Help MPG Shocking on W219 320CDI

[Jordan24]

Hello

so i hadn't driven the car properly in a while as couldn't get it to pass an MOT, before i stopped driving it the car could get anywhere from 29-38MPG depending on city driving or on the motorway the car had a new DPF, new spring and a propshaft bearing the car is now getting about 19-22 city driving and I'm lucky if I get 27-28mpg and that's driving with a very light foot the only thing that seems to come up on a reader is glow plug in cylinder 1 needs replacing which its getting replaced soon but I'm wondering what could it be surely the glow plug wouldn't cause that much if any MPG loss and the car seems to drive perfectly fine? I think I heard a squeaking noise from a belt? today from something but have no idea what it was it was very faint and seems to be a new sound I have had a look and all the fuel lines are bone dry no misting or anything round them.

Thank you in advance

 

[Trickythemerc]

Who fitted the new DPF is it genuine or after market ? You need to tell the ECU that the DPF is new, usually done via Star diagnostic session.

 

[DSM10000]

It is winter

Diesel engines take much longer to warm up than petrol engines as they are more likely to thermodynamically efficient.

What is the engine coolant temperature afte running for 15 minutes or so at 50 to 60 mph?
It may be that the thermostat is stuck open and the engine is over cooling.

 

[Jordan24]

it was fitted by a local mechanic and was a warranty change from BM Catalyst as the old DPF failed within 2 years i don't recall the ECU being told it was a new DPF when the first one was fitted but that could be why it failed so quickly? although the MPG was better with the old DPF.

 

[Jordan24]

coolant temp is roughly 1 or 2 bars above the 80 mark pretty much within 10 minutes and stays there, sorry if it sounds stupid but the screen that tells you the outdoor temp that's stuck on -40c but i don't know if that changes how the engine runs.

 

[W1ghty]

coolant temp is roughly 1 or 2 bars above the 80 mark pretty much within 10 minutes and stays there, sorry if it sounds stupid but the screen that tells you the outdoor temp that's stuck on -40c but i don't know if that changes how the engine runs.

Get on the motorway at a steady speed of 70mph . Reset the mpg trip thingy when the car is up to temperature. Drive for 20 minutes.
If the car isn’t up near 40mpg you may have a problem .
You can’t judge anything in the city stop start traffic in the winter .
Get the outside temp gauge fixed as well

 

[Jordan24]

Turns out the fuel injection pump was leaking so had that replaced along with the belt had it on a 200 mile motorway run and still only returned 33mpg

 

[BlackC55]

Thats perfectly normal MPG in the winter. My V6 diesel truck only gets 23mpg on a good day and its only done 23k.

 

[W1ghty]

Turns out the fuel injection pump was leaking so had that replaced along with the belt had it on a 200 mile motorway run and still only returned 33mpg

That’s way lower than my E with the same engine , I would be early 40’s easily on that distance . Weird that the CLS is so much worse than the E , maybe heavier with bigger wheels ?
I’ve got 16” winters at the moment

 

[Larkone]

Getting mid 30s this time of year on my CLS OM642 (18" wheels) for 40-50 mile runs - down from summer when it is 40.