Car: Mercedes SLK230 R170, year 2000, automatic
Problem: Engine very rough at idle
Solution: New ignition coils
Make sure you get the right coils! They change in cars manufactured from 2001 onwards. For this year 2000 car, I used these coils: amzn.to/4eHry7T
Description: The engine was running really rough. Big side-to-side vibrations, which were very noticeable when the car was going slowly or stopped (at traffic lights) and in gear. People kept saying it was the engine mounts (horrible and very expensive job) - and indeed engine mounts are a cause for engine vibration - but I had a feeling it was fuel system related.
When the car went for its MOT it had an emissions test which failed on Lambda (Carbon Monoxide) and misfiring. That was the answer right there. It was the fuel system. The mechanic looked underneath the car whilst it was on the ramp to check the engine mounts and they were fine.
So, if you've got to that point above then you need to work through three things:
1. New spark plugs (cheap and easy)
2. Clean fuel injectors (cheap but fiddly)
3. New ignition coils (not very cheap and very easy)
Doing the whole lot should cost you no more than £50 if you don't mind doing it yourself.
Happy fixing!