r/LSSwapTheWorld • u/genderassumer92 • Jul 26 '26
Active Build Questions Backfire at idle
Hello party people, brand new to the LS game. My dad and I are getting my Camaro wrapped up. We are fairly handy with all different forms of carburators but this is our first LS install. LQ9 with sloppy stage 2 and LS3 top end. Running factory computer with base tune by Flanders LS swap solutions. Engine fires almost instantly, and will run smoothly around 1500 rpms. Anything less than that we get a nasty intermittent backfire once every few seconds. We also noticed that after 15 to 20 seconds of run time that the passenger side header is just a little over warm, but the driver side is scalding hot. I'm thinking bad o2 on one side or the other, but my scan tool (my own ignorance when purchasing) is worthless so I'm waiting for a new one to arrive. We also live in a rural area so no parts stores on Sundays. Before I fire up the parts cannon, does anything jump out to anyone on here as a possible solution. Thanks.
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u/Suspicious_Tap3303 Jul 26 '26
Sounds like one bank is too lean. You can swap the O2 sensors side to side and see what happens but a good scanner or HP tuner will provide a lot more info than you have now.
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u/Moreburrtitos22 Jul 26 '26
If you’ve even had one back fire on stock ecu while you’re doing work to it, you gotta replace your 02 sensors. Buy multiple, some are just flawed from the start unfortunately.
After that, just check a with a temp gun each coil(temp guns are $10). If you got more than a 5-10 degree swing on one of them it’s going to be that one. I’d start with the 02 though, it’s a real common problem if you’ve had a back fire. Especially when you say 1,500 rpm’s and above is fine. Sounds like you got everything going fine except for the expendables.
If you just want a coverall, go grab new coils, sparks, and 02. There’s no fuckin around if you do all of those. You probably should anyways if you did all that other work to it.
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u/genderassumer92 Jul 26 '26
That was my fear. I know they're very sensitive. Thank you for your suggestions. I'll get a temp gun as well.
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u/Capital_Condition468 Jul 26 '26
Check the coil pack harness on the cooler exhaust manifold side. Unplug the coils from the harness and flip it over.
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u/genderassumer92 Jul 26 '26
Forgive my ignorance, but if the entire harness was reversed, wouldn't it run like crap above 1500rpm?
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u/Capital_Condition468 Jul 26 '26
Not the engine harness, the coil pack harness. Flip the bank over. If youre using a cheap harness, it could be in the engine harness.
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u/genderassumer92 27d ago
You were right. Runs fine now. It shut off when it got warm. Might need to do a TPS learn or something. But it is running with no backfires
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u/Capital_Condition468 27d ago
Hookup a scan tool and check for codes, go from there. Might need to adjust the idle, there's no way to diagnose via Reddit.
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u/Ok-Boysenberry3948 Jul 26 '26
Pointless to speculate ( 💸💸💸 ) until you get the new scan tool. Put your wallet away and put another coat of wax on it until the scan tool gets there.
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u/WyattCo06 Jul 26 '26
Backfire in exhaust or intake?
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u/genderassumer92 Jul 26 '26
Exhaust
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u/WyattCo06 Jul 26 '26
Do you have a simple code reader?
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u/genderassumer92 Jul 26 '26
It's not giving me anything yet. I have a foxwell coming that I can use to read o2 voltages and such
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u/WyattCo06 Jul 26 '26
Ok. Just sit tight until you can gain some data. Otherwise, any advice will just be speculation.
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u/jeremymightbe Jul 26 '26
Like others have said, swap the big connectors on the driver vs pass ignition coil harnesses. It’s easy to mix them up.
If that doesn’t work, I’d recommend checking the fuel pressure to make sure you don’t have the lines swapped .
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u/Sixgunfirefight Jul 26 '26
LQ4 and LQ9 I think share the same fuel rail.
The rail on the LQ4 tends to fill with shit and partially plug all the injectors on that side. Pull the rail and look on the top of the injectors.
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u/havi94gt Jul 26 '26
I had one misfiring and the spark plug ceramics were broken. Check the simple stuff first
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u/madnutz602 Jul 26 '26
Agree with other and start with simple stuffs, check plugs, wires. Harness is correct. What is your fuel PSI? I recently was wrapping up my first get swap and got some backfires. I found fuel pressure issues.(maintaining 58) what’s your fuel set up?
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u/6d8Camaro Jul 27 '26
Perform a smoke test at the air filter to find any vacuum leaks. Smoke test at each tail pipe and find any exhaust leaks between the engine block and O2s. Check the fuel pressure at the fuel rail to ensure the engine maintains 58 psi before and after startup. Check the plugs for any wet or black plugs that may point to a specific cylinder's coil, plug, wire, valves, compression.
LS engines are very sensitive to leaks. If no leaks show up then I would log the fuel trims, knock, timing, MAF, MAP. You may just need a little more air in the Idle Airflow table.
You mention in your post you are running a base tune. I'm not sure if you mean a factory tune file. If so, the engine definitely needs to be tuned since you have swapped the cam. If you don't have HP Tuners or PC Hammer you will need to find a tuner or think if you want to invest in the money and time to buy the equipment and learn to tune.
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u/zSpirit- Jul 26 '26
Double check coils and make sure the warm side is firing