OP sorry to hear the stealership was useless, who's vehicle is the other one that is working properly why are they revving to red line with cold engine oil and transmission fluid (anyways I digress).
I did notice the two vehicles might be running different software versions, your vehicle likely also has the newer version that may or may not affect the behaviour. As the shift up indicator is gone, such as: https://mbworld.org/forums/c450-c43-...ml#post7556104

 

Sorry to hear that OP let's wait for the smoke test you mentioned coming up hopefully that might provide some more insight for us.

 

The other one may or may not have been me test driving the dealer's used vehicle with permission. I have never had a shift up indicator in my car. Is an early production 2017 supposed to have that? My VIN comes back as released from the factory September 29, 2016. I just texted the dealer as I write this and they say they checked for software updates and my vehicle is on the latest version, and they would have requested permission to update if one was available. I believe that was a change made between the 17-18 model years

 

no longer having it simply means a newer version of software is installed, not showing what is wrong with the vehicle though. As your dealership confirmed to you, you have the latest version. I mean, it might not explain the differences in behaviour of both vehicles but it was something I noticed.

 

Software updates can be installed to all C 43 regardless of production date and model year where applicable so even if you have an early C 43, it can still have the latest software update if it was installed say by the previous owner (if you are not the original owner) What is the history of this vehicle, you purchased used or have it since new? This issue occurred suddenly or you noticed it after purchasing?

 

I purchased the vehicle with 82k miles on it in late January of 2025. I've put approximately 22k miles on it, and only noticed this issue in the last few weeks when I was out on a spirited drive with a few friends trailing behind. I noticed the car felt sluggish, opened up the AMG menu, and noted that boost was only building to about half when I got on the throttle in 3rd. Previously it felt fine and boost went up as it should. I have never taken it to the dealer before now, so it should not be a software issue.

 

Hmmm, anyways here is another thread to read in the meantime https://mbworld.org/forums/c450-c43-...-no-codes.html

 

How are the tires and air pressure?
If the car is detecting slip it will pull throttle/boost. I see this mostly just in 1st gear but I could see it happening in 2nd or even 3rd on the 9spd if one tire is rotating at a different speed than the others due to low tread or psi being dramatically off from others.
Also hows the transmission fluid at that mileage? It too will limit tq by pulling power.

 

.

 

Tires are in good shape. All replaced at the same time with Continental DWS06+ tires on OEM 19" wheels in OEM sizes and have plenty of tread depth. Plus I test this issue with Traction control fully off. Pressures are all where they should be with regard to loading as marked under the fuel door.

Trans Fluid I'm not sure about. I've been meaning to change it because I don't know when it was last done. If there's a sensor in the trans detecting fluid, that could be it. But I imagine that would be a parameter showing in Xentry by reducing the target boost.

 

Here again, different car but their car was fixed with intake diverter valve replacement, not saying it is the cause or applicable to yours but worth looking into https://mbworld.org/forums/w213-amg/...hing-else.html

 

Quick update again!

Tried a smoke test. No leaks in the intake system. I picked up a Launch Creader to see live data and it confirmed the MAP sensor is only seeing ~20psi/138KPa absolute (at 1atm of 15psi/100kPa that's 5lbs/35KPa of boost) under wide open throttle. When I brake boost it, the boost builds plenty fast, it just seems to have a rough ceiling at 5-6psi/35-42KPa over ambient. Could be the MAP sensor, but regarding @p_gill 's graph, I DO see an accurate representation of a throttle blip in my pressure graph. The sensor is cheap to replace, so I'm doing it anyway tomorrow, but at this point I'm starting to think the issue is with the diverter valve that I'm having a hard time getting to beneath the coolant reservoir for the intercooler, because that's the only thing left other than failed wastegate bushings.

 

Is there a way for me to directly graph Requested Boost against Actual pressure so I can tell if the ECM is pulling power or if there is a physical damage?

 

Now that you can see the output of the MAP you are getting closer to the solution.

one thing to consider is that the diverter valve can fail in ways other than the torn membrane. The post that was linked above likely had this happen. Note: they were getting an intermittent diverter valve and not a consistently broken one

but to confirm the valve you need to check it electrically

do not give up your are going to find the solution

take care

Paul

PS if you want me to I will post a link to a YouTube video showing what I am talking about

PPS fingers crossed that the MAP sensor is broken and a new one solves the problem

 

Looks like we are on the right track.

 

Bought the wrong sensor! Got the one for the airboxes, not the intake tracks. "Boost Pressure Gauge Converter" is what I need. I've also ordered one Vent to Atmosphere DV+ from GFB in the hopes to replace a faulty Diverter valve. I'm going to check the drivers' side again. The diaphragm isn't torn, but if the internal spring has gone weak and the valve isn't holding shut all teh way anymore, that could do it too.

