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Not read the thread then or all of the OP?
Not really, I was concentrating on taking the wee.
Now I have, for politeness sake... moving from a very good Conti Sport Contact to an Avon will make a difference... and temperature... for all the above reasons but including that summer tyres give worse rolling resistance in the cold as the rubber stiffens.
It's ten degrees warmer this week up my end, probably same all over so test again next trip to iron out.
Mine last week (not similar) went from normal easy 40mpg to 33mpg in temps around zero, no idea why but there's so much complicated gubbins going on in a modern engine. I rest assured that spring is around the corner.
Continuing to ignore that mercedes disagree with your statement?
I ignored your post cos I couldn't work out what you meant and couldn't be bothered to follow it up 😉
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.
@doomanic if you're friendly with your garage ( are they a merc specialist, or just local indy?) I'd be tempted to go in for a chat about the service, not to throw any blame around just to find out if they have done an ecu update/reset. Worth seeing if there's anything else they may have adjusted, it's clearly stems from something they've done but not necessarily wrong, could they have adjusted the hand/foot brake? maybe it's a shade too tight and drags when the discs/pads have warmed up.
Did the service involved a calibration / ECU update? Might not have been explicitly mentioned but just done in the background.
There are some updates doing the rounds on various cars as a result of "dieselgate" which result in MPG reductions.
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.
the drag and weight are similar but the parasitic losses of the 6 pot engine are far greater, which is why the economy is lower. For every rotation of the engine the car is having to suck squeeze and blow a further 1l of air through restricted spaces, which causes a large amount of drag and lost energy. Think about how fast your car slows down in gear vs in neutral, that effect is increased by 50% on the larger engine, let alone the fact it weighs ~80kg more (total weight for a c250 vs c350. might be a different generation but its probably indicative)
Think about how fast your car slows down in gear vs in neutral
Surely that's mostly caused by the drivetrain?
if you’re friendly with your garage
but I can’t return to the garage for reasons that aren’t worth wasting many pages on
Doesn't sound like he is on very good terms, so maybe.....
the drag and weight are similar but the parasitic losses of the 6 pot engine are far greater, which is why the economy is lower.
Yeah I get that, the comment was in reference to TINAS comment that a more powerful car always uses more fuel going up a hill. I should have been clearer.
See, as soon as someone gets all "ignore the well intended but utterly useless advice" , then goes on to talk about vacuum leaks, in a diesel car. - with no throttle to create neg air pressure i just think the well intended advice might be more helpful.
My om642 has the aftermarket silicone inlet tract so has no pesky O rings to fall into the turbo vanes, its a 20min job to see if the turbo is blocked - borked, but i would expect to see some grey or white smoke on acceleration.
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?
Because heating a small amount of air a lot Vs Heat a lot of air a small amount. Take it to extremes with an infinite mass of air and you get no change in temperature/pressure during combustion. In addition to about 80% of the energy goes straight out the exhaust valve with the hot gases.
What you're describing is the inverse of an Atkinson cycle engine which is where you deliberately underfill the cylinder prior to combustion, which effectively means the engine is operating slightly closer to atmospheric pressure at BDC after combustion, as any leftover pressure at that point can't be recovered (except by the turbo, but that's not very efficient).
i would expect to see some grey or white smoke on acceleration.
But the DPF blocks that, no?
Because heating a small amount of air a lot Vs Heat a lot of air a small amount.
Tangent, but if the cylinder were the same stroke but a larger bore, then it a lower pressure would exert the same force on the piston. If the stroke were longer, it would exert less force but for longer. In this case, the cylinders are the same volume (not sure about the bore) but there'll be less fuel and more air compared to the 4cyl.
I know this goes against the request in the OP...but why can't you sell it?
Tangent, but if the cylinder were the same stroke but a larger bore, then it a lower pressure would exert the same force on the piston. If the stroke were longer, it would exert less force but for longer.
That's just a long winded way of saying work done = force x distance.
In this case, the cylinders are the same volume (not sure about the bore) but there’ll be less fuel and more air compared to the 4cyl.
Exactly, which is less efficient. Because you're pumping that air through, heating it up, then blowing that hot air (and energy) out the exhaust, which isn't doing work.
Caveat - assuming the same gearing. Logically you would assume the 6cyl engine should be able to run at 66% of the RPM to get the same firing intervals (by time, not engine rotation), and if you did run it slower, you're back to the same amount of air being pumped, bigger fuel pulses etc (more torque, lower RPM, same power). But then you run into issues with valve timing as low RPM = low port velocities, which means the timing of the valves opening and closing needs to be closer to TDC/BDC as you can't take advantage of the momentum of the gasses which would mean the engine wouldn't run efficiently at higher RPM.
Exactly, which is less efficient. Because you’re pumping that air through, heating it up, then blowing that hot air (and energy) out the exhaust, which isn’t doing work.
