Mercedes C250 CDi Coupé (204 bhp) Intermittent lack of power

mikes48

My wife's car - it's a 2011 face-lift model, 2.1 4-cyl turbo diesel, 7-speed automatic, 43,000 miles, main dealer serviced from new, bought it late October 2014 with about 27,000 miles on it, with (fortunately) 12 months M-B used warranty.

Nice car, everything fine until driving to S of France in August this year, when the auto box developed a fault - wouldn't change up higher than first gear. Contacted M-B who had it low-loadered to an M-B dealer in Chantilly, just north of Paris. They fitted a new auto box control unit, cost 1,400 Euros fitted, and with taxis, hotels etc the total cost was about £1,300, and 5 days of our holiday lost. M-B reimbursed me under the warranty. This fault may or may not be relevant, of course.

Seemed fine until about a month ago, when every now and then it would feel as if you were driving with the hand-brake on, massive loss of power, and struggling to reach 70 mph. Happened 2 or 3 times, but intermittently, and stopping and re-starting the engine seemed to clear it - until the next time. While it was showing the fault it wouldn't rev higher than 3,000 rpm in neutral, even on full throttle. When it's working properly, will rev up to the red line as easy as pie.

Then it started happening more frequently, and with the warranty expiring on 28 October I booked it in to the local M-B main dealer on 22 October. They had it just for the day, phoned to say the exhaust back pressure sensor was showing a fault, they had a spare sensor in stock, so they changed the sensor (!) and said it was now fine. We collected it that afternoon.

We had only gone about 300 yards when my wife flashed me to stop, and she said it was just the same as it was before - no power! So we swapped cars and I took the Merc straight back to the dealer while she carried on home. The technician took me for a spin and he could tell straight away (luckily) that there was something seriously amiss with power output. He asked whether there had been any on-screen messages, which there hadn't, and he was clearly a bit flummoxed.

So we returned to the garage and he asked if they could hang on to it, as it might be temperature-dependent. OK, no problem.

It was eight days before the car was ready for collection - that was last Friday, 28 October. They had apparently been awaiting advice from M-B UK, or Germany, on how to solve the problem. The eventual diagnosis was an intermittently shorting wire under the car, which they had replaced and "all was well". Drove home, everything did indeed appear to be well.

Didn't use the car on Saturday. On Sunday (1 November) my wife visited her parents in Salford (about 35 miles from here). Outward journey, no problem, return journey - you guessed it, no power. She was even embarrassed at a set of lights by an elderly lady driving a Citroen Picasso, that's how bad it is when the power just isn't there.

Back to the dealer this morning, they must be sick of the sight of me. It's booked in for tomorrow morning (3 November).

Any of you guys any idea of what it might be? I'm flummoxed too

enigma1701

Not that I can help but you might have yourself a Friday car!

But on a serious note I do hate it when it's a gremlin that's at fault and the garages have no idea what's really wrong! Hope they find the issue!

outoftheknow

And presumably being MB they are not suggesting that a fault reported during warranty is not now covered? I'm sure they aren't but whilst it may not be anything to do with your fault, I have seen diesel ECUs that have some kind of get you home mode. This is triggered when the ECU failure light comes on and basically by-passes all the electronic control gubbins and usually the turbo charger. Hard wired fuel and air control and as it says on the tin it will get you home. Actually on a Vauxhall it got me everywhere for the week it took to get a service slot impossible on a petrol engine due to sparks and other electronic stuff that makes it go but a definite advantage for a diesel

70mph is certainly about the top end in my limited experience not on your make and model. I presume the system may be getting the signal and the warning light doesn't show - or the signal is all there is so the mode is switched on with no indication.

If the mode was activated though I would expect a record to appear somewhere in the MB software logs etc.

Anyway one possibility if MB have any such system in the electronic control systems .......

un1eash

Have they done a pressure check on the inlet side as could be a boost leak?

FZR400RRSP

Petrol cars have a 'limp mode' too.

outoftheknow

Yep but not when the entire electronics have packed in such as the stuff that gives a spark and electronic control of fuel and air. Diesel doesn't need this stuff to actually go. All I am saying is that on diesels I have owned there is a 'hard wired' mode that means the engine will start and run with no ECU at all. The waste gate is opened and turbo blocked off and the fuel and air is as per the hard wired system. These flaps are fail to open and fail to close as required. It results in black smoke when accelerating and total lack of power with a dead end around 60 - 70 mph on cars I have experience of. Just a suggestion for the OP to ask MB if their diesel could have this and if it malfunctions could it produce the described symptoms. Sound exactly what other makes do if the system thinks the ECU has failed but of course it may be entirely incorrect

FZR400RRSP

Never heard of this.
I'm of the impression a common-rail diesel engine needs the ECU to start/run.
I'm assuming the Merc is a common rail, most/all modern diesels are.

outoftheknow

The good old Zafira and many others have this. The electronic gubbins is modern and diesels have run  mechanically for over one hundred years. However the common rail systems with electronic control of injection etc does indeed bugger the whole simplistic beauty of compression ignition up. Common rail being modern in cars is a bit weird to me TBH - marine diesels were common rail yonks ago (British of course ) and the individual fuel pumps kind of took over. All was mechanical
Until relatively recently....

Anyway common rail can be mechanically controlled but as you say the modern car systems do muck this up.

The described lack of power just sounds like some kind of get you home system as I know exists in some diesels. As you say petrol can have similar as long as the ignition electrics are alive. It is as good a guess as any other at the moment I reckon

car-man

All modern diesels need the ECU to make them run.
There is nothing mechanical that can take over the electronic systems.
I would expect this problem to be a fuel pressure regulator type of problem, a MAF problem or an EGR valve problem....or even a MAP sensor fault.
If an engine goes into limp mode it's the ECU that controls what is happening.

mikes48

Agree that this is what seems to be happening, but no warning light/message. I've had similar in years gone by on a Rover 75 CDTi, where it turned out to be a faulty MAF sensor, and on a Jaguar S-Type 2.7 D Sport. In both cases the engine management light/"Restricted Performance" message came on though.

Took the car back Tuesday morning, they rang this afternoon (Wed) to say they've referred it back to M-B again. However, the lack of any messages/lights on the dash are leading them to think that the main ECU may have an intermittent fault.

Unlikely to get it back this week, it seems