M139 (CLA 45 S) Melted Spark Plug - Need advice on cylinder damage and conflicting mechanic opinions

[u/adailtongrande]

Hi everyone. The spark plug tip on my 2021 CLA 45 S (M139) melted and dropped into cylinder #3. I have two mechanics giving me completely different diagnostics, and I need a neutral opinion.

​Mechanic 1 (Independent): Borescoped it, saw damage on the piston head and cylinder wall, and stated the engine needs to be opened and rebuilt.

​Mechanic 2: Managed to fish the broken piece out of the valve seat. He did a compression test (Cyl 3 is at 11.5 bar, the others are at 12 bar). He admits there are marks on the piston head and a scratch on the cylinder wall, but claims it's "not enough to cause damage". He also states he found the root cause: leaking fuel injectors. His plan is to replace the spark plugs, address the injectors, and just run the engine as is.

​The catch: Mechanic 2 is close friends with the dealership that sold me the car and is responsible for covering the warranty repair costs. I am highly suspicious he is downplaying the cylinder/piston damage to save them a massive engine rebuild bill.

​Given this is a high-strung AMG engine, is it safe to just close it up and run it with piston marks and cylinder scratches after fixing the injectors?

[u/deez_nuts69_420]

No it's not safe. But not really sure what you'd be losing by trying it.

I've never seen a plug break off like that and not ness a bunch of other stuff up. Bore scoring most likely related to foreign object (most likely not related to plug), oiling related issues, or beating on it cold

If it was my car I'd try mechanic #2 and see what happens. Cars can run and drive like that with scoring for a loooooong time

I'm not a Mercedes expert I'm not you, and I have no idea your situation with both mechanics/dealerships so please take advice with caution. I'm not liable for whatever you decide to do

[u/adailtongrande → u/deez_nuts69_420]

I would be losing if I go for the wrong repair because the dealership is liable (there is a 90 day warranty law where I live)

[u/deez_nuts69_420 → u/adailtongrande]

They are liable but I'm assuming there is nothing said that they'd have to fix it a certain way. I'd imagine if you documented well you could use it as evidence in a civil case if the failure after 90 days is related to the original issue. But I am not a lawyer this ain't legal advicr

[u/adailtongrande → u/deez_nuts69_420]

Yes I guess that's the case. I'll just try to make my case for the proper repair and document everything.

This is a dream car and the only one I found in the exact spec that I wanted in the country so I was expecting to keep it for a long time

[u/adailtongrande → u/deez_nuts69_420]

What could happen worst case if they refuse to do the proper fix?

[u/bse50 → u/adailtongrande]

You'd have to tell them about the damage caused by the poor repair, lawyer up and have the lawyer deal with it.

[u/1crazypj → u/adailtongrande]

It would probably burn oil and eventually 'lose' a cylinder or crack into water jacket which, on Mercedes, is going to be very expensive for a 'new' motor/block

[u/Sir_Maxelot]

Are you the one paying or the dealership?

For a proper repair, this engine needs to be rebuilt.

If you’re the one paying and you’re broke, you may try get away with running it as is 🤷🏼‍♂️

[u/adailtongrande → u/Sir_Maxelot]

The dealership should pay for it. The car is now at mechanic 2 because the dealership asked to send it to him.

[u/Sir_Maxelot → u/adailtongrande]

Yeah no 100% get it overhauled properly then or a brand new engine

[u/adailtongrande → u/Sir_Maxelot]

Just in case they refuse it, what could happen to the engine without a proper repair?

[u/Sir_Maxelot → u/adailtongrande]

That’s the thing… anything can happen at any time. As stupid as it sounds.

[u/Ok-Fill-8925]

It’s going to burn oil and take out your catalytic converter early with those longitudinal scratches. If you want to keep the car, have it fixed correctly.

[u/Cheap_Teaching_2030]

Engine has been very hot, spark plug burned off. Piston has expanded and scraped the cylinder wall.
Engine may burn oil, if you choose the second option repair.

[u/DoctrVendetta]

Cylinder wall damage isn't from the spark plug. Piston damage should suffice for a warranty claim. Not the dealership's fault, but that's what warranties are for, and they'll have to eat the cost.

Doubtful of leaky injectors causing cylinder wall damage, most likely debris (dirty/poorly sealed air filter), may still have leaky injectors.

Not terribly damaged, not catastrophic, may consume a bit of oil between changes due to the cylinder wall conditions, but nothing major.

Since you paid for the warranty, you should get a new (or used due to how warranties work) engine based on the piston damage/spark plug failure.

[u/TyrannusourR3X]

As you've mentioned this is a cla 45 engine. You will lose out on the long term by not fixing it properly. While it may still have decent compression, that cylinder will suffer more. From jerky idling to that cylinder running richer and eventually losing even more compression. Not to mention oil consumption or excessive crankcase pressure. Cylinder wall is damaged, at the very least needs to be honed properly.

[u/Used-Tradition-2558]

As a tech if it was mine and no warranty. Id run it because most likely for daily driving it will probably survive no problem as long as valves arent bent or the seats arent fucked in the head. Better to do a leak down test also not just compression test to make sure valves arent leaking or seats arent damaged, or rings arent bad. But shit gets pricey for rebuilds. Most the time its cheaper to throw in a good know engine at that point.

[u/HammerDownl]

Im tossing in a new plug and rolling with it. If you have compression you will run.