the AC is on it still gurgles but the extra load eliminates the “pops”.
EDIT: here’s one -
https://www.corvetteforum.com/forums...tion-6spd.html
Last edited by pedricd; Aug 28, 2022 at 02:41 PM.
Thread Starter
7th Gear
Joined: Aug 2022
Posts: 7
Likes: 1
Cool, thanks a ton for the additional input. I'll drive it with A/C on and see if there is a difference, as suggested.
Yes, it's a 6 speed, by the way. I should have said the in my OP.
Also, it seems to be better with air pump plugged in likes its supposed to be ( the seller had unplugged it to see if that helped). More driving necessary. What a bummer. 😁
Yes, it's a 6 speed, by the way. I should have said the in my OP.
Also, it seems to be better with air pump plugged in likes its supposed to be ( the seller had unplugged it to see if that helped). More driving necessary. What a bummer. 😁
Safety Car
Joined: Jun 2017
Posts: 4,786
Likes: 425
From: North Georgia, USA
I've seen a dirty MAF sensor cause that issue on many GM engines from the 1990s.
You might try some MAF cleaner on the MAF wire. Use only actual "Mass Air Flow Sensor cleaner." I like the CRC stuff, which is easy to find. Follow the instructions on the can. Usually it's just spray it on and let it drip off and air dry.
It can't hurt and it might help
You might try some MAF cleaner on the MAF wire. Use only actual "Mass Air Flow Sensor cleaner." I like the CRC stuff, which is easy to find. Follow the instructions on the can. Usually it's just spray it on and let it drip off and air dry.
It can't hurt and it might help
Last edited by C6_Racer_X; Aug 28, 2022 at 07:15 PM. Reason: I hate autocorrect!
Pro
Joined: Aug 2020
Posts: 647
Likes: 126
From: Liliha Bakery stuffing my face with coco puffs!
I've seen a dirty MAF sensor cause that issue on many GM engines from the 1990s.
You might try some MAF cleaner on the MAF wire. Use only actual "Mass Air Flow Sensor cleaner." I like the CRC stuff, which is easy to find. Follow the instructions on the can. Usually it's just spray it on and let it drip off and air dry.
It can't hurt and it might help
You might try some MAF cleaner on the MAF wire. Use only actual "Mass Air Flow Sensor cleaner." I like the CRC stuff, which is easy to find. Follow the instructions on the can. Usually it's just spray it on and let it drip off and air dry.
It can't hurt and it might help
'90 converted to a 6 speed with 6 speed PROM also batch fire, doesn't backfire on decel.
Drifting
Joined: Feb 2016
Posts: 1,384
Likes: 339
From: Simi Valley CA
I have a 92 6 speed and mine does that pop too. I have been chasing an issue where I have a lot of knock retard from idle to 3500 RPM. She gets a turbo like boost after 3500 when that KR goes away. I will have to look into this air pump myself. Now that I think about that after I got her 10/21 I had her smogged and the vacuum line to the air pump was disconnected and plugged. After she passed I did get a new vacuum canister and hooked everything back up. I didn't really push her prior to that as I was still getting all the bugs worked out.
Intermediate
Joined: May 2021
Posts: 25
Likes: 11
From: Yooper
Wait hold on. I am guessing you have a 6 speed right? What happens if you decel with the AC on? You are talking about the pops and occasional rifle shots once everything is good and hot especially if you downshift decel?
If so there is likely nothing wrong! 92-93 batch fire the injectors, this allows a bit more fuel when the engine is unloaded/engine braking to make its way into the exhaust and “pop”. It may also be “tuned in” by GM in the OEM tune.
I have a 93 and it does this gloriously through my corsa. When I had the stock cat back it was much louder in the cabin (resonator resonating through the floor).
I would have to hear/feel it to know if you are experiencing something abnormal, so you could quite well have a problem. I changed the O2 sensors on mine for good measure and it made no difference…you might want to consider doing the same…also perhaps leaky injector(s).
Again SOME of this should be expected on a 6 speed 92/93.
If so there is likely nothing wrong! 92-93 batch fire the injectors, this allows a bit more fuel when the engine is unloaded/engine braking to make its way into the exhaust and “pop”. It may also be “tuned in” by GM in the OEM tune.
I have a 93 and it does this gloriously through my corsa. When I had the stock cat back it was much louder in the cabin (resonator resonating through the floor).
I would have to hear/feel it to know if you are experiencing something abnormal, so you could quite well have a problem. I changed the O2 sensors on mine for good measure and it made no difference…you might want to consider doing the same…also perhaps leaky injector(s).
Again SOME of this should be expected on a 6 speed 92/93.