[etyrnal]

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!

 

[etyrnal]

bump!

 

[magnuman]

It's not likely to be in 4 cylinder mode unless there is something very wrong, as there are too many parameters that have to be met before it'll start working. A couple of the key ones are engine temp. needs to be a little over 110*F and the vehicle needs to be moving at more than 17 mph. I'd start by putting an advanced scanner on it and then start it up. Try to capture live data and go from there. Also, check the fuel trims and whether or not it is transitioning from open loop to closed loop in a timely (about 45 seconds or less) manner. If it's not, you'll remain very rich and probably get misfires and black smoke out the exhaust. When is the last time the O2 Sensors have been changed? A good scanner can also check to see if the heaters in them are working properly. Once it warms up, does the problem go away? A stuck/sticking EGR valve and the fuel purge valve should also be investigated. A good scanner can check the EGR Valve operation and whether the pintle is opening/closing properly. It can also check the the opening %ages of the Purge Solenoid. You'd have to manually check to see if it is leaking though.

Please keep us posted on your findings.

 

[etyrnal]

Once it warms up, does the problem go away?

COMPLETELY. Yes. (p.s. thank you for your insights!

 

[etyrnal]

Please keep us posted on your findings.

It feels strange that no codes of any kind appear for such a noticeable condition.

 

[magnuman]

Does your scanner show stored codes, especially if it goes away once it warms up. If you can check your fuel trims and see if you have any black smoke coming out of the exhaust when this is happening, that could be a clue.

I had a similar problem when I installed Bosch O2 Sensors and they didn't heat up fast enough for it to switch from open to closed loop. Sometimes it wouldn't go into closed loop at all. If this is the problem (O2 Sensors),yours is probably eventually going into closed loop and not throwing code(s). A check of fuel trims from cold start to warm up will tell you or, just change the two upstream O2's, using only NTK made by NGK. They were OEM, at least on the early LX's. If you have more than about 90K on it and they've never been changed, that's probably what I would do. I've had them go bad (lazy and slow to respond) as low as about 60-65K.