I've got a 2015 GL 450 4MATIC - 130K Miles. New Spark Plugs. Not new coil packs/boots, recent timing chain change.
The car shudders when the engine is in gear and idling, like braking at a stop sign or light. It's pretty consistent, sometimes after driving a while it'll stop doing it at stops. Putting it in park takes about 95% of the shudder away. It completely goes away above 800rpm. What could this be?

How many miles? What is your ownership and maintenance history with this car besides the spark plugs and timing chain?

Read codes.

Does "shudder" include vibration felt in the floor, seat, center console or steering wheel?

Could be coils or oil leaking into the wiring harness from the cam position sensors or cam magnets.

Also could be engine mounts or transmission mount, this would explain vibration in gear, but no vibration in park. These soft rubber components are normal wear-out parts for German luxury vehicles.

There’s 130K miles… maintenance is up to date. No other issues.

the vibration is mainly through the seat and body, not in the steering wheel much. It can get to the point I can hear body rattle as much as feel the vibration, but I wouldn’t call it violent.

If the vibration is noticeable in gear only, it points to engine and transmission mounts. Failed mounts are gradual and become more noticeable over time.

Read codes to rule out ignition problems. Any retail car parts store can read codes for free. Post them there.

I have my own OBDII scanner, there are no codes, stored or pending. Active/live reads look normal.

it just shakes a bit at a stoplight ¯\_(ツ)_/¯

What is your opinion about engine or transmission mounts?

What is your opinion about replacing coils?

What is your next step?

? My opinion is if it needs done it needs done ? ¯\_(ツ)_/¯

My 2014 GL450 with 92k just started vibrating today. I pulled it out of the garage and as soon as it was warm, rpm started to bounce and car was vibrating. Shortly after, the check engine light started flashing and then it became still.

Yeah I think that’s more serious. Get that code read. Might be something serious or it could just be the EVAP system.

Spark Plugs and coil packs have never been replaced. It happens sometimes and now I checked it's fine but the light is still there. Do you think it can be a misfire related to the spark plugs or coil packs?

You didn't state an opinion or a next step, but rather a question. What is your next step?

You come off like a bot, so please avoid this thread. Thank you.