Or I have this Mercedes and I have questions, part 2.

I have a 2013 133K mile CLS Shooting Brake with the 3L V6 and the fuel economy is dire. The best I’ve ever managed was 42 on a long steady drive back from Gatwick. It would do mid 30’s with bikes on the tow ball rack and high 30’s without.

Yesterday I did a steady 70 mile drive, mostly A road and Mway and it did 33! It’s been worse since the last service, but I can’t return to the garage for reasons that aren’t worth wasting many pages on, so don’t bother asking…

I’ve checked the obvious, like tyre pressures and I don’t think the brakes are binding; the centre of all the wheels feel similarly warm to the touch after a long drive and there’s no burning smell.

I used a mate’s Snap-On code reader and there were no codes that would indicate any issues.

Where do I go from here? Am I just looking at huge bills while a different garage chases their tail hunting for an issue?

What was included in the service?

Edit: I’m surprised the fuel economy is as good as you say it is to be honest. That’s quite impressive.

The best I’ve ever managed was 42 on a long steady drive back from Gatwick. It would do mid 30’s with bikes on the tow ball rack and high 30’s without.

Yesterday I did a steady 70 mile drive, mostly A road and Mway and it did 33!

what more do you expect from a 10year old 3l V6?

Other than lose as much weight as possible (empty the boot, remove spare tyre etc) and done have thing’s attached to the outside. The only other thing I can think of is gentle acceleration and use the brakes as little as possible by lifting off the accelerator early when slowing down

other than that the only way of reducing cost I can think of will be use the car less.

Sounds like you’re getting bang on the average?

https://www.honestjohn.co.uk/realmpg/mercedes-benz/cls-2011/cls350

It’s a 3.0l, turbocharged, V6, it’s never going to be economical, just slightly more economical than a petrol one.

The only service related thing I can think of is check the oil level, having the oil at the max level can have an impact on the MPG as the engine does a not-negligible amount of work just sloshing the oil around and pumping air back and forth under the cylinders.

Your fuel economy looks good to me!

But if you’re confident it used to be better and you don’t think it’s just down to weather/traffic/ different routes, then I’d start by looking at the air filter, it’s a bit clutching at straws,but check it’s clean and check it’s a genuine Mercedes Benz one.

My gut feel is that there is nothing wrong, but that’s where I would look if I thought fuel economy had got worse after a service.

3L v6? hmmm..

My V70 2.4t struggled to get more than 30mpg mixed driving, 40 on the motorway (under the speed limit, diriving super peogressive).. It would return about 20mpg avg when ‘making progress’.

How fast are you driving on the motorway? If you drop the speed from 70mph to 55ish it will make a significant difference to the economy…

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.

Om642 lump or petrol?
My om642 7g is about the same. Cruise set at 60ish obc reads 42mpg, reality its 38mpg.
Cold starts seem to hit the mpg really hard. Mines got aux cabin heater and i think thats a drain on the battery and alternator. Plus 9c amd its 4mpg better straight away

Headwind? That’s can make a decent difference especially if you’re doing 70mph or more.

It’s still better than the mid-20’s mpg I used to get from my 3.0l, flat 6 AWD Subaru Legacy!

You’re trying to beat physics, I’m afraid – high 30’s to 33mpg isn’t that much, as above a headwind, or a heavy right foot and it’ll easily done.

How many miles are you actually doing a year? 33 Vs 38mpg is £300 a year difference if you’re doing 10k miles a year.

My best with that engine in my old Vito was about 32mpg

A carbon clean or terraclean might help.

Folks used to get 35 out of their 350cdi. E class coupe

For 240bhp it was never very inspiring and was quite choppy on the road so not even a good cruiser.

The c coupe 220cdi they had before would get 45 mpg and both handled better and was comfier somehow.

How are you measuring it? What the dash says or full tank to full tank?

