hpilot

I have a 2015 C300 4Matic. I have developed an intermittent problem I can't solve because I can't demonstrate it to Mercedes Dealer. About once a month or so, either the engine or transmission cuts out very abruptly for a split second while accelerating. I must emphasize it is very violent when this happens. It feels like the engine or transmission is going to destroy itself. I can't determine which it is. It's sort of the worst downshift you can imagine when really stepping on the gas. It lasts for about one-fourth second, and then everything is normal. I had the dealer check out the computer for codes, but they said there was none. They test drove it, but of course found nothing because it is so intermittent. This is driving me nuts. I have not found a way to make it happen on demand. Again, it is very, very violent when it happens. Any thoughts?

ArtyV

How violent exactly?

hpilot

It almost gives you whiplash. There is also an audible bump. It's like you rear-ended the car in front of you, and then instantly you were hit from behind.

Cosworth2000

When they went to drive it, did they also use the code system do see what the car is doing? Look like you may have a problem with your gas pump but it's hard to know without see or hearing what the car is doing.

hpilot

Yes, they said they did not find any error codes. That's why this is so frustrating. Unless whatever is broken fails completely, I may never be able to fix this problem. The next time I take it in for service, I will force them to consult with the Mercedes head office.

ledukash

Sorry just re-read the post. Yeah sounds like fuel or air issue

Cosworth2000

If you drive it in Manual does it do the same thing?

hpilot

I don't know, but I would have to drive it in manual for a month or more to hope to find out.

Cosworth2000

okay, let us know how it all works out and if it still cuts out when you are in manual mode.

CaliBenzDriver

I feel sorry for the bad spot you find yourself in. Your Merc is working poorly and your "factory trained" dealer is acting clueless...

Besides a possible issue with lazy fuel pump you can also consider a crazy alternator control.

​​​​​​During accelerations the ECU effectively lowers the alternator load by dropping the voltage. Unfortunately this can result in soft-crashing unstable modules with at that point voltages swinging well below 12.2VDC.

It's entirely possible no fault codes get logged through this disruption.

hpilot

Thanks for the suggestions. However, with so many possibilities and no way of demonstrating it on demand to the dealer, I don't hold out much hope. I love the car, and it is perfect in every other way. In fact, nothing else has broken since I bought it. I have a scheduled service soon, and I will really push them to get it fixed. Unfortunately, I know what will happen. They will just keep my car for a month, and nothing will get fixed. I might have to sell it

CaliBenzDriver

you're very wise with realistic expectations. So you understand where in the MB ownership game you got cast.

Dumping your premium lemon at the Lexus lot is a guaranteed large lose...

Now if you use half that budget to get it fixed out of pocket , you will still save a lot over trade-in.

Summary of possible clues:
- low fuel pressure
- deep drain by alternator
​​​​​​
With a scanner and a little investigative work you can show up at the service lane with the proof you need.
Any relevant TSB may help tremendously.
🤞

hpilot

The cost of repair is not the problem - I have a warranty. The problem is it is not fixable...

CaliBenzDriver

... did you seriously believed the Sce Advisor that his dealership with all the remote factory support can not fix the best cars in the world??

Here is the relationship between: NO FIX & WARRANTY.

Dealer is not fixing your car because they don't want to spend any time figuring what is wrong with your property on their own dime.

Dealer knows if their tentative fix don't last, they themselves will be out of pocket, not the warranty and not you either.

As soon as your warranty expires, dealer will apply multiple fixes @ $2500 Ea. including "free car washes" until reaching the next limit: fixed or not fixed.

hpilot

Well, not sure they spend that much time planning all that out. It's not on their own dime - it's on the third-party insurance company.

CaliBenzDriver

To bypass the warranty stand still, simply agree to pay for diagnostic work on dealer terms.

Sticking your neck out will get your problem worked on. Dealer may then even decide to bill all work as warranty claim.

hpilot

The dealer doesn't have to decide anything - all work is covered under warranty except obvious wear items like tires.

CaliBenzDriver

Yes your right the warranty coverage sounds generous.... but your car is not getting worked on.
What gives ?

hpilot

I hesitate to leave it there for a long time. I will try to combine it with scheduled maint. coming up soon.