Two nights ago I added some Liqui-Molly Ceratec to my GLS. At first, I was shocked with how well it worked. Over the course of 30 seconds I could feel the engine become dramatically smoother. I made a short drive on Christmas Day and the engine was running much, much better.

Today, the car is running rough at idle (600 RPM). Two interesting things to note. When I put the car in reverse, RPMs fall to 500 (without any throttle input, foot on brake) and the engine smooths out quite a bit. Any speed between 800 and 2500 is similarly very, very smooth - I haven’t gotten the engine over 2500RPM so far - lmost no vibration whatsoever.

Im at a loss as to what to think. I’m not sure the engine mount diagnosis I got from the dealership is valid - I don’t think the mounts would lead to reduced vibrations at higher and lower RPMs (unless there is some weird harmonics issue at play). I placed my hand on the idling motor and it felt just about as rough as it does from inside the car. Again, that wouldn’t lead me to believe that mounts are the problem.

The GLS has 42k, has been driven very, very gently, is always kept in a temperature controlled garage (maintained at 55F in the winter), has experienced very few cold starts, etc.

 

I should note that, at idle, it’s rougher than my G550 or SL550 (also an M278 with approximately the same number of miles)

 

Easy enough to check the mounts yourself. Check out a video if you’re unsure how.

 

Don't ignore advice. Higher engine speed reduces the sensation of all vibration. Your description is consistent with, among other things, bad mounts or mounts on their way out.

Scan codes and post them here.

Please give a full rundown on this car:
miles
how long you have owned it
services done in the past 12 months
list of codes

 

I'll try and go to Autozone tomorrow to borrow their OBD.

That said:

Two oil changes in the past 12 months. 42k miles in total.

 

I appreciate your diagnosis which I am starting to feel is, indeed, correct. Makes me feel much better about the dreaded cylinder scoring issue.

Cheers!