Massive drop in mpg

[Sector-9]

Ok, so I used to get an indicated 60mpg+ (whether that's accurate is irrelevant) doing a steady 50mph on cruise up and down the M1. In the last few months however I've been struggling to get much more than 50mpg indicated. It also feels rather flat and dull though foot to the floor isn't so bad if you let it rev. My question is: what things have the biggest effect on the diesel's economy?

The brakes don't seem to be binding
Tyre pressures are good
EGR vac pipe disconnected
Engine temp is between 85-87degC
New intercooler O rings fitted recently and Torque shows boost pressure reaching 17psi
Air and PCV filters replaced beginning of the year

My gut feeling is MAF related or some kind of leak, though there is no smell of diesel fuel. Car has always been smokey on acceleration and doesn't appear any worse now, but always starts fine and injector correction values were declared good by BigRuss a while back so I don't think it's excessive leakback.

[FrenchMike]

Lot of people would like to get 50 mpg (including me)...:shrug:

[Amsca2]

Take the roof bars back off. :D seriously though I get 36 mpg. Turns out driving style plays a big part.

[KeithA]

Clean air filter, pcv filter, run some redex etc through, restrain your right foot if required!

Sent from my LG-D855 using Tapatalk

[stevenicks]

50 MPG!! I wouldn't worry or touch a thing :drool4:

[hotrdjon]

My 75 cdt auto averages 47 - 51 mpg unless I really boot it when it drops to 42 mpg. If you reckon you get 60 mpg you must be driving round at no more than 45 mph.

[andymc]

As KeithA says, replacing the PCV filter and air filter may be useful, ditto a dose of injector cleaner. Even if you don't smell diesel, check the fuel cooler behind the OS rear wheel for drips. Also, would the undertray be catching any leak?
However it does sound like the MAF might be the culprit - even if you reckon you're still getting 50mpg, which a lot of people would be happy with, obviously you've noticed a drop in performance and economy. Dunno whether TOAF is something that can help you narrow that down for yourself, if access to T4 is limited.

[clf]

Ok, so I used to get an indicated 60mpg+ (whether that's accurate is irrelevant) doing a steady 50mph on cruise up and down the M1. In the last few months however I've been struggling to get much more than 50mpg indicated. It also feels rather flat and dull though foot to the floor isn't so bad if you let it rev. My question is: what things have the biggest effect on the diesel's economy?

The brakes don't seem to be binding
Tyre pressures are good
EGR vac pipe disconnected
Engine temp is between 85-87degC
New intercooler O rings fitted recently and Torque shows boost pressure reaching 17psi

My gut feeling is MAF related or some kind of leak, though there is no smell of diesel fuel. Car has always been smokey on acceleration and doesn't appear any worse now, but always starts fine and injector correction values were declared good by BigRuss a while back so I don't think it's excessive leakback.

When was the last time you reset the IPK? To obtain that kind of figure, I have to reset it at the beginning of a constant speed run. Which I will get, but it drops over a period of time of 'regular driving'

[klarzy]

Check the intercooler rings and top hose for swelling and porosity..
Also the egr flexi for small leaks that would reduce turbo boost.

[Sector-9]

Ok, just as an update, I've done my fortnightly round trip to Leeds again tonight and sat at 50mph (digital speedo indication) and averaged an indicated 62.7mpg home - this was after resetting the fuel computer whilst on the motorway near Sheffield and on cruise. I think that last time I went, because I was later leaving and because they'd closed parts of both the M1 and A1, I'd set the cruise to 60mph instead, and was getting low fifties, and then high forties at 70mph. Doing some local driving brought the economy down to low forties which seems to be the consensus among those who use their 75's as their daily driver, so probably about right.

To surmise, it looks like 50mph on cruise is the best speed for economy, though I expect few would want to slow down by that much. Perhaps somebody else would like to corroborate by doing a reasonable distance at the same speed, resetting the computer and seeing what kind of average you've achieved when you get off?

[Union Wheels]

Hi Sector-9,
you mention in your first post that you checked the boost with "torque app". If you suspect the MAF why not check it with "torque", values have been posted before.

Here are mine:-

780rpm = 12.50
1000rpm = 16.90
1500rpm = 25.60
2000rpm = 36.50
2500rpm = 49.00
3000rpm = 65.00

Cheers, Pete.

[Sector-9]

I was aware a few members have/used Torque, but didn't realise any MAF figures from it had been posted - many thanks for this. :bowdown:

[hogweed]

Lot of people would like to get 50 mpg (including me)...:shrug:

And me...

Oh dear, that message is too short to be allowed to post, blah blah blah...

I do wonder about the mpg claims that are often made here. Best I can get is maybe 45, driving under ideal conditions - T4 doesn't reckon there's anything wrong with my car... either there is regardless, or we have fantasists on the forum...

[klarzy]

And me...

Oh dear, that message is too short to be allowed to post, blah blah blah...

I do wonder about the mpg claims that are often made here. Best I can get is maybe 45, driving under ideal conditions - T4 doesn't reckon there's anything wrong with my car... either there is regardless, or we have fantasists on the forum...

Have had High 40's in my auto on a brim to brim with a tour of Cumbria... 600M mostly motorway...

[hogweed]

Have had High 40's in my auto on a brim to brim with a tour of Cumbria... 600M mostly motorway...

Wow - and on an auto too... must be me then!