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I asked specifically for a torque converter drain when I had mine done. It's not that hard, there are videos for it, it's just an extra few steps.
How quick is the car? It should be fairly rapid by normal car standards. When mine was in limp mode it was still quite driveable unlike the Passat which would only do about 40mph on the flat and much less uphill. It's just about possible that with that much displacement it's been in limp mode all along and he hasn't noticed if he's not sure what to expect from it.
According to Parkers, the fuel economy is 46 for the 350 and 52 for the 250, so approx 13% difference. I could just about get 52 on my original wheels, so the OP should be getting around 46 if he's careful.
How quick is the car?
Very. If I hoof it it squats it’s arse down and takes off like a scalded cat.
I’ve been in limp mode once (long before the service) so I know what it drives like then.
I was thinking air filters as neither schedule mandates changing them but that wouldn’t explain the marked difference that started at a specific time.
I was thinking air filters as neither schedule mandates changing them but that wouldn’t explain the marked difference that started at a specific time.
It would explain it if a hose or sensor plug has been removed and not put back properly.
On the Passat if you unplug the MAF it uses a default value, so it still drove apparently fine but not optimally. That's a possibility. There are procedures you can do to test a MAF apparently, but I've only tried this on VAG.
Air and fuel filters are on a separate 45k mile schedule so they don't necessarily get done at the regular service.
I do recommend looking for new wheels in a different size next time you need tyres. It's expensive though, since they are a rare size and you then also need to change all four tyres at the same time to go with. 18s are much more comfortable though.
I missed the part where you said it was getting 33 after the service. I would take it to somewhere with STAR to get a full readout - generic code readers may not give you all the information.
I don't know Merc's, but does it have a DPF? Assuming it does, how frequently is it regen'ing? As the filter fills with soot, it'll regen more frequently, which increases fuel consumption.
Usually an expensive fix though ☹️
OM642 owner here in an E Class Estate, done 37mpg average over more than 30'000 miles from when I changed the battery and it reset.
Your figures are not 'abnormal' at all however theres a few things to consider
- Dieselgate software scam - it is widely reported that on cars with OM642 and 7GTronic, post your 'free software update' that gearbox does not now change into top until well over 70mph. Which is appaling. And once you've had it done, they wont undo it (coz emissions, init).
- 7Gtronic does have lockup n every gear, but it could well be due a fluid change (They are NOT good for life!)
- Thermostat is a common failure, sometimes not so obvsious due to heavily damped temperatue gauge. Should be rock solid once up to temp, and fluctuation at all indicates duff stat. Very easy fix, about £100 for Genuine Merc part. Front and centre on the engine.
- Check the batwing, the Orange turbo seal and the smaller PCV seal should be replaced at every service as they fail but crucially its very easy to badly fit them. Turn your radio and heater off, boot it, and see if you can hear a faint boost leak noise from just in front of the dashboard... Common! Again, dirt cheap from Merc.
- Fuel filter is every B service. Make sure its been done. Make sure its a decent component.
- Tyres can and do make a difference.
- Fundamentally, we get the same figures. For me, 37mpg over >30000 miles is brilliant, frankly. Its a 2 tonne car once loaded. It'll hi tthe limiter at 155mph, and launch off a sliproad onto the motorway faster than my Lotus does. You can sit at 70+a bit for 5 hours down the length of England no problem and not a hint of fatigue. I think you might have slightly unrealistic expectations, buy a miserable 4 pot Octavia DI if you want mega MPG but it wont do any of that stuff as well.
ETA - in the short term, relating to your recent drop post service - check the batwing. Theres 2 MAFs whch could be damaged or not plugged in correctly (although this was flag a CEL), and turbo seals is an obvious one. Check the coolant running temp with an OBD on a long run if you can too.
And please tell me that it didn go to Merc for the service, as if so, your probably a victim of a 'software update' and guess what, your stuck with it.
@snotrag I'm in Evesham, Worcestershire.
Re Dieselgate, the service was done at an indy, not a main dealer, so I would hope it hasn't been done. However, I have noticed that it seems "reluctant" to shift into 7th on A roads, but OK on Mways.
Temp gauge is rock solid.
