For about 8 months, my new to me 05 CLK 500 cabrio has had an irregular shaking at idle and at stop lights. It felt like an intermittent misfiring, no rhyme or reason to the rhythm, just a bunch of random spikes peppered onto an otherwise smooth rhythm of combustions. No codes on launch. Fault counter at zero for all cylinders. I remember thinking it felt a little more aggressive on acceleration when I first picked it up, but it also wasn't slow. Maybe 10% down on power according to my *** dyno. No fluctuations in rpm. No smoke out of the exhaust. No vibration while driving. Nonetheless, it drove me crazy. No matter what I tried, it was always still there to torment me. Plugs and wires were new, and it doesn’t stall, and there’s no codes or faults so it must be vacuum leak. Ok, let’s replace all the disintegrating vacuum lines. (INCLUDING splitting the intake without removing from the engine in order to get to that one line underneath.) Maybe a little better but still shaking. Coil plug connectors! They’re not seating well! I remembered thinking how brittle they were when I did the spark plugs! Proceeded to solder in 8 new connectors and almost no improvement. Ok, let’s clean the MAF and replace the seals on the air box and intake elbows. Ehh, no difference. Ok let’s check the MAF readings. 16-20 but slow as hell, let’s replace it. Mild change but STILL shaking. What else can it be? Crank and cam position sensors? Checked them, working beautifully. Battery or alternator? 12.6 and 13.9, solid I guess. Maybe injectors? Replaced them with a new set after cleaning and refurbishing my original set but breaking them along the way (rookie, what can you do..). No noticeable difference. Belt is perfect. No rattles. Could it be engine mounts? It is only at idle.. Replaced with Corteco mounts for the engine and uro parts for transmission mount. Smoother, definitely, and the ones in there were really collapsed, but still shaking! Ok.. think, think, think.. engine ground? Checked all the grounds and replaced one with a THICK gauge ground strap. No change. At this point I was ready to take this thing out back and shoot it. What was even left? Just fuel filter and ecu really. And then last night something happened.
Remember I said plugs and wires were new? A little while after first buying the car, I replaced spark plugs, plug wires, and a couple coil packs that were bad. I put my spark plugs in with some anti-seize and as it turns out, due to some misinformation and my personal lack of experience, I didn’t torque them down enough. Last night I decided to go back to the basics and look at the spark plugs to see if the combustion was even actually happening and I wasnt lean or rich. What I found was that some of the plugs were loose, and one was even leaking. All black threads from exhaust gas shooting out of the cylinder on the one that was leaking😑.

Im gonna chalk this up to lack of experience. I've had to remove seized spark plugs out of a diesel before with my dad so I wanted to prevent them seizing. And my dad always taught me to tighten spark plugs slightly more than hand tight, but thats on diesels and older iron engines. But, with newer spark plugs and this engine which has aluminum heads, they have to be torqued to 25 newton meters (according to a forum user and chatGPT. Used my torque wrench for the first time and have all of them in there perfect now, after cleaning each spark plug’s threads several times with brake cleaner and a soft wire brush. Let's see what happens when I start it later.
The most remarkable parts of the story that made me think it was worthy to share:
A) Most of the things I did to the car weren’t a waste. The vacuum lines were all leaky. The coil plug connectors were barely holding. The mounts were in desperate need of replacement. The MAF wasnt cycling quickly. Intake seals felt like plastic, definitely weren’t sealing. Plug wires were super brittle. All these things were not working optimally. And yet this beast of an engine was STILL GOING STRONG. Incredible. Bravo. Despite everything, it still pushed you back in the seat if you floored it. Simply incredible.
B) I never suspected the spark plugs because there were no faults, the engine never had any faults show on the fault counter, it started right up, and it never cut out. That’s because of there being 2 plugs per cylinder! That’s why 2 and 2 never added up for me! This thing never did misfire. It was just not getting good burn from 2 spark plugs like it’s supposed to. What a lesson to learn. That’s some experience for ya right there..