rsmooth

Hello all!

I could use some help on troubleshooting this issue.

I have a 2015 C400 that I've been driving around for a while that has lost a good amount of power on acceleration. I suspect the turbos have stopped working since when I floor the gas pedal, the car accelerates very slowly from what I'm used to. I'm not seeing any other issues and the car does reach highway speeds, but it just feels as the turbos are not working. No check engine light or any other codes pending. Seems like this just one day happened as the car was running perfectly.

Here is what I've done and I don't recall if this issue occurred before or after these repairs I performed:

• Changed battery - I've heard that it's supposed to be programmed after it's been changed and I've also heard it's not a big deal if you don't have it programmed. I never programmed it after the change.
• Changed cabin filter under the hood and both air filters.
• Changed spark plugs.

I've been driving the car around in this state for a while now, but have a little time now and would like to fix this if I can and get the missing power back. Again, no check engine light or other codes pending and the car otherwise runs normally.

Thank you for any tips or suggestions that can help me solve this mystery.

TwoC400s

This issue started some time AFTER you changed the plugs?

As a free diagnostic step you could pull them and see what they look like. Google images is your friend on "reading spark plugs."

Your best bet is to get an OBD diagnostic tool that's capable of real-time logging of the sensors. Check your boost pressure, fuel trims, ignition retard, etc.

It is kinda weird you don't have a code, but sometimes these cars just don't throw them for low performance issues.

There are a myriad of sensors and control solenoids that could possibly be the culprit; you would be the first M276.8 motor I've heard of to have the turbos go bad.
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PeterUbers

How are you scanning the car for codes?

youre not making boost - do you have active exhaust flaps on the c400? If so those could be toast

would love to see a log of your boost and wastegate duty, timing ....

MatthewJ

C400 didnt get any active exhaust.

First thing to check would be that the intercooler pipe hasnt disconnected from the intake causing all of your boost to "blow out".
Log of the items above would be most helpful if you have no other codes on the car.

rsmooth

Yes, my best guess is this started happening after the plugs were changed. The plugs look fine.

I do need to look into getting a better OBD diagnostic scan tool so I can grab those additional logs. I will search on the forums for recommendations, but any suggestions?

rsmooth

I was using a smartphone app with a Bluetooth OBD2 adapter. I'll look into getting a better tool.

rsmooth

Thank you. I will check on the intercooler pipe.

I'm going to look into getting a better OBD diagnostic scan tool. Any suggestions while I'm looking?

rsmooth

***Update***

I borrowed another OBD2 diagnostic scanner and well... there were definitely codes.

P0299 - Turbocharger low boost
P0236 - Turbocharger boost pressure sensor A range/performance problem
P0237 - Turbocharger boost pressure sensor A low input
P0238 - Turbocharger boost pressure sensor A high input

I'm guessing the boost pressure sensor A needs to be replaced? What do you think?

If it needs to be replaced, should I buy the part from MB or another source? Lastly, has anyone done this or can point me to instructions?

Thanks a lot!

MatthewJ

Do you have an aftermarket intake? There are air sensors on both intake boxes. Failing to plug one of those in may cause the low boost codes you are seeing.
There are also MAP sensors on the intake plenum. The JB4 install guide has a photo of where they are. I would double check nothing came unplugged when you did the plugs.

https://www.jb4tuned.com/instruction...tall_Guide.pdf

edit, try to get MB parts when you can. check out FCPeuro or pelican for OEM stuff.