I did the math on the 27mm plunger to get a total area of 0.8875 square inches, multiplied that by twelve psi (stock targeted boost in pounds of force required per square inch multiplied by area of plunger in square inches) to get the minimum force the valve needs to hold against to keep boost from leaking past the seal and back into the turbine. It's just over 10 pounds, which seems quite high for that little spring in there... Obviously it should hold more than that for tuned vehicles that use stock diverters with no issue. In theory even if the diverter isn't torn, because the spring is supposed to hold it closed and the ecm commands it when to open, if that spring is weak, you will lose boost pressure.

If it isn't the driver's side, I need to figure out how to get the passenger side done, which seems impossible from the top. I can't get even a hand in there to get the old one out, and getting the new one in will be an even bigger *****. I think draining the intercooler loop and pulling the reservoir out will let me get a hand in there enough to replace it, but I'm not sure. If there's any tips out there, now's the time.

 

Ive not replace the DV before, but I have pulled the intercooler out of the way for the belts and pulleys.

You dont need to drain anything to get the IC pipes out of the way, but the bolts can be a PITA to get to.
I highly recommend replacing the belt, idlers and tensioner while you have that out of the way.

 

How did you get the pipes out of the way without taking the reservoir out? I can't seem to figure that part out.

 

The way that the valve works is not obvious.

Boost pressure is on the back of the piston as well as the front of the piston.

Because of this the spring isn’t what it holding back the pressure.

For the DV+ the manufacturer says that you can remove the spring and the valve will still work but with reduced performance

Take a look at the video and let us know if it makes sense

Take care

Paul

 

Oh interesting. I appreciate the video. The sales rep at GFB told me to order the T9488, which is the type with the full replacement solenoid for vent to air, not just the DV+ on it's own. So when the diverter goes bad, it's that big ring seal that leaks from the back.

Pressure travels up the holes through the pison
Into the back of that seal
Escaping past/through it
rather than pushing back the whole piston itself. Very neat, but seems a bit complicated! So to know if it's bad, I should blow into the piston itself... I tested this wrong when I pulled it out of the drivers side!

 

I followed this guide from TwoC400

Let us know how the DV+ BOV performs and sounds. $600 is a lot for those if you dont have improved throttle response or better boost response.

 

After taking a look at that one, I don't know how much that will realistically help me. I think removing this reservoir will create plenty of space for me to get in there if I can manage to drain it with a fluid extractor out the top. Pulling that out (depending on hose connections, they may be the crimp type that i can't put back on) seems like it will make a major difference in how easily I can get my hand in and onto the valve to get to all 3 bolts and pull the valve out, not mention how much easier it will be to get to the connector because it's on that side.

 

Sorry for the late reply, sounds like another forum member is currently helping you out. Thank you to the other forum member.

 

Update for everyone following this thread:
Found the problem!
The passenger (right hand) side diverter valve had a torn diaphragm. Replaced with a GFB T9488 Blowoff valve and while "low" boost hasn't completely vanished (it got worse the last few days, I fear I broke the other one a bit romping on it during testing), and boost is up from 5psi to 10ish! Photo attached.


Torn diverter diaphragm

I was able to get to it after work by removing the intercooler reservoir with the steps below:

1. Remove engine cover, passenger side intake tube via clips
2. Use E10 socket to undo one bolt on the turbocharger, unclip MAP sensor in the airbox, and lift out the filter housing and turbo elbow tube.
3. Undo the three plastic clips on the front plastic air dam that holds the three holes for intake and ECU cooling and lift it out, being sure to pull the cable out of the three channels molded into the back.
4. Remove the cap and use a siphon or fluid transfer pump to evacuate as much coolant from the reservoir as possible (easiest at this stage)
5. Undo the 2 E10 bolts holding the reservoir to the valve cover, and the T25 down in the bottom right corner. If you have a magnetic snake, this T25 will come out easy using that. I did this whole job with the car on the ground, so I had to make sure I didnt drop tools or bolts into the engine bay, never to be seen again.
6. There is another T25 screw on the top of the boost solenoid. Remove that screw, the electrical connector, and the vacuum lines. This should enable you to lift it out WITHOUT completely undoing the bottom T25 that holds the solenoid in.
7. Beneath the solenoid at the bottom of the reservoir, there is ANOTHER T25 screw holding a hose bracket to the reservoir. Remove this as well.
8. There are now many electrical connector clips holding wires to the reservoir. Undo all that you can find.
9. The only thing holding the reservoir in now is the hoses! Unclip the top hose, lift up, and undo the bottom hose. Coolant WILL spill out here, but not a crazy amount.
10. Use various arrays of extensions and a magnet snake to get a 5mm allen socket into the three bolts and you can remove the diverter valve and undo the electrical connector.
11. From here, it's the reverse order, being sure to put the new solenoid in using the correct orientation. IT IS NOT AN EQUILATERAL TRIANGLE AND ONLY GOES IN ONE WAY. I used a bore scope to fasten the top bolt and check if the others were lined up.

The left hand side (driver's in my case) is MUCH easier to do. When you do this, let the car cool off for an hour or so first with the hood open. A lot of heat builds up down there and a gloved hand wont be able to feel around to make this happen.

 

Congrats in finding the issue!