MPG problem

[just_william]

Morning all
325i Touring 2003 Auto. Doing 18MPG. Wife drives is like Miss Daisy free wheeling down hills etc. Surely this cant be right. Doing 249miles to full tank. It is due a service but still sounds as sweet as a nut, no funny noises or smoke or smells??
Any thoughts appreciated
thanks

[Mr Ben]

MAF at a guess?

[Fox-]

just_william said:

Morning all
325i Touring 2003 Auto. Doing 18MPG. Wife drives is like Miss Daisy free wheeling down hills etc. Surely this cant be right. Doing 249miles to full tank. It is due a service but still sounds as sweet as a nut, no funny noises or smoke or smells??
Any thoughts appreciated
thanks

Does she mostly drive around town? I get about that driving quite normally around town etc in a 530i Touring Auto.

[Vee]

Has it just started to do this ?
My E46 330i Auto used to give me around 240-250 per tank and that was normal.

[halo34]

just_william said:

Morning all
325i Touring 2003 Auto. Doing 18MPG. Wife drives is like Miss Daisy free wheeling down hills etc. Surely this cant be right. Doing 249miles to full tank. It is due a service but still sounds as sweet as a nut, no funny noises or smoke or smells??
Any thoughts appreciated
thanks

How do you freewheel down a hill in an auto out of curiousity?

If its only started recently there may be a problem like MAF as suggested above.

[8Tech]

Doing short journeys could easily return that sort of mpg. Do you have a fuel computer on the car? If so reset it and then do a short drive from cold and check the consumption, you may well see less than 10 mpg for the first couple of miles. Then try a 50-60 mph cruise on the motorway from warm, resetting the computer and see if you can get around 30 mpg.

The main culprit would normally be the MAF if you do still get an awful mpg, ie less than 25 mpg on the steady cruise when warm.

[anonymous-user]

Vee said:

Has it just started to do this ?
My E46 330i Auto used to give me around 240-250 per tank and that was normal.

OK, that's blown my getting a 330 idea out of the water. Currently driving an E46 318 manual (M43, 1.9 8 valve) and get 420 miles before the light comes on (8 litres left). Think if I was brave enough to run it to empty I could get 470 to a tank.

[_Nathan_]

Stuck thermostat?

[OldSkoolRS]

_Nathan_ said:

Stuck thermostat?

Stuck wheel caliper (though I'd expect some signs of this such as heavy brake dust and possibly pulling to one side if on the front).

[Blue Oval84]

Sounds about right to me, my mam gets 21mpg from a Z4 2.5i Auto and has done since new.

I bet your wife does a fair few short journeys of less than 10 miles, mostly in town rather than entirely on a free flowing dual carriageway?

[Vee]

Joey Deacon said:

OK, that's blown my getting a 330 idea out of the water. Currently driving an E46 318 manual (M43, 1.9 8 valve) and get 420 miles before the light comes on (8 litres left). Think if I was brave enough to run it to empty I could get 470 to a tank.

Very dependent on the type of mileage. That figure was pretty much driving around London. A long motorway journey would get 380-400 with a little care.

[just_william]

Hi Sorry for late reply. Thanks for thoughts.
She does indeed do quite a few short journeys. You can knock an Auto into the neutral possition. Doesn't have a computer but a needle thingy, which only shows MPG at the exact time rather than an average, so figures quoted are calculated at fill up times.

It is a new car to us, so am suspicious that it maybe why the guy shifted it! It is due a sevice and it will be an inspec 2 service so will see what that brings up. Chap at garage suggested do that first and see what happens. Will then see if new plugs and filters and wheels off will show anything, and then ill go down the MAF route to see if things can be improved further.
Thanks

[Fox-]

It does have a computer - just cycle through the OBC options and there is an average MPG display. Reset it by holding the the OBC button on the stalk.

18mpg from short journeys in a 325i auto sounds about right to me.

[Luca Brasi]

Mine is exactly the same, they like a drink when doing this sort of thing.

[300bhp/ton]

just_william said:

You can knock an Auto into the neutral possition.

Am I understanding you right that you are putting the transmission into neutral while moving and then presumably back into drive, again while still moving???

Can't see how that is anything but bad for an auto tbh.

[Super Slo Mo]

300bhp/ton said:

just_william said:

You can knock an Auto into the neutral possition.

Am I understanding you right that you are putting the transmission into neutral while moving and then presumably back into drive, again while still moving???

Can't see how that is anything but bad for an auto tbh.

Indeed, you'd also be using less fuel by leaving it in gear when rolling downhill.

[pilchardthecat]

e46 autos kill the mpg - for example there 330d e46 manual is about 20-25% better on fuel than the auto

[just_william]

Interesting thoughts about the free wheeling. I think she has been watching me in my manual free wheeling home and wants the same. But surely it would use less fuel on tickover than being in gear going down hill? Appreciate that it prob not good for the box though. Will reprimand her tonight.... after she cooked the dinner though!!

[eastsider]

_Nathan_ said:

Stuck thermostat?

+1 check this. If it never warms up, or rather thinks it isn’t up to temperature, then mixture will be permanently richer = poor MPG. The thermostats and water pump are a known weak point, both can be changed for £120 ish by a decent indy.

[Super Slo Mo]

just_william said:

Interesting thoughts about the free wheeling. I think she has been watching me in my manual free wheeling home and wants the same. But surely it would use less fuel on tickover than being in gear going down hill? Appreciate that it prob not good for the box though. Will reprimand her tonight.... after she cooked the dinner though!!

No, on the contrary, it will use no fuel at all in gear going down hill, while on tickover it'll use a little. Admittedly, the difference probably isn't worth bothering about, but still, there will be a difference. It's because with modern fuel injected cars, they cut the fuel supply on the overrun (ie when the car is driving the engine, like when going downhill).