Hello Scotty and the community, I have a 2006 Mercedes S65 W221 generation with 5 speed auto and now 75k miles. Have been driving the car for the past 15k miles and recently there have been a few repairs. Obviously, I knew what I was doing when I bought the car... Anyways, I have recently had problems with various cylinder misfires. That's a bit of a weak spot with these things. The two mechanics that work on the car are shop foremen at an official Mercedes dealer and have their private garage on Saturdays, because they need money to feed their family. Honest guys who know what they are doing and don't charge much despite all the knowledge and Mercedes dealer scan tools. I'm lucky to know them. But even they wonder what the issue with my car is. Before I bought it more than a year and 15k miles ago, it was standing for almost 10 years inside a dry hall. Anyways, has been trouble free for the first 10k miles when some age related problems started. I've put in a new left intercooler (old was leaking from inside), new ignition coils on both banks, new spark plugs, a new drive shaft, new axle bearings, a new activated carbon filter and some maintenance things like changing ABC oil, differential oil, transmission oil and engine oil obviously, with the filters. Fuel filter as well. At first I thought that the car has an electronical speed limiter, because it wouldn't go past 190 mph, but the mechanics told me, it is disabled, and I found out that it actually has a moderate stage 1 tune with about 660 hp instead of 612 hp. I tested that when the mechanical things (like intercooler etc.) were fixed and the thing went 200 mph when the check engine light came on - without blinking though - and the car was shaking a bit. So it had misfires and shut off some cylinders, sometimes it's cylinder 5, 6, 11 and 12, sometimes it's 4, sometimes it's 9. No idea what the actual top speed is, but I won't find out because I absolutely shit myself at 200 mph, even though it was 2 am, and nobody was around in Germany. Scary speeds. After pulling over and shutting the car off and immediately back on, it ran absolutely normal. Another day when travelling through Germany, I was going about 100 mph when a free stretch came and I tested if there will be misfires, as last time they came when giving full throttle for a longer time. This time, they already came at easy 150 mph. During the last 200 mph run, I eased on the gas carefully and floored it after around 180 mph. So I assume that it misfires now after long periods of full throttle. Sounds a bit like fuel related, BUT it had some random misfire counters on the live data scan tool while idling. So it can't be a fuel pump? I have forgotten to change the voltage transformer, which was 20 years old, so we have done that as well, but I haven't tested if these misfires still exist. At least they don't when idling anymore, but this was the case as well after changing the activated carbon filter, until *bam* (during long full throttle). My mechanics said, it's the (new, OEM) spark plugs that are taking damage from the tune, because it produces too much heat. But I wonder then why this didn't happen when I was testing the top speed a few times (because the limiter was disabled, but the car wouldn't go over 190 mph) a few times half a year ago, when there was much longer much more full throttle, on top of that with old parts and not new, unlike now. It all started when the whole ignition coil on the left cylinder bank went out, after changing that I had tons of other misifires, before that never. I don't care about the tune or speed, as I have learned that it scares the crap out of me, and if the purpose was speed and tuning, I wouldn't have bought a 20 year old luxury sedan, on top of that with a v12, which produces this much heat. I don't live in Germany, but in Switzerland where the speed limit is 75 mph and the car is way too fast anyways. It never misifires at any halfway reasonable drive and speed. So, do you think the voltage transformer has fixed the issue, or is the tune really a problem or might it be something else? If it's the tune, then my mechanics can just set the thing to factory settings, even the stock 612 hp are way too fast anyways. If it's too expensive to find out the exact reason, I'd just leave it like that, it runs great and perfectly healthy when not reaching speeds and accelerations that can only be reached on a German Autobahn.
Have a great week and with kind regards
Stefan
@chucktobias Thank you! I'm not sure about the codes, I only remember the live data counting cylinder misfires. Usually it's 0 of course, but before changing the activated carbon filter, it counted various misfires live. Cylinder 1, 4, 6, 11, 12, 8... there was no rule to it. After getting this new filter, it disappeared entirely and it was counting 0. Well, after testing it in Germany if it disappeared, it misfired again under longer full throttle, but I think the speed itself didn't really matter. On a long entirely empty stretch, I eased in on the gas and accelerated to around 180 mph with part load, until flooring it after these 180 mph. Then it misfired at 200 mph with check engine light and shut off some cylinders, which apparently this time were cylinder 4 and 9. But after shutting the engine off and back on, everything was normal again. Then I tested flooring it at around 100 mph, which caused the same misfire at around 150 mph. It didn't do that under part load. I thought maybe it is fuel, but the car never lost any power and had some very sporadic misfires even when idling. It really never had any trouble with that before messing with the ignition system, but I had to do that because the left igmition coil pack was actually broken. That's why I thought it might be wires and plugs, what do you think? Thank you for the FAQs!
