P0301

I have a 2007 300 with a 2.7-liter/6-cyl engine and 154.5k miles. She is my pride and joy. In early 2024, the motor began shuttering (or misfiring), which a former Chrysler mechanic fixed by replacing the #1 fuel injector. Current status: a mild rumble presented again in February and continues now with solid engine light coding "P0301". The problem happens:

- while in “D” / but not moving most of the time
- while braking - nearly at complete stop - sometimes
- a half second when foot is back on the accelerator - sometimes
- Putting the car in neutral at red lights smooths out the rumbling somewhat

Again sometimes never at all - even during stops but usually presents every ride. So it’s somewhat intermittent. No apparent loss of power. No stalls.

Repairs performed: The coil was replaced (supposedly) in November by a local shop (we’ll call number one). The repair (also supposedly) included spark plugs, new valve cover gasket and an Oil Pressure Sender Switch. It not only didn’t fix the problem - an attempt at a different shop (shop number two) revealed that shop #1) didn’t put the “egr tube gasket” back on so they focused on that. They said they couldn’t compression test but they did switch the cylinder one plug and coil to see if the code changed but it didn’t. In fact, given the miles, they recommended putting her down…. That was in March. I'm still driving it.

Oil changes were always synthetic and usually on time. Gas - always Hi-Test. There has been a very mild oil leak somewhere on the crankcase for years.

Questions:

1) What are your suspicions?
2) Is there a possibility that an engine cleaner, oil cleaner or injector cleaner could help?
3) Am I looking at a MAJOR repair?

Thanks