Raptor285

I recently purchased an 09 Cobalt SS sedan, but it has developed a mysterious issue. It randomly misfires (any and all cylinders) during cruising and decel, along with some backfires. Also, the voltage occasionally drops to the low 11's. The previous owner put a new battery in it and I just replaced the alternator, which didn't help. I'm at a loss since the car has no codes and no cel. I did the blow through test on the purge solenoid and unplugged it but it didn't make a difference.

Mods below:
K&N sri
charge pipes
TR10 IC
OR downpipe
Tuned (don't know who tuned it)

I also have HPTuners but I'm not great with it and I haven't seen anything obvious (to me) on the logs...I would post them but I'm not sure how besides just posting the log file.

I've done a lot of reading but haven't found much on the low voltage issue that wasn't battery or alternator related.

Thanks in advance, this is a frustrating issue!

tomj77

iTrader: (2)

Well I'd say check ur grounds and all ur connections. The battery connection pos and ground, make sure they all snug, check the connection on the under good fuse panel. See if there are any damaged wires at altanator or batt and fuse box. Ect. This could be ur issue, find that and maybe fix everything. Ur decel and cruising issue sounds like it may he that, or ur maf. Could also be a bad ecu. If u can hook a bolt meter straight to batt and monitor while u drive that would narrow it down some

SSlobalt

You can pay a tuner to look at the log or maybe find a very generous one on here to take a look at it.

Raptor285

Well, I found a red wire split about halfway through in the trunk. It looks like it goes from the battery to the subwoofer. I taped it up but have not had time to cut out the broken part yet.

I had a buddy that builds engines take a ride with me while I scanned, we're thinking the O2 sensor might be going out. I'll try to attach a screenshot of the chart below. If anyone can help let me know. I will be contacting some tuners too since I'll be moving to California soon.

Thanks all!

09CobaltSS1

Car is likely missing because it's going WAY rich. It's trimming in tons of fuel, for what appears to be no reason. With the voltage being as low as it is in the screen shot (10.8), that could potentially be contributing quite a bit to some funky things happening, but it's tough to say for sure. I'd start with getting the running voltage issue situated first, and then move your way up the diagnostic ladder from there.

ECaulk

iTrader: (6)

You need a def config file, that doesnt have all the info you need. First glance looking at your LTFT at 8.6 you def need some help from a tuner on here

ECaulk

iTrader: (6)

Also depending on how long he's been running that rich he could be fowling his plugs or building up unburnt fuel letting it stumble down low

Raptor285

Greatly appreciated guys, I will get the wire fixed and scan again. Hopefully that fixes my voltage problems.

I send Term2 a PM about getting the car retuned as well.

Any specific parameters I should add to my scan?

cmiller8006

iTrader: (3)

Also what's the plug gaps, I have never seen it really tackled about here, but now that I am on the vette forums people talk about the give and take off small gaps for power with boost, but you get more misfires when cruising or at idle. Not sure if it applies to us or not

ECaulk

iTrader: (6)

Thats for high load/high boost misfires, his are down low or at least when looking at that log it looks like idle misfires

cmiller8006

iTrader: (3)

No they talk about it for down low stuff. I know all about high load, rpm misfires, but maybe it was high load, low rpm they talked about

ECaulk

iTrader: (6)

Figured those guys would have issues with metering the airflow correctly with the maf causing misfires more than gap at low load.

09CobaltSS1

I mean nothing can really be completely ruled out at this point considering how little we know about his situation, and plugs could possibly play a small role here, but there are definitely other issues at play. My first 2 immediate concerns are the low voltage and random big trim spike there.

If for some reason the upstream o2 was sensing a lean condition (or even sending a false lean signal), we would subsequently see a very lean dip on the o2 graph that would correspond with the large trim spike. In this case though, we do not. We see a large +38% overall fuel trim, yet the o2 sensor is also reporting a very rich condition as well which the ecm is not responding correctly to.

I'm honestly not convinced that a retune is the solution here.

Raptor285

I remembered seeing a lean signal from the O2, so I went back and checked my graph. I was trying to clean it up before I posted it and inadvertently deleted the O2 reading. It's on there now, it actually reads about 1.9 lambda at one point. The 0.85 is the commanded lambda (Cmd).

I apologize, I shouldn't have posted this late when I was tired

Edit: I added part of the graph that precedes the original; This is where the O2 starts reading lean about 7 seconds before it adds fuel to compensate.

Terminator2

Me neither. 10.8V means most likely the voltage from the MAF is out of wack as well which would cause that crazy +30 STFT out of no where. OP most likely bad battery, bad ground, or bad alternator causing the low voltage. Voltage should be around 13.8-14.4 when driving and 12.6 when just measured at the battery with the engine off. If voltage reads 13.8-14.4 at the battery and the alternator with the engine running then there is an issue in either the wiring harness near the ECM or the ECM itself.