What do your fuel trims look like when it's misfiring? (Not sure if you mentioned that, the post is a pretty dense block of text that is difficult to fully parse.) Assuming the engine is otherwise sound you're looking at a fuel delivery or ignition problem.
If you have codes, the repair manual for that car linked below has a list of codes and troubleshooting information for them which may help. (This is for the U.S. specification model so there will likely be detail differences):
As Scotty said there may be an issue with what was done to remove the speed limiter but there are a lot of things that can cause misfires and you're pushing a 20-year-old car, albeit one designed for high performance, pretty hard.
@chucktobias Thanks a lot, Chuck! Yes, I'm sorry about the text, I should have written it better lol... The repair manual is very helpful! Unfortunately though I don't know what my fuel trims look like, should have that checked out. Except for the fuel filter, which is new. I guess I'll see what I can do with the manual and also the wires and plugs, and then just leave it. It never misfires during normal or even reasonable-fast ride. That car is awesome for cruising... driving 200 mph in games is super cool, but in real life I've learnt it is scary and your heart ends up beating, even without traffic... or at least mine does, and the top speed wasn't even reached
Paragraph breaks are definitely our friends! 🙂 (In my case I have an eye problem which makes it a bit more difficult to get through a dense block of text.)
Fuel trims can tell you a lot of what's going on, as can codes when you're armed with the troubleshooting information for them. I wouldn't want to try driving that fast myself even on a track let alone a public road even though I believe there are still parts of the Autobahn with no speed limit. There used to be a few states in the U.S. that had roads with no speed limit but I don't think that's the case any more. (I think my old SAAB turbo tops out at only about 140 miles per hour anyway).
@chucktobias That's right, yes! I will make a picture of the codes next time with my mechanics, if they come up again.
Yes, I see what you mean. No speed limit means, you won't go to jail, but that in itself doesn't make it safe. Some people go ridiculous speeds, if only the fast lane is free, but I only feel safe if the speed difference between me and other cars is max. ~30 mph. The only time with no traffic is very early in the morning or during night times.
Some parts in Germany have 3 lanes, fences and stuff, so it's unlikely that something happens when there's no traffic, but even so, it's more about the subjective feeling. You never know if there's something on the road or whatever. Just the feeling that you're driving at a deadly speed, but have basically no control over what might kill you. Some people love that feeling, I absolutely hate it, which is the reason I stopped doing that.
Speeds of around 170 mph are already extremely fast, but the difference between 170 and 200 is actually scary, albeit it being "only" 30 mph. Can't even imagine what 260 mph feels like what these Bugatti youtubers have driven...
My old 2004 VW Golf with 170k miles tops out at around 140 mph as well, just like your awesome Saab. Plenty fast enough, lol.
Thanks for the help my friend, keep it up!!!
Well, I'm assuming you had the speed limiter removed because they won't go over 200 unless you do that. And whoever did that work. I have no idea if they knew what they were doing.
@scottykilmer Thanks Scotty! Might be, but I thought it might also be the old plugs, cables and stuff. I'll probably change that since it doesn't cost much and if it still misfires, I'll just reset it to factory settings and enjoy the car, lol. Keep it up my man, you are a legend!!!
I'll probably change that since it doesn't cost much
The parts cannon approach does not make sense, only replace those parts if they test as "bad". Were they confirmed to have failed?
@fishingandcarsenthusiast123 That's a very good point, though testing and measuring costs almost as much as replacing these plugs and cables. After all, I wouldn't know what else it could be. Fuel? Ignition coils have been changed, spark plugs, voltage transformer... and nothing is leaking. What I haven't mentioned, is that all of these sporadic misfires have started ever since the original left coil pack went out. After messing with these things and changing actually broken parts, these sporadic things started. I thought it might be fuel, but why would it then misfire when idling? I just don't see what else it could be other than these 20 years old wires. What do you think?