M44 Fuel questions...

[John_B]

Hi folks,

Because I am a sad git, I keep a spreadsheet for my cars recording when I fill up. Since I bought the car last year, it has always ran cool, yet I managed an average of 31.2mpg. I finally sorted the coolant system out last week and now it runs with the temp gauge bang-on the middle. I did a 210 mile trip at the weekend, the majority of which was a steady motorway cruise (with a little stint of trying to keep up with various other more powerful cars on country lanes in the middle), I filled up when I got home but was a little disappointed to see I only achieved 28.9mpg over the trip - is that about right for the M44 and what other owners are seeing? Or will the ECU now need some time to adjust to the correct operating temps? That works out at a 345 mile range per tank.

Another thing that has been bugging me is the fuel gauge - possibly due to the tank shape? I brimmed it before the trip and had done around 50 miles before the needle even moved off the stop, it then lost 3/4rs of the gauge over the next 160 miles, leaving me with 1/4 of the tank (indicated) remaining when I got home and filled back up. The fuel tank is 51L, right? And I filled up with 33L, meaning there should have been a third left. Does that all sound normal for the M44 Z3?

And final question - Should the low fuel light come on with the ignition like the overheat warning does? (I presume the red part of the fuel gauge lights up, same as the temp?) I haven't seen the light come on yet since I've had the car and I am developing a mild anxiety that it doesn't work

 

[Ianmc]

My M44 averages around 33mpg on local running, high 30s on a long run. Best was 42mpg on a run to Spain a couple of years ago.
Your fuel gauge acts the same as mine; low fuel light comes on with about two gallons left, say 9 litres (55 miles left on OBC).
The low fuel light comes on in key position two for a second or two, then goes off assuming you have fuel in the tank above the two gallons or so.
By the way, I always use 99 octane.
Hope this helps.
Ian

 

[John_B]

Thanks! I think that confirms my suspicion that the low fuel light bulb has blown in my cluster, I'll look to get that fixed.

It's a bit concerning my economy is so low by comparison. I'm doing 300 miles down to Cornwall later this week, so I'll behave on the throttle and see how get on. It's too late to fill it with the good stuff for the journey down, my local garage (Texaco, soon to be BP) currently only does standard grade petrol and diesel - still better than supermarket stuff though. I'll treat it to some premium while I am down there and before the coming home.

My clock (pre-facelift) just has the H & M buttons, I think that means I don't have an OBC (adds to the list of potential upgrades & retrofits)

 

[Ianmc]

I understand that the ECU can take a while to adjust things, learning from how the car is driven and the effectiveness of components.
Your run to Cornwall sounds just the thing.
Agree you seem to have the low level warning bulb out; an easy enough fix.

 

[Ianmc]

Picture of the prefacelift OBC for reference.

 

[Mike Fishwick]

The Z3 owners manual tells us that while use of 95 octane in an M44 will produce the rated 140 bhp, the engine is optimised for use of 98 octane, which will improve performance by about 10%. Of course, Teutonic 98 octane of the mid-nineties was far bettetr than any of the claimed super fuel such as V-Power etc which are available today, and an excess of ethanol will further degrade the fuel. The last time I drove in the UK (thankfully a long time ago!) I found that V-Power gave 25-28 mpg at 70 mph on a motorway, while Esso 97 octane gave about 38-40 mpg, It is the same in France, where local supermarket fuel gives 35-36 at an 80 mph cruise, whereas V-Power gives 25-28 mpg,

Try different fuels, and calculate your consumption from tank full to tank full, ideally using the same pump. Of course, driving style can have a big effect on consumption.

 

[John_B]

Thanks Mike. Interesting about the difference you've seen between V-Power and Esso 97. Out of dumb luck I think I have covered most of the big brands on my travels and premium Esso is the one I haven't got an accurate number for yet, but . I just did 808 miles to Cornwall and back and found the following:

Fuel	Miles Covered	p per litre	MPG	Driving style
Texaco/BP 'standard'	241	147.9	29.1	Motorway (70-75) - mix of roof up/down
BP Ultimate (97)	242	189.9	29.8	Motorway & a bit of local running around - roof down
Shell V-Power (??)	195	181.9	31.8	Motorway (70-75) - roof down
Esso Synergy Supreme+ (??)	130	182.9	30.0	Motorway (70-75) - roof down

On a related note, Andy Spurs fan has sorted me out with an OBC, so I will install that in the coming weeks and see what that says!

(Edited to add calculated MPG with Esso)

 

[Mike Fishwick]

I regard the fuel gauge as a rough indication, which is about right at 25% increments. After about 65 years of motorcycling I automatically calculate the fuel consumption from full to full. I have probably been spoilt by the quality of european petrol, and the ready availability of good 98 octane at any supermarket.

I am not sure whether or not a used OBC has to be coded to the car - worth checking first. Remember that the fuel consumption readout can be adjusted to almost coincide with the calculated mpg figure - I think I put the procedure in the 'Articles' section.