Gearbox serviced last year.
Batwing? Is that the intake piework from the dual air boxes?
It's less the mpg figure that concerns me, but the sudden change in efficiency.
Yes - theres two air filters tucked down the side, then the 'batwing' has a Mass Air Flow sensor each side and the flow joins, feeding the turbo which is in the middle at the back. Pop the plastic engine cover off its pretty obvious.
To one side is a smaller conection for the PCV system.
Both have orange silicone rubber seals which A)fail and B) are easy to badly fit.
See here: https://forums.mbclub.co.uk/threads/om642-cls-350-turbo-intake-gasket.271288/
The two MAF plugs ar an obvious potential maintenance error, along with the seals. I'd have a noseround for anything obvious if I were you.
Not local unfortunately otherwise I'd have a look for you.
Sounds a bit low but there could quite possibly be some weather related influence.
I did this (chester - London - Chester) trip last summer but I get about 37 on a long steady cruise control run at this time of year (3L v6 Diesel)
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And this was when the car was about 75k miles younger but a long motorway trip (Glasgow - Chester) in colder conditions - so it's not necessarily an age thing!
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The only turbo car I had would change from 32mpg (ish) to 18mpg (ish) just by ‘cruising’ 5mph faster.
The key to getting the best mpg was staying below the revs where the turbo kicked in.
Similar here .. my old BMW 3l flat 6 could jump from 50mpg to 40 mpg (or lower if I tried) just based on almost imperceptible changes. The key for me was watching the instantaneous MPG and changing the acceleration and speed .. there was a proper quantum step (actually 2) that were probably not just "Turbo on/off" but the more complex engine management modes call them (dump/normal/lean) and the key was sticking in "lean". (Pump mpg also tied well to trip)
If you have an instantaneous maybe try that .. if not then maybe try an ODB2 reader that will give you that and more...?
The other part was winter/summer and journey direction....
In winter the big 3l block took a while to heat up and I had a fair but of non motorway/dual carriage way on the way out and almost straight onto the motorway on the way back (2 different locations but similar characteristics).
On the way home it was still warmer after 8hrs than it would have been in the morning and got to the better efficiency quicker.
My car goes into top gear at around 58mph. It drops from 1600rpm to 1400rpm, because of the silly adapting throttle it takes a minute of steady state cruise after a boot to get onto a mway or dually to change up, or it will jold at 2000rpm then change up.
Tyre pressure can make a difference on the E, 32 to 34 seems to be a sweat spot, with lower softer and 36 plus spoiling the ride, mine on comfort suspension tho with 16in wheels
The other part was winter/summer
Warmer air has less mass and drag.
Previous posters have listed the many other factors that could affected the MPG. It's probably a combo of the tyre change (have you double checked pressures?), fuel variation, driving into the wind on your worst MPG example and even the computer calculating incorrectly. Have you tried manually calculating MPG from a full tank? I used to get around 35mpg from a 3L diesel BMW, so you're in the right ball park.
Aside from trying to find the issue, it's worth noting how MPG is sold to the public as a figure to obsess over, the difference in actual running costs for 33mpg v 40mpg is £350 a year for 10k miles. Considering running a car like that Mercedes is likely to be £2500+ per year in servicing, insurance, tax and depreciation alone; a few MPG is a minor difference.
42 to 33mpg is much greater drop than any of the factors listed here apart maybe from boost leak.
Usually an expensive fix though
The fix for a DPF becoming blocked is not a new DPF. It's fixing the thing causing the excess smoke.
Dieselgate software scam – it is widely reported that on cars with OM642 and 7GTronic, post your ‘free software update’ that gearbox does not now change into top until well over 70mph. Which is appaling. And once you’ve had it done, they wont undo it (coz emissions, init).
I searched for downsides to the emissions fix and there are a few furious posts reporting this with the 9sp box that the OP doesn't have. But it seems to be because the fix resets the shift points, and they re-learn with time.
I've had the fix, it made the throttle response nice and nippy for a while, but it went back to normal. No discernible difference in economy, power, noise or anything. It's not like the VW fix; Merc weren't pulling the same scam.
