Hello everyone, I'm new over here and just had a couple questions re some problems my g/f has been having with her 850.

On cold starts, for about 4-5 seconds the motor sounds very rough, kind of like a very reluctant idle with a bit of metallic clacking (sounds kind of like tappets or worn valve guides or something). It goes away after several seconds, but seems to be consistent every morning and has gotten worse over the last few months. The car gets regular oil changes, but it has been having problems with sludge. 120k miles, and its a 1997 GLT for reference. What do you guys think?

 

sounds like there may be some clogging going on inside the engine. Search for seafoam cause i think that is what will help you fix this...

 

Okay, will do....actually I'm familiar w/ using the stuff from the working w/ older Mercedes

 

These cars also have plenty of valve tick when they first start. The owner's manual says this too.
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+1.

And I would also try full synthetic 10W-30 with OEM (MANN) filter. Some sludge would be cleaned out by synthetic oil.

I would also give it a full tune-up if it hadn't received it within the specified intervals.
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JPN