Poor Start and Random Fuel Economy

[Beastlee]

Sorry, long post.

I got my 2019 C300d in March and as part of the test drive, etc. I checked the long term average fuel economy. This showed 55mpg over 8k miles, which I was surprised at but also pleased about. I knew the car had been owned by someone who loved it, based on it's condition and the extras they added, but don't know how it was driven.

The day I picked up the car I took it for a good run with mostly 50+mph dual carriageway and motorway driving and I struggled to get 35mpg out of it, even taking it steady. I mentioned this to the dealer when I went back that day for another reason but said I'd see how it goes. It was fine for a couple of weeks and I was getting an average of 50mpg on my 40 mile trip to work on the M6 so was happy. Then the same journey randomly dropped to 35mpg but the car drove fine, it was Ok again in the evening so I ignored it but since then I've had it happen multiple times on similar runs in similar conditions. I only drive 1 or 2 days a week but they are always longer runs and start with 1-2 miles on a 50mph dual carriageway so car gets warmed gently and has a good run. I occasionally give it a hard drive, no thrashing, but mostly I'm watching the economy meter and keeping it cruising. I don't hang around joining motorways, etc.

Then, last month I noticed that when I first start the car and try to drive away it stutters for a few seconds with no power. There's no EML/MIL or anything on the dash and it drives fine after that for the rest of the day, even after 8 hours in the office. I eventually tested it by starting the car and attempting to gently rev it, instead of drive off, and the same problem happened so I knew it wasn't gearbox related. I suspect glow-plugs or controller based on this but I booked it in with Mercedes under warranty and they found a hard pipe near the air box and replaced a NOx sensor.

Initially the car seemed fixed and the next time I drove it there was no stuttering. Then, on the day I went to the office, it was worse than ever and took until the end of our road, ~80m but was fine the rest of the day. I booked it back in but it didn't do it the next few drives, until Saturday. It did it for a moment when I started it and I couldn't get more then 35mpg out of it whilst just cruising at 30/50 in 5th-7th. Got back to the fan at full speed but no exhaust noise. This happened again the next day and on a 7 mile journey at mostly 50mph I averaged 24.5mpg!

Yesterday it started fine in the morning and I drove to work, the usual 50mpg over 40 mile run. I went to leave work to head to Mercedes and, for the first time after having driven the same day, it hesitated again for a moment. I drove to Mercedes and it managed a fantastic 56.5mpg despite a traffic jam.

This morning they called me up and said nothing wrong with the car, we took it for a run and it did a DPF regen which still achieved 35mpg with them hooked up watching it. 'This is why your fan was running at the weekend.' Given I'd only done <100 miles since Sunday this seems wrong to me, especially as surely it would have done it on the run the night before.
They hadn't remembered the starting issue and said there wasn't anything wrong but I've told them that if the car had been started before they did their seatbelts and mirror checks it could have already cleared. They're going to check this later but I think the car will be too warm still.

Has anyone else had any experience of these issues? Does a DPF regen really last for hours and drop the fuel economy so much? Should it be happening this often? I'm really wishing I'd kept the old car at 115k miles with maybe a NOx sensor needing doing again than go through this. I'm just glad I bought the 2nd year of MB warranty cheap when I got the car.

 

[wongl]

This does not sound right to me. Many years ago, I had a petrol Mercedes that would hesitate and that turn out to be the MAF in-take air sensor. Not sure if the diesel engine has a MAF with all the "dieselgate" technology these days?

Personally, if you haven't already done this already, I would give it a heavy dosing of a diesel fuel additive such as Redex Diesel Injector Cleaner or the Millers Diesel additive (or such like) to get the injectors and fuel system a clean up and a purge. Then use normal doses on a regular base.

 

[tonysmb]

How many miles has the car covered?

55mpg over 8000 miles would suggest that, over that period of time, it was probably driven under good conditions and wasn't thrashed. A DPF regeneration does make quite an impact on fuel consumption but shouldn't take hours; it's difficult to put a specific time on it but 10-20 minutes maybe. It might he shorter, or longer, depending on numerous criteria, ie how blocked it is, temperature etc. etc..

If they're saying 'nothing wrong' that presumably means no error codes were showing on STAR, which doesn't help much. What temp. does the gauge show under normal running conditions and how quickly does it warm up?

It still might well be a faulty NOx sensor but it could also be a number of other sensors. The previous suggestion to run some diesel cleaner additive through it is sensible, or a tank or two of something like Shell V-Power diesel or similar and see if that makes a difference.

You can also buy dpf-cleaner fuel additives but I have no experience of them or whether they actually work.

As an aside, dpf regenerations are a pain, particularly if the process is stopped part way through, and it would be a great help if manufacturers fitted a light to inform the driver when it's happening (I know that factors such as holding revs up etc can be a clue); after all, they fit lights and buzzers for everything else these days.

 

[Beastlee]

Just been and picked it up again and as soon as I started it it hesitated straight away then ran fine. As an old school engineer I hate that dealers just go with what the digital diagnostic says. If it wasn't under warranty I'd be in the engine bay reviewing components that may cause this.

Car has done 43k miles, engine temp sits bang on 90 degrees, heats up within a few minutes. When up to temp it runs perfectly, definitely no lack of performance when I chuck it in S+ and floor it!

It's run on either Costco Diesel, which I ran on my 250d for 6 years flawlessly, or Shell Diesel if we're really stuck. I must admit to being wary of cleaners, etc. we used a Redex cleaner on the wife's Fiesta Zetec TDCi and it resulted in it smoking forever more.

Has previously had a replacement EGR valve so has had some kind of issue before. Overall I'm still at 44.3mpg over about 3k miles so It's not bad, and it was doing 56mpg going on holiday in May, but I just know there's something wrong with the starting if nothing else. I'm going to try and video the hesitance at least so they can see I'm not crazy

We have a 600 mile round trip coming up in a couple of weeks so will get an even better feel for the overall running and it should give everything a good clear through!

 

[sautah]

I had similar stuttering when cold on my 250CDI OM651 which is obviosly not the same engine...... After changing the exhaust pressure sensor loctaed in the engine bay that issue has vanished.

I would add that Its good practice to put the gearbox into sport or manual and use the whole of the rev range once in a while instead of hypermiling everywhere at low RPMs

 

[Beastlee]

I had similar stuttering when cold on my 250CDI OM651 which is obviosly not the same engine...... After changing the exhaust pressure sensor loctaed in the engine bay that issue has vanished.

I would add that Its good practice to put the gearbox into sport or manual and use the whole of the rev range once in a while instead of hypermiling everywhere at low RPMs

I think I preferred my OM651 250d, even if it was a little rougher

It did it again yesterday morning before a good run down the M6. Used Sport+ a couple of times and gave it some loud pedal, certainly surprised the Integra Type-R in front when we changed motorways and he couldn't get away until I hit 70 and started cruising.