Take out a couple of pistons? 30 doesn’t seem at all bad for a car of that type, and it’s pretty common for economy to fall off a little as cars age. A carbon clean <might> help, but I’d not put much faith in it, and the cost of the clean buys a fair amount of petrol. It’s probably nothing to do with the service that you mentioned, the oil used can make a little difference but not much at all.

Tyres can make a big difference- pressures too, but a sensible efficient tyre can give good performance and add a few mpgs compared to a sticky or just less efficient tyre. Ovicously expensive, one for when they need replaced.

In the end there’s generally not a lot you can change, that’s cheap enough to be worth it when you offset it against the price of petrol, except for thngs like tyres where there’s no real additional cost, or things like driving style. Like, a good remap can often get efficiency improvements but takes ages to break even.

Is it just the difference between winter & summer? I get worse mpg in the winter.

How are you measuring it? What the dash says or full tank to full tank?

What the dash says, but I’m comparing all readings from that.

Is it just the difference between winter & summer? I get worse mpg in the winter.

That has been suggested, but it was the sudden change after the service that has me worried.

Om642 lump or petrol?

Diesel.

What was included in the service?

Edit: I’m surprised the fuel economy is as good as you say it is to be honest. That’s quite impressive.

Merc schedule B service which is basically oil and filter.

It used to be better…

what more do you expect from a 10year old 3l V6?

It used to be significantly better. If something is wrong, I’d like to get a handle on it before it becomes terminal.

Headwind?

It was an out and back trip, so unlikely.

I’ve asked the exact same question on the owner’s page on FB and most people are claiming much better figures.

Edit, you’ve just said diesel 😀 i was guessing petrol…so what i wrote below is from that angle

Sounds ok, much better than people are getting in their golf r’s etc
I don’t believe its that good either… Im going to say you are going by the computer and not tank to tank
Auto or manual? for reference my old 530d wouldnt beat 32mpg in auto form, i got 10mpg by converting to manual, had so much of an impact
anything you get back will literally be 1 or 2 mpg which is going to make bugger all difference on your wallet.
In perspective, my old 525 petrol wouldn’t beat 23MPG (mixed) iirc and that was after me spending a shed load of time tweaking…

Oh, and switching to premium diesel made no discernible difference, other than to my credit card bill.

More clutching at straws from me ,but the wrong type of oil can make a noticeable difference to fuel economy,so if the fuel economy genuinely is worse after the service and all they did was change oil and filter then it could be the oil.

Thermostat on the way out? When I bought our Galaxy it was shot, and that knocked 10mpg off the car with it stuck open and cold engine.

One other variable was new front tyres at the time or service. It had Continental Sport Contacts on before and Avon ZV7 after.

The Conti’s are rated C for fuel efficiency, the Avons are rated D, could the tyres really make that much difference?

Can you define significantly better? What is the drop we are talking about?

It went from high 30’s on a run without trying too hard, to 33 while trying really hard to maximise efficiency.

Really taking a stab in the dark here perhaps wrong diff oil or transmission fluid? Failing lock up clutch in the torque converter (I’ve never heard of this happening but I suppose it could lead to really poor fuel economy).

Oil burner you say, oooh also consider partially clogged cat or dpf. In your diagnostic report does it state ash and soot levels?

Another thing; before the service, there was a noticeable difference to fuel economy when using the tow bar rack (3-5mpg) now it’s 1-2mpg lower with the rack on.

could the tyres really make that much difference?

Possibly, in the absence of anything else I’d blame the tyres,but it’s an expensive experiment to swap back and see if things improve.

It’s more likely to be due to tyre than the wrong oil

Is the tow bar rack electrics properly integrated? On VWs plugging anything into the towbar electrics changes DSG engagement routines and shift points – effectively the gearbox and engine switches into a different, sloppier map. My how we laughed at my brother when his (seriously fettled) superb lost 100hp and many torques from its custom map after plugging in his bike rack. Has it become stuck in towbar mode?

Is the tow bar rack electrics properly integrated?

No idea, it was fitted before I bought the car.

Has it become stuck in towbar mode?