Point you all seem to be missing is that the economy dropped after a service. His method of calculating economy hasn't changed, his driving style hasn't changed, etc etc. He's had the car a while so should know what to expect.
Assuming 10,000 miles a year, 33mpg will cost £4700 (£2200 fuel + £2500 minimum running costs), 40mpg will cost £4300 (£1800 fuel + £2500 minimum running costs). All of a sudden £350 is not significant, especially when realistically a 3L diesel isn't going average 40mpg all year. So if a realistic 'good mpg' is say 37mpg, we're talking £200 on a near £4.5k spend to run a Mercedes per year. This all assumes no other running costs/breakdowns/tyre replacement.
If you want low running costs, a 10 year old 3L Mercedes isn't a starting point, so a couple hundred pounds is irrelevant.
Also bear in mind that a "headwind" isn't canceled out by a round trip.
Drag is proportional to the square of speed so to put numbers to that
(70+20)^2+(70-20)^2=10600
70^2 + 70^2 = 9800
(figure is just illustrative, you'd need to multiply by Cda and time as well)
So a round trip on the motorway on a windy day is using ~8% more fuel.
I'm doubting there's anything "wrong", you're just driving a massive engine and comparing the best day (worn tyres, low pressure, windless (or even a one way tailwind), hot weather, etc. Vs New tyres, full pressure, freezing weather, new + inflated tyres, etc.
If you want to feel better, do/replace everything suggested, wait a few months for the ECU to adapt, and it'll be better, because it's summer.
All the 'it's a 3.0l diesel' comments make no sense to me. In a diesel, it only injects the fuel it needs to power the car at the speed you request. The displacement doesn't make much difference, unlike in a traditional petrol car that has to fill the cylinder with a minimum amount of fuel. That's why bigger petrol engines are less economical.
The two extra cylinders and the V configuration add more friction. And there may be other airflow factors, I don't know. But regardless of any of that - he's driven it in for a service getting low 40s, and driven it out getting low 30s. The displacement of the engine and the number of cylinders haven't changed....
Warmer air has less mass and drag.
Not so you'd notice at the speeds to get to the motorway/dual carriageway... but the other thing I'd notice is the heater came on "properly" (as in switched) as the instantaneous mpg went up... presumably in the fairly sophisticated engine the ECU is trying to get to operating temp ASAP using every "trick" ???
Once I noticed/realised the two seemed linked it was even more noticeable.
Point you all seem to be missing is that the economy dropped after a service. His method of calculating economy hasn’t changed, his driving style hasn’t changed, etc etc. He’s had the car a while so should know what to expect.
*Insert The Office “Thank you” meme here…*
To be fair, this has been more useful than the responses to the exact same question on the FB owners page, not that that is particularly surprising… 🙄
The computer could be calculating the MPG incorrectly, and a service/ECU reset (or battery disconnect) could have affected that. I have previously noticed car MPG gauges aren't consistent and the best way is to check on a full tank to empty. The last time I did that the car was out by 10 % and there was even a hidden menu to override the gauge offset. This offset could have been reset at the service.
Also for reference, I looked at buying a 2014 E class with the same engine a couple years ago and didn't because the reported MPG from several sources showed realistic MPG of just 35mpg. Which is partly why I'm suggesting the original MPG might have been inflated.
Not so you’d notice at the speeds to get to the motorway/dual carriageway
So what's the explanation for my two examples above?
Two long motorway trips with very similar average speed but with 15-20c difference in air temp. The journey in warmer air was actually 5mpg more efficient even though it included much more urban driving and for half the trip was carrying more weight.
To be fair, this has been more useful than the responses to the exact same question on the FB owners page, not that that is particularly surprising
Unless you are doing the same journey in the same weather/time of day etc. I think it's really hard to say and its easy to cherry pick "I did x mpg on that trip last year" and potentially spend money that doesn't make a significant difference.
I know you said you've done 3000 miles since the service but on the van I noticed (or thought I noticed a drop in MPG after the service) that didn't seem to be reflected at the pump and was perhaps a "reset" occurred in the service???
he’s driven it in for a service getting low 40s, and driven it out getting low 30s.
No, he said he used to get high 30's and now it's mid-low 30's - so it's about 15% which could be explained by a combination of factors mentioned above.
