1996 Impala SS LT1 (Intake backfire) no check engine light

Hi Everyone thanks for checking out my posting.

I have a 1996 Impala (around 800000 miles) that has me little stumped.

When I snap throttle I hear a big intake back fire.
(here is a video showing the issue)


There is no check engine light on. I did scan to see old codes and got

-P0141 (02 Heater circuit Bank 1 sensor 2)
-P0161 (02 Heater circuit Bank 2 sensor 2)
(see pics below)

I checked fuel pressure at the rail it was low. (I think it was 22psi at idle) Common on these cars for the fuel pumps to fail.
- I replaced complete fuel pump and sender also new fuel filter and fuel pressure regulator.
-Another common issue is fuel injectors on these cars. I was able to find brand new (Bosch injector set OEM spec)
Now my fuel system is up and working nice
38psi vac line connected and 46psi vac line disconnected at idle.
with engine off fuel pressure holds at 35/36psi
this video shows good fuel pressure and also you can hear intake back fire and fuel system stays correct during the pop


Here is some repairs/maintenance have done to the car
-I removed the cap and rotor to inspect while I was in there I Installed new OEM AC Delco replacement parts
-New Ignition wires and spark plugs OEM
-I remove all 4 O2 sensors to get camera inside cats to inspect both sides looked fine to me also installed 4 new AC Delco 02 sensors (see pic below)
-New CTS sensor
-Lastly K&N Filter washed and I installed a brand new OEM MAF unit

Tools I have to help with testing

-Mulitmeter
-Innova 3130b Scan tool

and new to me
-Uscope
-Autel MS906 Scan tool
hoping to be able to get her back on the road this year

thank you for your time.

Popping through the intake is it going lean. How’s the mass air flow sensor look? The old gm you can clean them real easy pop out 4 screws take maf apart clean it. Bet it’s dirty from the j&l filter

K&n

Forgot to menton that in my post will edit it now

yes thats sounds right intake backfire is lean usually but not always

the K&N was washed and I did the New style Maf update so its brand new again OEM parts

Well maybe that update. Is the problem? Pcm isn’t programmed for that style??

the original metal maf is discontinued the replacement is plastic (LS1 style now)

How does fuel quality look? Check alcohol content?

I usually run 91/94 octane
I have not completed the test you suggested.

I don't think its a fuel quality issue simply because I drained the tank to complete fuel pump installation afterwards new fuel added and problem still the same.

Popping through intake would be distributor timing, valve adjustment/timing chain or valve spring issue

Yes sometimes it is I hope it’s not on my car it’s a pain to check timing chain markings

just check distributor timing cause that bolt can come loose or the knock sensor wire can get under the distributor clamp if not being careful

I have heard that inside a distributor there are some screws That can come loose. when I took it off to replace a cap and rotor I did inspect everything in the distributor and use blue Loctite seal things up a little bit better

For me to check chain I have to re remove Opti spark water pump etc

well i meant for you to check base timing of distributor advance or retardation of spark timing with timing light. sorry should have made that clear.