No idea if it even has a towing map to be honest.

i think merc had to fix a lot of cars because of dieselgate, so it might be that? has it been in a merc dealer recently?
my 2010 v6 viano which is a 2t van with windows and seats etc can do mid 30s but you have to drive very carefully.
my wife gets 30 out of her v70 d5, but if i drive it on a motorway in granny mode i actually have managed 50mpg. i was tooling along at 60 though.

if it wasnt a main dealer, what exactly did they do? just a oil and filters?

is the lock up clutch lokcing up?

OK ,
Its unlikely to be the oil as long as its 5w-30 fully synth. B service wont have touched gearbox , that 40,000 for a partial change . Diffs are another long interval and wont have been done on a B .
The Batwing mass air sensor is a tricky bugger to refit so I would take a good look at the airbox and the PCV. The diaphram fails on these so it may be related to that. On the E you have to remove the airboxes to refit the filter although its above the MAF if its been put back wrong it might make a differnce.
Tracking done with tyre swap ? That might not help.
I dont think the Daimler 7g tronic has torque converter lock up , at least either mines borked or it drives with the TC in suish mode permanently.
DPF full ? My car dosent have a DPF so cant comment on the longevity, but with the chassis and box all contributing to wafting its exactly the type of car that will soot up.

has it been in a merc dealer recently?

Merc Indy.

just a oil and filters?

Pretty much. Plus looked at a load of stuff.

is the lock up clutch lokcing up?

No idea, how could I tell? The same garage changed the gearbox oil several months before the service.

Edited to add – most of these have been covered above.

Mass air flow sensor working? I’m guessing on yours it would flag a fault code but my HDI Berlingo MAF was dead and it resorted to a default fuelling map that overfuelled.

Tracking? I do my own tracking by the string method and set my cars up with no toe in or out. Again my little Berlingo rolls so well.

Tyres? Again….my stupid little Berlingo, it’s on Continentals and they must be rock hard as they last for ages yet still manage to grip and must have good rolling resistance.

Oil viscosity – has the garage filled it with 5-40w or something equally more viscous than should have been used?

Deiselgate update could be a good one.

Similarly could it already have had an aftermarket tune that has been wiped during an update?

There are sometimes sensors that can be out of spec without bringing up a fault, a decent auto spark or mechanic might be able to run through them to check.

Things like the engine cooling fan permanently running can also cause a drop in efficiency, I’ve heard the one on my van can pull about 20-30bhp from the engine at high revs (it certainly makes a difference to engine braking when it switches on and off)

May require dremel:

I’ve got a 2011 CLS (same model as yours but the saloon) 350 cdi 3lv6 with 64K on the clock and my average over thousands of miles is 38mpg. On the motorway I get low to mid 40s if I do 70 ish, nearer 50mpg if I do 60, I managed 55 by sitting with the trucks at 55mph on a 200 mile round trip…but only the once, mpg has been a bit off lately as driving with the heater on burns more juice.

Your mpg sounds a bit meh.

Plan your journeys to be downhill.

That doesn’t seem very good. My old 350cdi estate would easily do +40mpg on a run. Even my last e43 (3.0 petrol) would do mid 30’s on a run.

I’d get it to a Merc indie with the right software to get it checked over.

One other variable was new front tyres at the time or service. It had Continental Sport Contacts on before and Avon ZV7 after.

The Conti’s are rated C for fuel efficiency, the Avons are rated D, could the tyres really make that much difference?

There’s your answer!

New tyres are probably 6-7mm more thread, and probably have more air to boot. And car tyres, like bikes, are woefully inconsistent between brands which could compound it further.

13mm on a tyre is about 2% of it’s diameter.

Old ones underinflated by 5 psi or a slow puncture? maybe another 5%?

The bigger the wheel, the faster you actually go for the same speedo and odometer readings. The fuel economy is dictated by the actual speed though. You could very well be driving quite a bit quicker than you were.

Does it have “learned driving style”? Could this have been reset during service. If so it could take a few hundred miles to sort it out/settle down.