All the ‘it’s a 3.0l diesel’ comments make no sense to me. In a diesel, it only injects the fuel it needs to power the car at the speed you request. The displacement doesn’t make much difference,
I wonder why mercedes report a 15mpg differential between their e200-250 and 350 then ? Surely by your theory all should use the same amount of fuel.
So what’s the explanation for my two examples above?
Two long motorway trips with very similar average speed but with 15-20c difference in air temp. The journey in warmer air was actually 5mpg more efficient even though it included much more urban driving and for half the trip was carrying more weight.
You weren't doing 20-30mph seems to be the obvious one?
I was simply pointing out that my 3L flat 6 got considerably different MPG with an engine at operating temperature than below operating temperature and that the 3L diesel block takes a while to heat up at 20-30mph.
I understand the BMW "M" petrol engines of the same age were rev limited before operating temp is reached.. so I expect the diesel ones also have some change in mode ???
The drop in fuel efficiency is reflected across all my driving and can be tracked back to a specific event; the service. Pre-service it would reliably return 37-39 on a run without too much effort, now it's doing 32-34 and I'm consciously trying to improve that figure.
Next time I know I'm going to do a decent run I'll brim the tank before and after and see what the fuel consumption is that way.
The computer could be calculating the MPG incorrectly, and a service/ECU reset (or battery disconnect) could have affected that. I have previously noticed car MPG gauges aren’t consistent and the best way is to check on a full tank to empty.
That seems to be what happened to the van... I was actually in the same sort of space as doomanic as I was actually hoping for the inverse but whatever got reset seems to have recalibrated over time.
No, he said he used to get high 30’s and now it’s mid-low 30’s – so it’s about 15% which could be explained by a combination of factors mentioned above.
I have had to change my driving style significantly to get over 30 now, driving like I am currently would have seen low 40s.
Then either it's the fresh tires giving you an effectively larger diameter wheel, or they've left something like the MAF unplugged.
But as molgrips said, diesels are simple machines, they suck in a lot of air, add fuel in proportion to your right foot, and blow it out again. Which is why they'll happily run with no MAF, manifold pressure sensors, etc. But seeing as you've plugged a code reader in I'm guessing it's not that as they'd still flag up unless the reader wasn't compatible with the car's computer (they aren't universal, the manufacturers hide a lot of the detail in their own protocols).
Have a look for the obviously unclipped sensors under the bonnet, take it back and ask them to plug it into their Merc code reading machine, and if it's none of that then it's the tyres.
All the ‘it’s a 3.0l diesel’ comments make no sense to me.
You still have ~50% more pumping losses than a 2.0 i4, it's not as bad as a petrol, but it's still bad (which is why you get 3.5t vans with 1.5l diesels and 2x turbos).
+ if I absolutely floored my old Berlingo at 3500rpm (or wherever peak power was hiding, it was hide and seek champion), it's still only sipping fuel at whatever flowrate gave 90Hp. A 270HP engine has the potential to drink fuel at 3x the rate. Now bearing in mind the Berlingo couldn't even drag itself over the Ridgeway on the M40 at 70mph, there's going to be a lot of time when more powerful cars are burning more fuel even if they're just on cruise control. Plus every time you dab the noisy pedal to make progress, get upto speed on a slip road a bit quicker, beat a Berlingo away from the lights to the merging lanes etc, cumulatively that's all adding up.
I was simply pointing out that my 3L flat 6 got considerably different MPG with an engine at operating temperature than below operating temperature and that the 3L diesel block takes a while to heat up at 20-30mph.
My point was that air temperature makes a difference given similar trips.
I have had to change my driving style significantly to get over 30 now
OK..... you hadn't mentioned that before. Something might be up then! If it was petrol I'd consider the change from E5 to E10 but diesel is different.
The drop in fuel efficiency is reflected across all my driving
When I had 2 regular commutes my fuel efficiency was between 49.5 and 50.1 and because it was something interesting to do whilst driving I'd try and hit the 50mpg... and reset it in both directions. Winter temps aside that had no relation to my MPG across all driving. Another regular commute of a similar distance I'd get maybe 35 mpg.. depending on time of day.. I tried pushing that to hit 40mpg but other than very late at night I'd not even get close so it stopped being "interesting".