No, that's exactly how the work is done - making the gas in the cylinder really hot so it expands and cools a bit in the process. How else is the engine doing work?
The inefficiencies of the V6 come from more pumping losses as shown above (I'll accept that) and the friction within the engine due to more cylinders and I think the effects of the V angle, not sure about that one.
I don't think displacement volume has much effect on economy in a diesel. We pretty much settled on 2 litres for most cars, whereas petrol engines have become smaller and smaller.
I know this goes against the request in the OP…but why can’t you sell it?
1. I was forced to buy a car at pretty much the high point of the second hand boom so I suspect I'd take a real bath if I sold it.
2. I'm not comfortable selling a car with a potential fault. I know, stupid, right?
3. I really like the car. Getting low-mid 40's on a long steady run was a bit of a shock after the easy 50 of the 2L 5 series on similar journeys but the grin factor when I hoof it and the phwoar factor when I look at it is worth it.
That’s for SteveXTC, in case anyone was wondering…
Just pointing out that I had similar differences in MPG on commutes that didn't seem so different... it's not really very cool that the height of excitement each for me for 3 years or so was the 0.5 mpg difference I could squeeze out.
You seem convinced it was the garage and it might be, or something else or a combination or a reset on the MPG measurement but unless you're really doing the same journey it's the same sort of variability I had with a not dissimilar diesel.
I don't have that car anymore but if I wanted to compare MPG because I felt it had changed significantly I'd very specifically go and do one of those routes with no queues at silly o clock as I know what I used to get rather than some average over however long before I spent more money that might have changed due to what seem to be small changes in use.
2/3 of the routes I know are the same 1/2 way... I'd not have thought the other half made that much difference unless I'd been playing my "see what MPG you can get today game"
I realise that's not what you or anyone wants to hear in terms of £££ draining away unless you can do something about it but if you have a route you know from before you can repeat in the same conditions it would eliminate or not one aspect for the price of the fuel
The fuel economy you get on a route isn't random. If you own a car for a while, and you pay attention, you get a reasonable idea of what to expect. I will be able to predict what economy I get within a few MPG. And I would know if my car suddenly started using 1/3 more fuel!
Steve, You seem to either be missing or ignoring my point.
I'm not talking about two different commutes of similar distance but dissimilar mpg figures, I'm talking about a marked change in fuel efficiency across all driving that started at a clearly defined point. I'm not convinced the garage has caused the issue, but it's a possibility given the timing of the change which is why I asked on here for advice/opinions on what might have caused the change.
It is amazing how many mfgs don't know capacity makes no difference yet routinely quote lower figures for bigger engines.
The bsfc charts do tell the story of the science but you'll not be bothered to look those up either I'm sure.
When was this service done doomatic. The wife's 1.6 berlingos showing 40.4mpg in the 2 k since service and reset when I fitted the winter tires IE just as it got cold. It's spent a while idling demisting etc also which has contributed along with taking longer to warm up..... But it will rise to 46 in the warmer weather as it routinely does.
November.
It is amazing how many mfgs don’t know capacity makes no difference yet routinely quote lower figures for bigger engines.
Yeah not what I said. But whatever, mockery is a noble pursuit and I wouldn't want to get in your way.
Just when the weather changed and it suddenly went warm to cold per chance?
I think we have the answer.
Bigger engine means more kg to warm up. 8.5ltr of cold engine oil takes longer to heat up.
Running with the ptc cabin heater and climate on auto at 22c draws more amps amd has to burn more fuel to get them. Heated rear window and mirrors all add amps too.
So do the auto headlights
Cold gearboxes are more draggy, esp auto boxes.
All the wheel bearings are stiff forctge first 5mins and create drag.
The roads are wet and create more drag.
Unless you have a 40 mile each way commute a much bigger percentage of your journey is in warm up mode. 6mpg is normal imo for a td engine, sometimes 8mpg between high summer amd mid winter.
Time to stop worrying and enjoy the car, mine hit 43mpg on the way home tonight as it was a warm afternoon.
Exactly. Also, your battery will naturally be at a lower charge state in cold conditions, even if it weren't being used to power all of those additional things but just from temp effect alone. So when driving as normal isn't there more drag on the system as the alternator has to do more work to bring the battery back up to charge?... or am I making this up? (don't actually know the answer to that one, but assuming it to be the case).
Just when the weather changed and it suddenly went warm to cold per chance?
Assume he drove it last winter as well?
Assume he drove it last winter as well?
But did he reset the mpg counter right before the cold weather ? 9 months at decent temperature will soon hide a couple low months due to averaging.
Wife's car would solidly report 45 all year round till COVID reset the service from spring to just before winter. I reset the master ave mpg trip meter at each service to keep an eye on it over time and use the trip meter for the tank-tank ave
No, that’s exactly how the work is done – making the gas in the cylinder really hot so it expands and cools a bit in the process. How else is the engine doing work?
