Hi everyone. Been reading this forum for a few years and finally get to post a question. I have a 2012 GLK350 base. I'm in SC so it has experienced some hot summers, but really no adverse winter conditions to speak of. I have recently replaced the AC condenser (leak), thermostat (check engine light) and the Electronic Steering Lock (that decided to go out in the middle of changing out the other two). Everything is back together and working great except I have a little shaking (my dad would have called it skipping) when accelerating, especially uphill. The check engine light is not on. I have changed the spark plugs, cleaned the MAF and MAP sensors and have reset the adaptive transmission, none of which made any difference. Without taking to a mechanic, my next step was to change out the engine coils, but they really don't look worn or dirty. Any thoughts?

We are to assume it didn't shake before the work, correct? If so, I would double-check the plugs for correct gap/type. I don't think it'd be the coils, they either work or don't and you end up with misfire(s).

If you've been running with a bad T-stat, you would've been running rich since most likely the T-stat failed in Open position, meaning your GLK never really got up to temp; but it was injecting more fuel trying to warm it up. You may want to try a good fuel injector cleaner perhaps? When was the last engine air filter change?

You mentioned you've cleaned the MAF and MAP sensors, that was probably a good idea. How many miles? Are you on the original oxygen sensors? Any EVAP issues? I would invest in a decent scan tool so you can monitor fuel trims and individual injector quantities, you may be able to better diagnose the issue.

It did the shake before changing everything but it seems to have improved a bit since the plug replacement. I changed out the air filters when I changed the plugs. The car has 93k miles and does have the original O2 sensors. Wouldn’t there be a code if the O2s were bad? I was wondering if there were any systems on the car not really monitored by the cars computer system?!?

Do you run premium fuel every tankful? If the maintenance/repairs you did helped the original issue, you're on the right track.

The O2 sensor(s) (there are two, one pre-cat and one post-cat) can be operating within spec, just dirtied up and slower in providing data to the ECU by being in the exhaust stream for 93K miles and 9 years. If they fail, you'll get codes for sure. I would maybe check for any vacuum issues as well as the PCV system. A dirty/clogged PCV valve would affect the way it runs.

Modern MBs have sensors and computers in all sorts of places because they're overengineered. Oftentimes it's not a physical fault with a part, it's a sensor that goes wonky for no reason. It's just part and parcel of owing aging German vehicles. Again, a good scan tool would reveal things we can only guess at. They're worth owning if you plan to keep a German car for the long haul.

Always premium (93 octane). And I have a scan tool that can look at live data but not exactly sure what would be in spec not to throw a code but still cause an issue

TBH, i'm not too sure either. My experience is with the diesels, not so much with the gassers.

I would try a few tanks for fuel injector cleaner and perhaps an "italian tune-up" as often as possible. Hopefully others can chime in.