Diesel Fuel Conundrum

[billynibbles]

I've puzzled over this phenomenon, but I think I've cracked it. Whenever I lift my foot off the accelerator in my 2.0 HDi, the instantaneous mpg shoots off the scale, showing 999 mpg. However, if I coast instead (OK illegal, I know, so spare me the lecture), I 'only' get around 350 mpg with my foot off the pedal.

 

Would I be right in thinking that lifting your foot with the car in gear cuts off fuel completely, whereas coasting which is essentially idling on the move, forces the fuel system to maintain the idle?

 

If this is so, it maybe goes part way to explain the slight 'flutter' I get when using cruise control when the car commences a gentle downhill slope, say on a motorway, in 6th gear. This would make the system alternate between a light 'foot' on the pedal, and cutting off the fuel completely.

[CoolCarbon]

slight 'flutter' I get when using cruise control when the car commences a gentle downhill slope, say on a motorway, in 6th gear.

 

My 1.6 HDi EGS 6 speed does that too... :(

 

Strange thing is that my previous 1.6 HDi 5 speed manual didn't, it was always smooth..

[billynibbles]

My 1.6 HDi EGS 6 speed does that too... :(

 

Strange thing is that my previous 1.6 HDi 5 speed manual didn't, it was always smooth..

 

It could be something to do with the fact that (well, I assume it's a fact!) 6th gear gives you even less engine-braking than 5th

[Stuey]

when mine does 999mpg it is in gear and just rolling along, it tends to do it down hills when you dont need any throttle...I did mange to get 100mpg over a mile the other day when I filled it and reset everything. most of the trip was downhill so was reading 999mpg but 100mpg from a petrol is pretty good I reckon (its back down to 39 now though!! :(

[C4VTS]

I averaged 70mpg after refuelling and being stuck behind a Nissan Micra at 35/40mph for 20 miles on a NSL road.

Not bad for a VTS HDi 2.0. Usual max average is 40MPG.

[Stuey]

I averaged 70mpg after refuelling and being stuck behind a Nissan Micra at 35/40mph for 20 miles on a NSL road.

Not bad for a VTS HDi 2.0. Usual max average is 40MPG.

 

you do drive it like you stole it though mate!! :unsure:

[billynibbles]

The other 'good' way (if you can call it good!) to get excellent fuel consumption is to join in the '50 mph wall of traffic' on the M3 early on a Sunday evening!

 

However, my original question was - does lifting your foot whilst in gear cut the fuel completely? That would explain the 999mpg (since it would in fact be 'infinite mpg' at that point) compared to the figures shown if you coast out of gear, which still needs a squirt of fuel for idling.

[Rich_Eason]

To answer your question, no it wont cut the fuel completley by taking your foot off.

 

Your engine would splutter and die if the fuel cut out when you took your foot off. Unless you drive a Prius or something....

 

Where as you take it out of gear and coast this is the same as being stationary in the vehicle with the engine idling, the only difference is you are rolling and thus recording a MPH.

 

I think the confusion is being applied in calculation of the MPG as you are moving and the computer is factoring in the MPH.

[billynibbles]

To answer your question, no it wont cut the fuel completley by taking your foot off.

 

Your engine would splutter and die if the fuel cut out when you took your foot off. Unless you drive a Prius or something....

 

Where as you take it out of gear and coast this is the same as being stationary in the vehicle with the engine idling, the only difference is you are rolling and thus recording a MPH.

 

I think the confusion is being applied in calculation of the MPG as you are moving and the computer is factoring in the MPH.

 

Well, I take your meaning, but surely, the only two factors needed to measure mpg instantaneously are current speed and current rate of fuel usage.

 

Therefore, if as you say, lifting doesn't cut the fuel off, both coasting and just lifting your foot off on a downhill section ought to give the same mpg, but they don't. In gear it rockets to 999, whilst coasting it is more staid at around 256 or some such.

 

This tends to indicate that there's a difference in diesel fuel flow between the two methods, especially since coasting may indeed see you speed up if the hill is steep enough.

 

The only real difference would be that coasting would see the rpm drop, so if anything coasting ought to show the higher result but it doesn't. Indeed in my past two petrol cars with computers, it's been coasting that shows the higher result.

 

This being my first diesel is what lead me to my question about fuel cut-off.

 

Anyway, before I get accused of agonising over how many angels can sit on the head of a pin, I'll drop it, especially since coasting is illegal! Clearly, the fuel computer has a few anomalies of its own.

 

It would be interesting to see if you get the same with your petrol version.

 

p.s. Still at least we've got this thread back on course. For a moment there it looked like it was going to become another 'I followed someone wearing a trilby hat in an Austin 1300 with his mother in law in the back for 200 miles at 56 mph and got 73 mpg' effort! Just got back from Norfolk and the place is crawling with 'em.