Rough Idle/Running Rich on idle only
 

This is my first post so I will try to not make it long-winded. 2017 2ss with 30k miles, no mods, original owner. LTFT for both banks is reading -8 to -18 on idle. STFT bounce from 0 to 2.6. It's an intermittent rough idle with random misfires: in 5 minutes cylinder 2 misfired 52 times, and a few other cylinders under 5 times where after a short drive cylinder 5 then misfired 32 times and #8 15 times in 5 minutes. I recorded the diagnostic tools readings (2 tools under $200) if someone wants to know more but here are some points: Fuel pressure 47, fuel rail pressure 550-650, MAF 0.6-0.7 lb/m, MAP 8 "HG, Oxygen sensor readings/voltage and cat monitors came back good on mode 6 data.

6 months ago I had a rough idle and backfired on a cold start-up for 20 seconds, LTFT never went below -8 at the time, so I changed all spark plugs and spark plug wires with just OEM, new fuel injector for cylinder #7 because the car threw that cylinder code, and changed PCV value since there was some oil in the intake manifold. The problem went away for a few months and now my idle is getting rough again with the problems listed above. Any input is much appreciated since I am totally lost right now.

Its not running rich. The car is pulling fuel so it doesn't run rich. If it was pegged at -25 and you were still running like shit, then yes at that point its actually running rich. What you are seeing is normal for the stock tune. The tune is commanding this rich condition and the fuel trims are doing their job pulling it back to keep it in stoich. In WOT openloop, none of that matters.

Do you have any ideas why I have random misfires on idle? They don’t clean up after a drive either.

What fuel do u put in it

Fuel is garbage or it needs to be run hard to clean the plugs. LT1/2/4 can load up plugs fairly easily if they are idled and babied a lot.

Strictly Chevron premium. There’s only a few other gas stations here. I will try stretching it’s legs tomorrow. Might just have to take a road trip I guess. That would make sense given I don’t drive it during the winter and I haven’t gone on a road trip for a couple years. (I used a fuel preservative for winter and fuel system cleaner beginning this year, but the cleaner didn’t help.)

As a general rule of thumb, if your LTFT is more than 10% either way(higher than +10% or lower than -10%), something is wrong. Usually the factory service manual will also have a limit on the absolute value of your LTFT and STFT, and typically it is around 10%.

Not sure if telling the OP to drive the car hard without exactly knowing the issue is a good idea. If the spark plug is loaded then I would at least pull the ones out of the offending cylinders to check.

There are also the usual culprits like air filter and MAF sensors, which are easy to check and address, so I would start there.

I think the spark plugs are being fuel fouled but I am not sure myself. The first picture is about 1 month (from new) and the second is 5 minutes of idling (also new). The plugs are OEM. I cleaned the air filter but I have not inspected the MAF.

Try chevron Techron

I tried red line total fuel system cleaner. I heard it was a little more orientated towards high octane burning fuel systems than Chevron Techron.

I thought the manual says only chevron Techron? Others could have things that can cause damage. Not sure

Redline Si-1 is good stuff. No trouble running that once in a while.

So I took a look last night at my OBD2 data and while I was hit idling, my MAF was reading 0.8~0.85 lb/min. Keep in mind that I live at near or even a little below sea level and I have the GM Performance air intake(with the calibration done). A MAF reading low should cause a lean condition, though, but it could be something you could look into.

I took a 300 mile trip yesterday and the car does idle better, but it isn’t perfect yet. I will just keep driving it until something else happens.

Sorry thats not correct. The car has the ability to pull or add fuel +25% to -25%. Anything in between those means the car is doing what its supposed to do. Keeping stoich. If its pegged at -25% or +25% then yes your car is running rich or lean. This is all controlled in the tune. The fuel trims will try to keep the car running at stoich.

My stock stock ZL1 on 93 sits between -15% and -8%. That's not bad. Its just set rich in the tune from the factory. During closed loop, the car runs fine. But in open loop, this is why some can pick up mph by running e30 on a stock car/stock tune. They are able to lean it out safely and pick up HP in open loop(WOT) because the car doesnt use the fuel trims.