Feels like it's running on 4 cylinders on cold start < 32degF - no codes - 2008 300c AWD

So, it's winter here. On a completely cold start, my 2008 300C AWD, which is showing no codes or dash lights, 'wobbles' in a way that feels like it's idling in 4 cylinder mode. The wobbling feels like it's around 300 bpm [sorry for the music reference]. If i let off the brake and let the car just start rolling at idle, or lazily accelerate, there's a a very rough feeling. If i accelerate at parking lot speeds it FEELS almost like there's a broken motor mount. I don't think there is, and i'll explain why. I think it just feels very rough due to being'4 cyl mode'. It'll stay this way for my whole drive to work.

Now, i haven't figured out the pattern perfectly yet as i've only had the car since newyear's eve. [Wanted this car ever since the gen 1's first came out -- only took 15 years]. But, in the same scenario described above, it seems like, once the car is warmed up, if i turn the car off, and then re-start it, it just runs smoothly. No rough idle. No feeling like it's in 4 cyl mode. When warm the car feels 100 perfect. When warm, there are no feelings of a motor mount issue. I just used that description to describe how rough it feels when cold.

Any ideas?

Is there any kind of treatment that could be added to the oil before immediately before driving it 2 miles to get the oil changed? Like a "clean out' sort of treatment to clean up the lifters/solenoids, etc? I'm guessing the answer is, "you really shouldn't", but i thought i'd ask just in case.

The car looks MINT in every way... interior mint... exterior mint... engine bay, mint... undercarriage consistent with a garage kept car that's been baby'd. The 'carfax' report shows the first owner got ALL of the recommended maintenance. The car was super well cared for.

When i first got it, there were two engine codes. One was oil pressure sensor. The other was a related oil viscosity code. After reading through the service manual and the owners manual, it seemed like these could possibly have been
a result of the 2nd owner having gotten an oil change with the wrong oil type. The dealer i bought it from changed the oil before i bought it. So, to test whether the problem was a permanent issue, i cleared the code, turned the car off, and pulled the PCM fuse (F2) for a minute. The codes never came back.

The engine, when running smoothly sounds perfect. No ticks, clunks, knocks, whines, etc. Even when starting up cold there's no tapping, etc.

Does anyone have any ideas?

Once the engine is warm, any number of re-starts just results in a smooth running engine with no shaking/wobbling.

Thanks SO MUCH for any insight.

This car has been my dream car ever since i first saw them in 2005. I finally got one. And i found a unicorn. Amazing car!

 

Sounds like a fun one to fix.... Sounds like a faulty PCM just from the description... But i wouldn't even place a bet on it.. Best of luck... Keep us posted..

 

if that's the case, i wonder if they can just re-flash the PCM at the dealer? Or at least check it to see if the current flash's parameters are all correct.

If it was an entirely bad PCM, i think the car would fail run run/drive. But if any of the programming is corrupted, maybe it can just be re-programmed.