[Rainfarer]

Hi, everyone

So here's the thing - my LGX v6 Camaro is idling not so smoothly and I think some amount of power is lost along the way - LTFTs are -11% and -3% for Banks 1 and 2, and the engine occasionally "skips a bit" which feels like a misfire. The other possible symptom is that according to a built-in computer, I have 11,5-12,5 volts on the battery. But overall idle is okay I guess - once warmed up the RPM floats around 610-650

Please advise what are the first steps towards diagnosing the cause. I have an HPTuners adapter and VCM Suite, I just have no idea so far what direction I should dig in

Thanks

[CYN_CRVR]

Need more info:
- How many miles on your engine?
- Do you have any power mods?
- Stock or aftermarket tune?
- Did this behavior begin suddenly out of nowhere? Or has it gradually gotten worse over time? Did you recently change your setup, or perform maintenance?
- Does the engine smooth out while accelerating, or does it still occasionally skip while driving?

An engine needs air and fuel in the right proportion, plus a correctly timed spark to operate smoothly, so your problem might be:
Air: vacuum leak in the intake system, or bad MAF sensor
Fuel: clogged fuel injector or low fuel pressure
Spark: bad spark plug or electrical issue
Also, a bad O2 sensor, or an exhaust leak near or upstream to an O2 sensor could throw off your air-fuel ratio and cause issues.

Years ago my C5 Corvette began to experience an occasional misfire. After detecting a drop in fuel pressure, it was eventually traced to a bad fuel pump wiring harness. It was an unusual issue that my builder hadn't seen before.

[Rainfarer]

Originally Posted by CYN_CRVR

Need more info:
- How many miles on your engine?
- Do you have any power mods?
- Stock or aftermarket tune?
- Did this behavior begin suddenly out of nowhere? Or has it gradually gotten worse over time? Did you recently change your setup, or perform maintenance?
- Does the engine smooth out while accelerating, or does it still occasionally skip while driving?

An engine needs air and fuel in the right proportion, plus a correctly timed spark to operate smoothly, so your problem might be:
Air: vacuum leak in the intake system, or bad MAF sensor
Fuel: clogged fuel injector or low fuel pressure
Spark: bad spark plug or electrical issue
Also, a bad O2 sensor, or an exhaust leak near or upstream to an O2 sensor could throw off your air-fuel ratio and cause issues.

Years ago my C5 Corvette began to experience an occasional misfire. After detecting a drop in fuel pressure, it was eventually traced to a bad fuel pump wiring harness. It was an unusual issue that my builder hadn't seen before.

Thanks for the suggestion, I'll try to check the wiring as well. I've gathered up all the info I could get so far:

1) 85000 kilometers (50000-something miles)
2) no power modes so far, only modded exhaust with sport high flow cats in it
3) No tune for the engine, only slight adjustments to the trans (A8), but I've rolled back to stock for the sake of getting clean data
4) I haven't done any significant adjustments or maintenance recently. As for the issue itself - it's occasional and that's what makes it harder to spot. The engine runs "smooth" in terms of rpm, nothing critical, but it has somewhat more vibration and occasional arrhythmic behavior. I know it sounds obscure, but I tried my best to explain it)

So far the clear sign of something's odd for me is that the LTFT banks difference is 15-20% and I think that should not be this way. The LT Fuel Trim graph in HPT shows -6 to -12 across the board with -9 in idle.

What should be the normal fuel pressure limits for idle and WOT?

[S.Macias]

I would start with the spark plugs. You can check all of them in about an hour of time. Plenty of YouTube videos that show how to do it.

From there I would look at the coils and see if damaged or just replace them. Six spark plugs and six coils cost me $200 usd.

[Keko]

Also super cheap to clean the MAF. Can also leave the battery disconnected for ~10 mins after cleaning so the ECU relearns new fuel trims.

[CYN_CRVR]

The delta in fuel trims between banks probably eliminates MAF, vacuum leak, and fuel pressure as potential causes, because they would presumably affect both banks.


That still leaves plugs/coils, fuel injectors, and O2 sensors as possibilities.

[Rainfarer]

Originally Posted by S.Macias

I would start with the spark plugs. You can check all of them in about an hour of time. Plenty of YouTube videos that show how to do it.

From there I would look at the coils and see if damaged or just replace them. Six spark plugs and six coils cost me $200 usd.

Thanks, done that already - new spark plugs, coils seem to be fine

[Rainfarer]

Originally Posted by CYN_CRVR

The delta in fuel trims between banks probably eliminates MAF, vacuum leak, and fuel pressure as potential causes, because they would presumably affect both banks.


That still leaves plugs/coils, fuel injectors, and O2 sensors as possibilities.

My thoughts exactly - if there were issues in one of these it would affect both banks. Tomorrow I'll be checking O2 sensor, I got to install AEM wideband to make preparations for Edelbrock supercharger anyway. As for the fuel injectors - so far my bet is on them. We had a reeeeeeealy bad gas last summer during the siege of Kyiv and I might've killed one or two injectors with it. The thing is that I don't know how to diagnose their fault and I will probably end up just replacing all of them. Any suggestions on how I can diagnose injectors?

[CYN_CRVR]

Originally Posted by Rainfarer

Any suggestions on how I can diagnose injectors?

I haven't purchased HP Tuners yet - Can you log the pulse width of each of our fuel injectors? If so, plot all six over time and see if one of them looks different than the others.

[Rainfarer]

Originally Posted by Keko

Also super cheap to clean the MAF. Can also leave the battery disconnected for ~10 mins after cleaning so the ECU relearns new fuel trims.

Will be doing it anyway, thanks

[Rainfarer]

Originally Posted by CYN_CRVR

I haven't purchased HP Tuners yet - Can you log the pulse width of each of our fuel injectors? If so, plot all six over time and see if one of them looks different than the others.

Thanks, I’ll try this later today and will have an update

[ctrlz]

What are short term fuel trims, specifically at idle?
I'm thinking leaky injector.

[Rainfarer]

Here's the rough picture:

[ctrlz]

Your purge solenoid for the EVAP system may be stuck open. This will cause a rich condition. This video explains replacement for an older car, but the ideas are the same.
EDIT: Just looked at the repair manual. purge solenoid is very easy to get to on the V6. Looking toward rear of the car, it is just left of the throttle body and looks similar to the older one in the video. You can try disconnecting and capping the lines to see if it changes your fuel trims. That is probably easier than testing the solenoid.