This is on an 04 gto. I'm at wit's end with this here. I've seen several threads but i cant find the answer. This all seemed to start after the idiot heatshield on the starter moved while i was driving and shorted out i believe the 60 amp main but i have a difficult time believing that caused this. It also seems to shake a bit when idling. Its does great any other rpm and pulls great ar WOT. Hunts for idle a bit at startup but stays pretty consistant otherwise. Me and the tuner were very confused about the dyno curve that looked more like a jagged line all the way up. Keep in mind this build had been running fine for 4 years.

Car has a cam, fast intake, lt headers, catback, cai, mild head work,

The thing isnt throwing any codes. Ive checked all the plugs and replaced the wires. Checked the intake torque and gaskets are new. Injectors are relatively new. Coil packs ohm out at 30k between 12v and ground which i dont know what to think about that. No compression issues or broken springs. O2s are all about 2 months old and between .3v and .8v. Plugged a high dollar scan tool into it and its apparently missing randomly on every cylinder. Fuel trims are right around 0. No vacuum leaks i can find. MAF is clean. Any insight would be greaty appreciated thanks

 

The PCMs in all LS powered cars from 1998 till now have PCMs that will not throw codes till a sensor or other item is totally dead/failed. Some PCMs pick things up earlier as a sensor or part is failing but has not yet failed. EXAMPLE: My car, the factory PCM will not throw codes on failing really bad 02 sensors, crank sensor, cam sensor, TPS or IAC sensors, etc.........you must scan it to see whats wrong...then after a 02 sensor is actually done, failed, gone...the code will come up. Friends of mine with all different year LS powered cars have PCMs that tell them the 02 sensors are starting to fail.

So you cannot rely on the PCM for help.

Backfiring of LS engines at idle is usually the Crank Shaft Position sensor or something wrong with its associated wiring or connectors. The code will say: P0336 Crankshaft Position (CKP) Sensor A Performance.

It happens to be located right above the starter where it mounts into the port on the block. Maybe you burned a wire or the sensor got hot when the heat shield issue happened........OR....you simply have a failing Crank Position Sensor.

I started a thread maybe 1-2 weeks ago about backfiring on my Trans Am......I felt it was the Crank Sensor but asked anyway. No codes. Then a few days later I was just idling in the parking lot and all of sudden it started backfiring randomly then clearing right up. I looked again for a code and BAM...there it was.....P0336 Crankshaft Position (CKP) Sensor A Performance.

If you're not getting the code.....swap that sensor. Takes 10 minutes. Drop the starter, its right there......Maybe thats it, check all wiring for heat damage.

Also, you could get these codes too:
DTC P0337 Crankshaft Position (CKP) Sensor Circuit Low Duty Cycle
DTC P0338 Crankshaft Position (CKP) Sensor Circuit High Duty Cycle
DTC P0339 Crankshaft Position (CKP) Sensor Circuit Intermittent

 

I would never have guessed that. Thats where I'll look next for sure. Thank you

 

Swapped the crank sensor. Had the tiniest knick in one of the wires. Fixed that, started it back up and let it idle for 25ish minutes, drove it, let it idle again and a few minutes later it backfired. Noticed one of the downstream o2 sensors was reading .050v which it wasn't doing before so i guess ill look there next. Might be an intermittent problem since it was definitely fine before.

 

I would disconnect the battery for 35 minutes. Let the PCM reset. Tap the negative wire on the frame of the car three times so it sparks, then 35 minutes disconnected.

Did it still show misfires in all 8 cylinders after putting in the new Crank Sensor?
Camshaft Sensor is the other one that can cause problems in all cylinders at the same time....but I don't think it causes backfiring?

Another thing I would do.....GROUNDS......loose grounds for the PCM wire bundle. Also, make sure the ground on the back of the passenger side head is tight....

Ground inside the PCM wire bundle:
Do this.....preferably with a scanner connected and the engine idling after its warmed up. Grab the entire wire bundle in your hand as close to the PCM as possible......and move it forward-backward...left-right...up-down. See if you get any reaction like a misfire or hesitation. There's a ground wire in that bundle that comes loose and needs to be reconnected (soldered)........ That wire is about 8-10 inches downstream inside that bundle in the wire harness.

 

I'm not sure if they're still missing. The scanner I used isnt mine but this is the longest its ran without issues. Off to hunt for bad grounds

 

Reset the PCM......some things need resets to work again.

Do that PCM wire bundle check......