Engine rough idle/Low RPM groaning

[NJ-JRS]

Hi guys, sorry for long post but desperately looking for some input on these two potentially connected issues.

1) My car started rough idling last summer (didn't drive it through winter); the RPMs hold steady, then slightly drop for a brief second before going back up, then 3 seconds later drop again for a brief second, and so on and so on. Happens once the engine is up to operating temperature. It only occurs in a small range of needle movement, say idling at 500rpm, climbs to 550rpm then does a quick drop down to 450rpm like it's going to stall but bounces back up to 500 right away. Repeat every few seconds.

I always ran regular gas, but switched to premium now. After running a couple tanks, the needle holds steady more often than it drops, but at idle the engine still has a constant pulsing surge that shakes the car with a rough idle that was never existence before last summer.

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2) Only recently noticed that when accelerating or just trying to cruise at a steady speed, the car feels like it's in a higher gear than it should be (automatic trans). When just cruising at 30-35mph the engine ends up sitting in 5th gear around 1k-1.2k RPMs and making a low groaning noise. I'll tap shift down a gear to get the RPMs a bit higher to say around 1,800-2,000 where there is no groaning like at the low RPMs. For the last 5.5 years the car would never go into this high of a gear at that speed, these are recent changes.

Also when accelerating , the vehicle is not down shifting like it used to. Previously I'd be coasting, could put the pedal down maybe an inch further and it would downshift for me to accelerate quicker. Now I have to put the pedal down almost halfway to initiate the downshift, otherwise it just tries slowly accelerating in the high gear that I'm in with no power behind it. Overall even accelerating to any speed from a stop the car feels like it just has an overall loss of power behind it.

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There's been no additions or changes to my engine in years. I checked fluids, drained my catch can, cleaned MAF sensor, and cleaned throttle body but nothing has worked. I've even taken the car to a reputable shop, and TWO dealerships - all have acknowledged witnessing the rough idle problem but none have been able to find the cause. One dealership eventually tried telling me they think it's a vacuum leak with my catch can, but this was after 3 days of them telling me they can't find the problem. Also tried hooking back up to stock PCV setup last year to remove Can from the equation and the problem persisted, but the dealer wouldn't listen to that.

Anyone aware of other similar issues? Any other possible suggestions at all? I'm really at a loss at this point and not sure what else to do; my cars become no fun to drive anymore since every minute behind the wheel with the rough idling, high gear/low RPMs, and loss of power, I'm constantly focused on monitoring these issues and wondering what could be wrong.

2012 Camaro 2LT
Rotofab CAI
RX Catch Can
Apex Air Scoop

EDIT:
Adding in mileage, only 49,500.
Also no CEL.

[TWOFORTYZ]

Do you have any codes?

Have you checked compression?

How much do you drive? Old fuel and fouled injectors can cause some misfire type issues and a loss of power, but will not make your rpm surge.

The surging idle symptom sounds a bit like a failing idle air control valve, but that can not cause a rough idle/misfire.

If you have an actual misfire, you "should" have a code for that.

I believe your car is drive by wire, it may also have no IAC and just rely on the throttle valve? You could check and see if the throttle plate is oscillating and causing the rpm fluctuation?

[RMEAV8R]

Just a thought since I had an issue with my torque converter on my G8 which the symptoms sound similar. Have you checked the trans fluid level lately? Have you had the trans fluid flushed? I had that done at a dealership and it destroyed my TC, the car made groaning and grinding noises, especially loud when starting out cold. It also had a vibration when coasting down a hill in gear.

[ChibiBlackSheep]

I was tracking down an issue similar earlier. It started off kind of like this, then I noticed my car running extra rich. Then my car started to run EXTREMELY rich (to the point where it would stay running after key-off), and the high pressure fuel pump in the engine went bad. They said I had a slight fuel leak for a while, then the pump finally caved.

Check your MPGs, make sure you aren't running richer than normal.