KristofVA

Hi guys,

I'm new here and I came to seek help for the issue that recently arose with my C43.
For a couple of months now, I've had this clear rise in fuel consumption while still driving along the same roads at around the same speed.
I know the instant consumption gauge is very sensitive, but I know my car very well and I can see a clear difference here.
When cruising on a flat road at a constant speed, it used to consume around 30 to 50% less. When changing gears, the consumption is normal for around one second, after which it flies straight up every time. The car doesn't do this when the engine's cold for the first 2 or 3 minutes though.
I can also feel it juddering ever so slightly, but when I try to show this to a passenger, they don't notice, so it actually isn't very noticeable.
I already took it to MB, they couldn't find any errors in the computer. They did an update on the engine software and a recalibration of all the values but that didn't help. It's starting to drive me crazy.
Last Friday, the problem stopped occurring for 3 days (never happened before, so I was surprised), to then reoccur today.
The only thing I can remember that could have influenced my car is that I was parked a couple of months ago alongside high grass and when I came back to drive home, they had cut it and my front bumper was covered in long grass and weeds, which I removed when I got home. I can't tell whether the problem started at that specific moment, but it might be a possibility. I've also noticed that, whenever I stop the car, the fan is always on full speed, even though I have driven it slowly, but I'm not sure if that is new or if that has always been the case. I think the fan didn't make that much noise when I first bought the car, but again, I'm not sure about this.
I hope that someone is able to point me into the right direction.

C43HeartBoost

Is this all based upon your view of the fuel consumption bar, or is it based upon the actual MPG over a full tank (miles driven divided by number of gallons consumed)? If not, then track what really matters - actual MPG.

If your question is based upon the actual MPG over a full tank (though not made clear in your initial post), what is the actual decrease in MPG you are seeing over a full tank?

Where do you live and what is the weather like versus when consumption was less?

Do you live in a state where ethanol content of gas varies by season?

KristofVA

Hi there, it's also based on real consumption when filling the car up with gas and on the average consumption after a trip. I live in Belgium, so excuse me for the use of "l/100 km" but before it easily did 550 km on a full tank, whereas now I have to be very careful to reach 500 km and the average consumption per trip when going to work for example shows around 13 l / 100 km. Before, this used to be around 10-11 l / 100 km. The weather conditions haven't really changed. I am 200% certain that something is going on with the car, I just can't find what it is. It's more nervous when driving at constant speeds and behaves weirdly on the amount of fuel that is being injected when cruising at a constant speed as well. When accelerating, there is no issue. The ethanol content is always the same here too.

SizzlCheez7

Spark plugs? Need to change them out every 40,000 miles.

SizzlCheez7

Also engine air filters...

KristofVA

it was serviced a month ago and all the filters and plugs were changed.

mols20

Did you ever solve this issue? I am experiencing the same exact thing as you. In case someone wanna see what this looks like: https://imgur.com/a/IJSUaNA

KristofVA

Hi, you are the first person I run into with the same issue. Unfortunately I haven’t found what it is. I went to the dealer twice but they didn’t find it either. They don’t even really see that there is a problem, but I am 100% sure that the car isn’t behaving normally. Is yours also turning its radiator fan at max speed every time you stop the car? They replaced the vacuum converter and one of the vacuum pressure sensors but it’s still boost spiking and over consuming. Sometimes you feel a jerky sensation as well, sometimes you don’t. Where are you located?

mols20

My dealer said the same thing. Nothing is wrong with the car. And yes the fan comes on after shut down. And the jerky sensation is there aswell - only when driving at a constant speed, like in the video i linked, It seem to only do this after 60kph which makes it even more weird.
I am from DK.

KristofVA

exactly the same. Also above 60kph more or less. Sometimes it doesn’t do it above 60 when I accelerate to 90, let it coast until 70 and then apply a small amount of gas to drive at 70. So weird. Dealer making me think it’s in my head but I am sure that it didn’t do this before and that the fuel consumption is too high, both at the gas station when refueling and on the instant fuel consumption gauge. Instead of 7-10 l/100 at 70kph it goes up to 13-14. Also, when shifting gear, it behaves normal for like one second and then jumps up. The jerky feeling isn’t always present though, I can only feel it from time to time.

mols20

I don't really feel anything strange with gears as such. It shifts normally, both in comfort and sport etc. But it can't be something with the turbo itself, i can still floor it, and it pulls just like it always has been. Boost also seem to build normally during this. The whole thing is really weird. I have an appointment next week at another workshop, hopefully they can find the issue.

KristofVA

Exactly the same here. Keep me posted when you are back from the workshop will you?

mols20

No luck as of yet. But atleast they are acknowledging that something is wrong. Have your battery ever been, or is "partially charged" by any chance?

KristofVA

That’s a good start. How were you able to convince them? 😅
My battery hasn’t been flat but I do have an ‘issue’ with it. It says low battery after not using the car for a couple of days and my Mercedes guy told me that there’s an error code for the battery, he said it’s because its health is getting weaker but still okay for now so that I would probably have to change it in the near future. It just doesn’t hold its charge as long as it’s supposed to.

mols20

Well they atleast went for a drive, so that helped. Reason i am asking is that if i recall, these problems started a couple of weeks after the battery went into "partly charged" mode. Doubt it has something to do with it though. I've seen someone mention the vaccum pump valve, is that something you ever got checked?

KristofVA

You mean that little plastic thing everyone is talking about? My guy said that this couldn’t be the reason, because of that fails, you loose turbo pressure or instant boost and that’s all fine in my case. They did replace the vacuum converter and one of the vacuum sensors but it didn’t solve it.

mols20

As expected they gave up aswell. Although they adviced me to get it on a dyno. Which is the next step. But wont happen for atleast 2 weeks sadly..

KristofVA

So I had it looked at once again and we managed to get 1 step closer to a solution. We have at least identified the problem now. So what happens is that the car wants to regenerate but can never complete the cycle. The gpf is pretty full and the last successful regen was 4000km ago. We now have to try to find out why the regen process can never be completed. But this is why the fan blows hard, why it doesn’t do it at lower speeds or with a low tank of gas, because the car doesn’t initiate regen under those circumstances. Hope this will also help you in your quest.

mols20

That is great news! i will definitely share that with the mechanic that is looking at it atm. The other one who looked at it, might have been onto something right then. He did mention something about regeneration, dont know if he actually ended up testing that theory or not though.

KristofVA

Yes indeed. I don’t know about your itineraries, but I almost never use the highway so it would make sense as the car apparently needs 80kph for at least 60 minutes to regen on a more than 15% full tank. Unfortunately I am always on shorter trips driving on secondary roads (70 and 50kph).

mols20

That sounds like me aswell. So have you tried driving for 1 hour now and the problem is gone or?

KristofVA

The mechanic advised me to try that this weekend. I hope this resolves the issue.

GermanCarShow

What is this regen you guys are talking about?