If I hadn't had a specific if inane interest in being a bit obsessive I doubt I'd think the driving was that much different between the 2 at 50mpg and the one at 35-38mpg..(I'd even stay a bit later at work sometimes). but when I was trying to get the 40mpg it became obvious that the on the face of it similar trip had much more stop/start or slow and accelerate and that was where I was losing the 12-15-ish mpg.
When I was doing the 50mpg commutes I'd also have some 35-40 mpg weekend journeys with bikes on and different places so my average across all my driving was probably more like 40mpg or something (tbh never really measured that I just let weekends be what they were)
there’s going to be a lot of time when more powerful cars are burning more fuel even if they’re just on cruise control.
Not compared to my car which is the same minus two cylinders. If anything the V6 would be getting more complete combustion with less fuel in each cylinder. Since both these cars can burn enough fuel to go up the hill at 70, then the 250 will use a similar amount of fuel to the 350 because the drag and weight are similar.
But in any case. It's behaviour has changed due to a specific event. Why are you all trying to tell him that behaviour is what to expect, when he's owned the car for a year or more prior to that?
It's just about possible that tyre changes can account for that difference when going from the absolute worst to the best. But I don't think he has?
AC unit switched off?
Would a remap help?
Cool story bro, I have no idea what it's got to do with my mpg though?
Not compared to my car which is the same minus two cylinders. If anything the V6 would be getting more complete combustion with less fuel in each cylinder.
That's not how combustion works. More air / less fuel makes is significantly less efficient as the heat produced is absorbed by the air, meaning less temperature and pressure rise. It's not analogous to blowing bellows into an open solid fuel fire.
It’s just about possible that tyre changes can account for that difference when going from the absolute worst to the best. But I don’t think he has?
It doesn't need to be best to worst tyre, it just needs to be a bald tyre that's not been checked for pressure in a few weeks/months, to one with new tred and inflated to the top end of the recommended range (and other stuff like the EU rating, variations in the actual size of the tyre between brands etc may be working against the OP too).
Not compared to my car which is the same minus two cylinders. If anything the V6 would be getting more complete combustion with less fuel in each cylinder. Since both these cars can burn enough fuel to go up the hill at 70, then the 250 will use a similar amount of fuel to the 350 because the drag and weight are similar.
Have you dropped mercedes a line to let them know they are selling them selves short in the empirical testing portion of the design qualification ?
Cool story bro, I have no idea what it’s got to do with my mpg though?
That's for SteveXTC, in case anyone was wondering...
Pump all tyres up to 60psi?
No, wait. Don't do this.
Impromptu fashioned carbon effect aero wheel covers?
Strip out all interior, carpets, sound deadening, audio, air con, unused seats for lightweighting gains...
Sorry, not much help.
Doesn't sound far off for a big capacity V6, in fact have seen much much worse.
Enjoy it for the few remaining years while we're still allowed...
More air / less fuel makes is significantly less efficient as the heat produced is absorbed by the air, meaning less temperature and pressure rise.
In diesel? The heat produced would cause the air around the injected diesel droplets to expand very quickly. The rest of the air isn't going to get gently warmed, the pressure is going to increase very rapidly which would cause it to get hot. But that heat is what's making the car go forward, how is that a bad thing?
Doesn’t sound far off for a big capacity V6, in fact have seen much much worse.
Not read the thread then or all of the OP?
Op it’s unlikely you’ll get much more that the well meaning but ultimately useless suggestions like ‘I emptied my ashtray once & then the performance was twelvety % better’ or ‘my Nan has a dog but his leads blue’
It could very well be that some kind of air / vacuum leak has been introduced by inadvertently disturbing some pipe work during the service / inspection. But that’s just another well meaning guess 😉
It might be worth taking it elsewhere & have the fault codes read, note them & reset & re-read after a week or two to see if any come back.
It might also be worth joining a MB specific forum / piston heads MB specific sub-forum & seeing if they have insight.
Not read the thread then or all of the OP?
Continuing to ignore that mercedes disagree with your statement?