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Sudden Massive drop in MPG

Guy.Pap

Guy.Pap

Active Member
Joined
Jan 25, 2015
Messages
117
Location
Rugby
Car
E280 Sport Wagon
I have a E280 Sport S211 2008 with 75500 miles on it, and love the car to bits. The car was remapped by MSL early in 2015 and the EGR was mapped out also the cat was removed the car drove lovely.

The car was treated to a new turbo and control unit last September (2016) and a new N.S.R ABS ring at the beginning of January 20017. My fuel consumption has always been around the 33MPG mark mainly due to the short journeys to and from work on a run it used to give and indicated 38+mpg, well that's what it used to show. Now it shows on a run no more than 26.5MPG and that is after a trip of 50+ miles at a constant 60mph and the trip being reset whilst at that speed. The counter quickly counts down to this figure regardless of what my foot is doing and the combined MPG is showing 24.5MPG, so I'm about 25% down overall. This was done brim to brim to confirm.

I have no codes showing as every thing was reset after it lit up like a Christmas tree when the ABS ring went, and then it was only faults relating to the ring ESP/Cruise/Adaptive steering/ABS etc nothing relating to the engine. This was checked again today There are no warning lights on the dash just an alarming drop on the fuel gauge.

The air filters & Fuel filter were replaced 12 months ago and I have only covered about 9000 miles since.

The motor does not seem to be lacking in power in any way.

I have checked that I am not towing my house around behind me :D but I am at a loss as to what could be causing this drop :dk:

Anybody suffered a similar drop and what did you do to fix it....
 
I had a similar massive drop on my C350. I simply reset the ECU by disconnecting the battery then not touching anything for 10 minutes, then reconnecting the battery. MPG returned to normal. I can't explain it, it just worked.
 
It seems to me there could be three things happening:

- your mpg really has dropped, or
- your car is overreading how much fuel is used, or
- your car is underreading how far you've driven

When you did the brim to brim check, you know how much fuel in real life, but did you take the mileage reading from the dash, or from how far you'd really driven?

The fuel and distance are electronically measured, so the reset knighterrant suggests might work.
 
Mine will only be 24mpg from cold start and doing short trips. After that and say iam on the motorway it takes an age to get upto 38ish mpg but if you reset it whist driving it will default to 40.5mpg then creep up if doing 70mph on motorway. Best I've seen was 46mpg iirc and mine is msl mapped but that's all. Put it this way it's got a better mpg than my w202 c230K so iam happy oh and a hell of a lot faster
 
Have you checked fuel lines / tank for leaks?
Could be a faulty MAF over fueling - does tailpipe smell of petrol?
 
knighterrant said:
I had a similar massive drop on my C350. I simply reset the ECU by disconnecting the battery then not touching anything for 10 minutes, then reconnecting the battery. MPG returned to normal. I can't explain it, it just worked.
Click to expand...

I gave it try yesterday, unfortunately it really hasn't made any difference but defiantly worth a ago

streethawk said:
if you reset it whist driving it will default to 40.5mpg then creep up if doing 70mph on motorway. Best I've seen was 46mpg
Click to expand...

Yes mine defaults to 40.5 but then just heads south down to the mid 20's :confused:

Flyinspanner said:
Have you checked fuel lines / tank for leaks?
Could be a faulty MAF over fueling - does tailpipe smell of petrol?
Click to expand...

Tail pipes are really sooty and do smell of Diesel No leaks from any fuel pipes or tank. I will take off the MAF sensors and give them a good clean with Servisol Super 10 which has worked before on other cars I have owned..
 
In cold weather 211s have an auxiliary heater that is electrical and huge. This causes a massive increase in cold start consumption. That combined with winter diesel makes for a big wandering in fuel consumption. Also the thicker fluids in winter don't help.
 
My MPG have gone much lower (400 miles a tank to 320) since getting colder. Perhaps it will be ok again when it gets warmer?
 
streethawk said:
Mine will only be 24mpg from cold start and doing short trips. After that and say iam on the motorway it takes an age to get upto 38ish mpg but if you reset it whist driving it will default to 40.5mpg then creep up if doing 70mph on motorway. Best I've seen was 46mpg iirc and mine is msl mapped but that's all. Put it this way it's got a better mpg than my w202 c230K so iam happy oh and a hell of a lot faster
Click to expand...

If you drove 12 miles at 24mpg then you have used half a gallon of fuel.

To then get to 38mpg average you would have to achieve 42.66mpg over the next 64 miles. Or a higher mpg to get there in fewer miles.
 
Winter temperatures, winter diesel will affect it greatly, especially if your thermostat is ticking at all.

Does it get to 90ºc within 6-10 miles and sit there rock solid?

Thermostat is the most likely cause, so test that first.
Today is a perfect day to test with fairly mild weather, just drive 10 miles, make sure it gets to 92ºc within that time and make sure if you then put your foot down it doesn't drop...at all.
 
Back in the days when this forum was blessed with diesel/Merc technology experts, I was educated that remaps can lead to over-exaggerated fuel economy.

This is due to something like the MPG algorithm counting 'pulses' of fuel over distance, and certainly after a 'chip' or even a remap (so I was told), the quantity of fuel in the 'pulse' is increased, so the MPG calcs are then based on flawed data, thus everyone thinks their remap has done wonders for their economy, but of course they are deluding themselves.

Could it be that the remap has actually been overwritten (due to reset/service etc.) and you are back to original?

The drop you think you have experienced never really happened.

Of course if you did brim to brim before, then maybe this theory is cr4p!

Also, have you changed the rolling diameter of your wheels? This will affect you distance recorder, hence MPG.
 
It is a 9 year old car, my bet is thermostats.

They need changing every 6 or so years from experience, and if they stick then your car thinks it needs to over fuel, it is a modern day choke, and until up to temperature it uses between 20 and 40% more fuel.
Oh, and if it is not getting to temperature you DPF can't do a regen, so this gets blocked and reduces MPG as well.
 
Temperature has been checked and sits at a constant 89-90 deg so I don't think it is thermostat related, although I will not rule it out.

The fuel calculations are backed up by my brim to brim recordings as I have recorded it ever since I purchased the car over 45 entries. this drop of about 20% is over the last three tank full's of fuel (V-power) wheels and tyres are stock Conti M/O 265/35x18 & 245/40x18 all replaced in September.

The car is not fitted with a DPF so no worries there..

The daft thing is the car is driving extremely well possibly feels even a little bit quicker, but that may be just because the turbo was replaced last year and it likes the charge of cold air during the winter months.....

I have a day off tomorrow so I will try and clean the MAF sensors and also fit a couple of new air filters and fuel filter, might as well as europarts have a 30% extra discount until the end of Jan
 
Last edited:
A little bit of an update on this..

Yesterday I had some free time and I took out both MAF sensors and gave them a good clean with some Servisol Super 10, whilst I was at it I checked out the air filters which seemed in good order. the whole assembly was refitted the car started with no issues or warning lights etc so I left the car alone until this morning. It started fine and a started the journey to work I drove nice and steady just the same as I had done last week when I was getting an indicated 25.6mpg when I arrived at work. Today it read 32.1mpg.
Yes I know that was just one journey and full results wont be know until I have used at least a couple of tanks of fuel but the early results are promising.
Fingers crossed :thumb:
 
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