A funny thing happened.......

[sfh3l]

This doesn't worry me too much, but seemed a bt strange and I thought I'd ask you guys if you knew what the cause might be.

I stopped for a fill up today and as I was driving off, the MPG meter on the dash of my CSL went from (a highly commendable methinks) 27.4 to '0.0 mpg' before my very eyes, and stayed there with no reaction to the throttle. This seems very strange, as in over 300,000 E46 miles, I have never seen zero - even if you reset te meter to '--.- mpg' it then reads directly from the throttle usgae till there is enought o work out the average historic, so even if you reset while tramping the throttle you may get a very low number (particularly in the CSL!), but never zero.......

I've now reset it and it is behaving as I would expect, but I wonder if anyone has any clues as to the sudden change of heart with no input from me?

[shane@mbtech]

Originally Posted by sfh3l

This doesn't worry me too much, but seemed a bt strange and I thought I'd ask you guys if you knew what the cause might be.

I stopped for a fill up today and as I was driving off, the MPG meter on the dash of my CSL went from (a highly commendable methinks) 27.4 to '0.0 mpg' before my very eyes, and stayed there with no reaction to the throttle. This seems very strange, as in over 300,000 E46 miles, I have never seen zero - even if you reset te meter to '--.- mpg' it then reads directly from the throttle usgae till there is enought o work out the average historic, so even if you reset while tramping the throttle you may get a very low number (particularly in the CSL!), but never zero.......

I've now reset it and it is behaving as I would expect, but I wonder if anyone has any clues as to the sudden change of heart with no input from me?

It will take readings from many places, fuel tank level sensor being one
I reckon it was just a sensor malfunction in tank


Does your fuel gauge sometimes play up?
Was you fuel really really low before fuelling up?
Did you brim it?

My Porsche turbo did this once or twice

[_Nathan_]

I assumed it just worked off pulse width of the injectors to work out fuel injected and between speed and that worked out mpg?

[shane@mbtech]

Originally Posted by _Nathan_

I assumed it just worked off pulse width of the injectors to work out fuel injected and between speed and that worked out mpg?

It also calculates range on fuel remaining

[sfh3l]

Cheers Lawsy,

No, the gauge has always been rock steady.

The car had been very low about an hour earlier, but when I stopped for V-Power I was confronted by £1.42 per litre and only put a tenner's worth in on principle. I then got closer to home and found my local Tesco at £1.345 for their 99 Octane stuff and did brim the tank then, but then I always do brim it and never with that result.

My relatively simple understanding would be that it takes data from the inectors, as already suggested, and probably the speed sensors (to determine distance travelled) and that would be about it for the actual MPG - sure it needs the tank sender to work out the range, but purely for MPG there isn't much more it needs.

Makes me think it might have had some kind of a spike that has wiped it's memory, but then again why go to zero and not just start again like it does when you reset?

Fortunately MPG was not the reason I went for a CSL in the first place, but it sees strange all the same.