And that's where it's inefficient to add more air. You keep dS the same but reduce dT. Sketch it out on a P-V and T-S plot if you still don't believe me.
It's got several trip meters, including one that resets after 4 hours of power off so I get a reading for every trip I do.
@doomaniac are you just adding an important point on every page?
Are we going to find out you added a roof box on Page 4 and started carrying around industrial tooling on page 5?
I was saving the roof box for page 6...
Assume he drove it last winter as well?
! bought the car mid-Feb last year and the best ever fuel consumption was achieved on 18th March.
It was 10deg when I left Gatwick around 6pm and around 9deg when I got home to Evesham, which is pretty much identical to the conditions on Sunday when I got 33.
Right.... So this if the first winter you've had the car.
Do you know your average speed for both those journeys?
If not it's hard to compare.
My money is on colder temperatures.
And that’s where it’s inefficient to add more air. You keep dS the same but reduce dT. Sketch it out on a P-V and T-S plot if you still don’t believe me.
Maybe you can explain to trail_rat how to explain something without mockery 😉
Although I'm not sure there isn't something else here because you aren't heating up all the air at the same time. Cylinder deactivation is a big thing for big petrol engines to save fuel, but not diesels. There it's used to heat up exhaust treatment systems more quickly.
But did he reset the mpg counter right before the cold weather ?
I assume since he's taking about the per-trip economy that he's using the per-trip counter rather than the long term one.
Particularly poor economy on one journey or two consecutive journeys can be due to the vehicle doing a regen on the DPF.
If it doesn't complete in one journey, or reduce soot to below a threshold then it will start again some way into the next journey.
With an old DPF it will have a load of ash inside it, so the regen will be occuring more frequently than a new or cleaned DPF as the filter cannot hold as much spot. This will cause economy to be worse than a new car
particularly poor economy on one journey or two consecutive journeys can be due to the vehicle doing a regen on the DPF.
Yes, but this is every journey.
If this is your first winter with the car, I've found winter diesel drops fuel economy by around 10%...
When did the UK finish the move to E10?
The change from Winter to Summer diesel mixes always impact FE. Once you add in the E10 standard into the mix, it gets even more noticeable.
Also, a decent indy workshop will have updated all the software, a crap one won't necessarily have done so.
edited: was going to say E10 hasn't affected diesel but I see there's B10 diesel now too. No idea what that stuff does but have worked out my car likes 99ron Vpower and the few tanks of E10 its had has noticeably nerfed engine power and returned around 10% less fuel economy, whereas the old jump from 99 vs E5 was barely noticeable (small capacity moderately highly strung turbo R56 MCS)... will be sticking to the good stuff from now on.
See, as soon as someone gets all “ignore the well intended but utterly useless advice” , then goes on to talk about vacuum leaks, in a diesel car. – with no throttle to create neg air pressure i just think the well intended advice might be more helpful.
Would that be the same person who said vacuum/air leaks, I’m assuming there’s no air fed to the engine or turbo(s) then?
Would that be the same person that caveatted their comment that it might be in the well meant but tosh category?
The same person who suggested a mark specific forum might yield some more substantive responses?
The same person who suggested a mark specific forum might yield some more substantive responses?
They are often even less conclusive than we are 🙂
Does the car feel like it’s working harder, are you needing more throttle? Is it dropping or holding gears more than it used to?
If none of those things are happening, it suggests it’s a fuel input/combustion mix problem. Is the exhaust or back of the car notably sootier?
I use a UniCarScan Bluetooth ODB adapter and the IOS Car Scanner app to monitor my DPF fill level and regenerations on my newer Vito with an OM651 engine. It can give loads of live data.
Also very good reading and resetting fault codes.
There are definitely profiles that should work with your engine. I use the E250 and it works great. The Sprinter one could work for you.
https://www.bmdiag.co.uk/unicarscan-ucsi-2100-bluetooth-obd2-adapter
https://apps.apple.com/gb/app/car-scanner-elm-obd2/id1259933623
Ooh that looks fun.
Did they RedEx it as part of the service? my old garage used to, and i dont know what that would do in terms of economy, but might be worth seeing if it settles down after a few tanks?
Or its hit a mileage threshold and the manual says to put a thicker oil in to compensate for enginer wear?
I like the look of that, thanks @colp
Might have to pick one of those up for monitoring the dpf on my Vito too.
edited: was going to say E10 hasn’t affected diesel but I see there’s B10 diesel now too.
Yes, typo, meant B10.
Has a similar effect on economy as E10. Increasing the bioethanol content = makes FE worse.
Apologies if this has already been asked, I've only read the first page and then skipped to the end.
Why is buying a different car not an option? Binning off a 3L V6 for something less thirsty is surely the obvious solution, no?