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Discussion Starter #1
Hello all, I’ve got a 2018 SLC 300 with a problem. When driving along at lets say 80k I give it gas to move up to 100k and I get a surging. Almost like a strong putt, putt, putt! Felt it when I bought the car new but wrote it off to not knowing the car. So today I’m in Sport + and giving a bit with the paddle shifters...suddenly starts running like crap and the engine light comes on. I actually thought I’d have to tow it to the dealer, it was running that bad. Stopped, restarted and the engine light is off and running normal. But I can recreate that misfire, surging, so something is up. Only 8,000 km on it so still under warranty. Any ideas? Thanks in advance.
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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Still under warranty? Let mb sort it :D
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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members will jump in
but they will also say....get the codes read
then you have to get a code reader......
mb has a code reader :D
@MartinatMersea
@Col
@VII7
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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I am using premium and it did sit from December until the middle of April without being started.
was it on a trickle charger?
 

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Discussion Starter #9
It was on a trickle charger and the tank was full so not much chance of moisture. Have run two tanks through it this year but like I said I did fell the surging during the test drive in November.
 

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Registered 2016 SLK55 AMG w/P30
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I'm no expert but 2 things come to mind: sitting over the winter allowed the fuel to foul (even without moisture old fuel can foul as I understand it), which then left residue on a spark plug leading to misfires.

Other possibility (although less likely since I only know of it happening on a couple of SLC 43s in the US) is a software problem that requires reprogramming to prevent misfires. My 43 had it and it was diagnosed by moving the spark plugs that had the misfire codes to different positions. Misfires happened again so they knew it wasnt the spark plugs but rather a software problem. I THINK it was specific to the transmission, though, so theoretically a 300 could have it with the 9g tranny. For me it always happened on hard acceleration (pedal planted) and moreso going uphill and hard accelerating at the same time.

Good luck getting it sorted!Make sure they give you a loaner. ?
 

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Discussion Starter #13
I’ve gone through a couple of tanks of fuel so any bad gas should be gone, but the plugs could be fouled. I wouldn’t write off the idea of a software up to stop the misfire. Thanks for your input.
 

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Registered 2009 SLK200
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I’ve gone through a couple of tanks of fuel so any bad gas should be gone, but the plugs could be fouled. I wouldn’t write off the idea of a software up to stop the misfire. Thanks for your input.
Doubt its fuel unless you often get iffy fuel your way ?
Only way you going to find out is get it plugged in and codes read if none then see if there is a software update for your SLC and poss gearbox etc .

If it starts and cranks over real good and no issues regarding battery doubt its that while driving .

Id go codes , firmware update then .........
 

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Discussion Starter #15
I dropped it off yesterday and on the way to the dealership I realized that there are two problems that might be related. The first is the engine light that came on while the engine was running on three cylinders. The other is a thumping that I’m sure is coming from the transmission. If you cruise along at let’s say 60 and slowly push the pedal down the transmission thumps while accelerating. Not while changing gears but midway through under load. Strange.
 

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Discussion Starter #16
So as it turns out there was a defective spark plug that was causing the misfire. It turns out the electrode was damaged and they had to check the cylinder with a borescope to make sure that it was OK. They said it was and hopefully they were telling the truth. As far as the transmission goes they changed the transmission oil and re-learned or reprogram the software. Running fine now...but! They dropped one of my beautiful AMG wheels on its face while doing the brake inspection. Of course they didn’t tell me and I noticed when I got home and gave it a wash. This wheel will have to be replaced...let the arguments begin.
 

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Registered 2016 SLK55 AMG w/P30
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I always take pics and video showing the calender on my wife's phone whenever I take my car anywhere for brake or tire work. For some reason despite their high cost, places are extremely careless even with AMG/Msport/bespoke rims. I guess they just ASSUME most people dont look at or care much about their rims. There are other stories round these parts about similar fights with MB dealers about damaged rims. Evem if they sort it by having it "machine refinished", it's always in the back of your mind after that, especially with painted rims where non-factory paint can start to peel or chip due to the wheels being such high exposure areas. Good luck with the wheel fight, glad you got the engine sorted though!

Might want to add fuel stabilizer prior to winter storage from now on, assuming it is safe in these cars. San Antonio is a year round drop top area so I cant speak with experience.
 

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Premium Member 2004 SLK350
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So as it turns out there was a defective spark plug that was causing the misfire. It turns out the electrode was damaged and they had to check the cylinder with a borescope to make sure that it was OK. They said it was and hopefully they were telling the truth. As far as the transmission goes they changed the transmission oil and re-learned or reprogram the software. Running fine now...but! They dropped one of my beautiful AMG wheels on its face while doing the brake inspection. Of course they didn’t tell me and I noticed when I got home and gave it a wash. This wheel will have to be replaced...let the arguments begin.

So the electrode felt into the cylinder? If yes then I will go for an independent inspection to be 100% sure that the wall is ok.
 

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Premium Member 2018 SLC43
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Other possibility (although less likely since I only know of it happening on a couple of SLC 43s in the US) is a software problem that requires reprogramming to prevent misfires. My 43 had it and it was diagnosed by moving the spark plugs that had the misfire codes to different positions. Misfires happened again so they knew it wasnt the spark plugs but rather a software problem. I THINK it was specific to the transmission, though, so theoretically a 300 could have it with the 9g tranny. For me it always happened on hard acceleration (pedal planted) and moreso going uphill and hard accelerating at the same time.
Interesting re-reading this post:
While away last month on our group Euro Road Trip down in the Alps, I got the Engine Management light come up and also the engine felt lumpy as though it wasn't running on all cylinders.... :surprise:
> It happened after applying power after a section of down hill Engine breaking..
> Was un-likely to be fuel quality related, as had ~25% plus fuel load & had filled it full with 98 octane fuel at the last fill a few days before.
(So could have been fuel starvation? as happened after a long sweeping 520' corner that spiralled under it's self through a tunnel - See map below..!!)
> Engine + Oil temp was normal & only ~23'c out side temperature.
> Pulled over straight away, took the key out and started again fine & haven't see the problem again..
> Luckily @Col was able to plug in His code reader in the next day & pulled the following codes - Google'd them but not able to say exactly what they are for My R172, which is newer that the SW on Col's code reader:
* P030022 - Possible coil pack..?
* P030200 + P030300 - Possible cylinder misfire codes..?


Will get MB to look at these at some point, as the car is still under warranty, but thought I'd document it here just in case other owners have or have had the same problem:

Map of the bit of road I was driving at the time the EML came up:
 

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