E400 cab shocking(?) MPG

[C270 CDI]

Hi all,
I don't know if I'm expecting too much here but my 2014 E400 (3.0) is getting pretty poor mpg. If I rag it I get what I expect, but on a run to work (11 miles, 10 of which on dual carriageway at about 65) I'm not even getting 30 - about 28.5 is the best it'll do.
Now I know it's a "performance" engine but even my 5 litre R230SL would do about that and that was 90s tech!
Is it worth getting it terracleaned or could it be something more sinister - or could I just have slightly higher expectations on this?!

 

[DSM10000]

Have you measured the fuel consumption properly and by brimming the tank, running it for a couple of weeks then refilling the tank and calculating the fuel used?

Does the engine coolant reach the normal operating temperature?

Also, have you reset the trip /fuel computer

 

[C270 CDI]

Yeah I've done that, on a really good run it's about 32 which I still think is a bit low. I'm just curious if anyone has had significantly better mpg or they're all bad like this. I know it's not a reliable source but if you see one on auto trader they're saying about 40mpg on a run.

 

[Petrol Pete]

OP ,Can you answer the question on engine running temperature.

It's important in this discussion

 

[fabes]

The engine is capable of mid forties ( I have the 3.5tt version)

As above, temps & thermostat could be a big factor
But also under inflated tyres, lots of stuff/ weight in the boot and whether servicing is upto date (a binding brake even)

RSVP to the post above

 

[C270 CDI]

Yeah temp gauge is about 85-90 - that's normal I would think?

 

[C270 CDI]

Also tyres are brand new and at correct pressure, and apart from me there's nothing in the car.

 

[Fastcar155]

I don't know what petrol you use but when I had a a C207400 with the 3.5 litre engine I always tried to use Shell V Power. Mercedes recommend 99 Octane in my car.The engine temperature gauge took a couple of minutes to reach operation temperature .The pointer always was above the centre line on the temperature gauge.

Fuel consumption is always tricky but I never ragged the car but I was always very pleased with the overall fuel usage.

Driving the car locally and on short trips the car generally averaged 20 mpg. Driving from the West Midlands to London the car easily would achieve 38 mpg and on really long trips West Midlands to North Yorkshire 45 mpg was easily achieved.

As others have said what is the engine temperature showing on the gauge ?

 

[C270 CDI]

My gauge is the same as yours - just above the centre line. And I always put super unleaded in it, I don't know if that's 99 octane or not but whichever is the most expensive is what I put in it!

 

[geoffus]

Is it due a service, plugs, air filter etc ?

 

[MCFastybloke]

Its not always the car

 

[geoffus]

Also the temp gauge should get to 90 within 10 mins, if any longer it could be opening too soon, or even stuck slightly open.

 

[C270 CDI]

I know it's not always the car, but ive been driving 40 years and can usually get good mpg out of anything I'm in - like I said at 65 on cruise control I'd expect better.
The temp gauge is at 85-90 very quickly so I dont think the cooling system or thermostat are playing up.
It was just a question really to see if I've set my expectations a bit high.

 

[geoffus]

I know it's not always the car, but ive been driving 40 years and can usually get good mpg out of anything I'm in - like I said at 65 on cruise control I'd expect better.
The temp gauge is at 85-90 very quickly so I dont think the cooling system or thermostat are playing up.
It was just a question really to see if I've set my expectations a bit high.

Serviced recently, or don’t you think that’s relative ?
I never use cruise, prefer to coast more on downhill slopes and gently ease uphill.

 

[DSM10000]

My gauge is the same as yours - just above the centre line. And I always put super unleaded in it, I don't know if that's 99 octane or not but whichever is the most expensive is what I put in it!

Super unleaded is 97 Octane or higher. Tesco's Momentum is rated at 99 Octane and Super unleaded is also no more than 5% ethanol.

 

[C270 CDI]

Update on this, I drove home this morning at 4am on cruise, 66mph, and got 27.5. Back to work today, no cruise and driving nice and easy but at the same speed and got 30.4 (both numbers off the cars computer).
So maybe using cruise less will help.

 

[fabes]

That's not right.....

I use 95ron / normal on mine and with cruise set at 70/75 get high 30s to mid 40s.

You need it looked at and plugged in ASAP.

It could be binding brakes or a problematic injector, but the £80 or so inspection fee at a good local MB indie will be got back within a few months at 10-15 mpg under what it should be.
And, you don't need this hassle.

Whereabouts are you based?

 

[C270 CDI]

I'm in North Lincolnshire, not far from Grimsby. I use a guy who's ex Mercedes and he's done good stuff for me before so I'll get it booked in with him.
You're the second person to say they're getting significantly better mpg than me so my initial question has been answered - thanks for all the replies.
Oh and it's still under (a very good) warranty so if it's anything serious it might be covered - fingers crossed!

 

[fabes]

Do let us know as it all builds the knowledge base on here

And a handy Warranty eh....?
Good luck

 

[markjay]

I would start with the basics, a bottle of Redex/Forte/Millers fuel injector cleaner, or just a thankful of premium petrol e.g. Shell V-Power, possibly coupled with 'Italian tune-up', then see how it goes. Terraclean could ne next.