[QLD]

This is a weird one.... so weird I'm not sure if Im imagining it or if its real...!!

But I have monitored it for the last 3 tank fill ups... anyone heard of something similar, or of what could be causing it??

Fill the car up with BP Ultimate Diesel - (always BP).... car is super responsive and 'toey'. Accelleration smooth and instant.

Car gets to quarter of a tank and car seems LESS responsive and simply not as 'toey' or sensitive to 'instant' accelleration... its weird (and annoying).

The car does NOT spit, splutter or anything - it just seems like I have to push the pedal further to get the car to start pulling, and when it does pull, it doesnt feel the same smoothness of when the tank is full.

Anyone??



-----------------

Quick update... the topic is not about less POWER - of course the car has the same power regardless of the fuel level... the noticable issue is "throttle responsiveness" at low diesel fuel levels in the tank..

[Bungle009]

it should get marginally more responsive as the fuel reduces. 60 litres weighs quite a bit...

[distortionist]

I thought that too, I took for a run flooring it before gas and after gas, and It still felt the same, maybe its because its performing better, but more fule to lug, ill try it when its 1/2 full.


I defenately do think it conserves gas when running low, but I need more tests, next time ill take her for a run when shes empty and get 1/2 a tank and see if i feel more power.

Whatever it is, my cars performance is better some times than other times, im trying to figure out why.

[kay-singh]

I Have a 330D, and i feel the same about this, when it low (under a quatre) its less responsive, and less rev happy! thought it was just me! but after reading this it might not be...

I also use bp ultimate diesel, as there is no local shell... strange!

[HighlandPete]

Years ago we could have this sort of issue when the fuel/lift pump was often more efficient on a full tank, didn't have to lift the fuel so much. but I wouldn't have thought pump performance would have the same influence today. But never rule anything out. Fuel filter doen't need changing does it? That could possibly be restricting the primary pump's performance, just noticeable as the fuel level drops in the tank..

HighlandPete

[BanziBarn]

The only thing I notice is that on a full tank the car responds well because the fuel is cold....

[kay-singh]

Originally Posted by HighlandPete

Years ago we could have this sort of issue when the fuel/lift pump was often more efficient on a full tank, didn't have to lift the fuel so much. but I wouldn't have thought pump performance would have the same influence today. But never rule anything out. Fuel filter doen't need changing does it? That could possibly be restricting the primary pump's performance, just noticeable as the fuel level drops in the tank..

HighlandPete

Don't think its the filter.. my cars only done 2500 miles...

Interesting thought on the fuel pump though!

[335iLondon]

Originally Posted by BanziBarn

The only thing I notice is that on a full tank the car responds well because the fuel is cold....

The best post yet in this thread.
Any confirm this is science?

[HighlandPete]

Originally Posted by 335iLondon

The best post yet in this thread.
Any confirm this is science?

Colder fuel. Wouldn't that be the same for a part tank, on a colder day?

Without a very serious and tested technical answer, I can't see the fuel's base temperature being of any relevance, certainly won't stay cold once compressed and injected. Plus we have diesel fuel heaters for warming fuel, if it is cold anyway.

HighlandPete

[QLD]

Given that diesels are reknown for their eco benefits and extremely good fuel efficiency.. seems the greater majority of folks believe (where this has been discussed) that it is a engine management restriction/feature.

Car recognises that it has less than half a tank of juice so tries to deliver even greater fuel saving performance by somehow 'governing' diesel flow... making it less responsive to normal throttle pressing.

With a full tank (or near full tank)... the car ecu focuses on delivering maximum power and performance..

Seems this is unique to diesels and NOT to BMW petrol vehicles.

[HighlandPete]

Originally Posted by QLD

Given that diesels are reknown for their eco benefits and extremely good fuel efficiency.. seems the greater majority of folks believe (where this has been discussed) that it is a engine management restriction/feature.

Car recognises that it has less than half a tank of juice so tries to deliver even greater fuel saving performance by somehow 'governing' diesel flow... making it less responsive to normal throttle pressing.

With a full tank (or near full tank)... the car ecu focuses on delivering maximum power and performance..

Seems this is unique to diesels and NOT to BMW petrol vehicles.

That makes no sense at all to me. Why restrict the performance just because the tank is less than full?????

HighlandPete

[rjs]

You might need performance when you have low fuel. There's a kid at work who was coming back from the cinema down the M42. He was running low on fuel & his great logic was to speed up - that way he could get back sooner before the fuel ran out!

[bodgerx]

Originally Posted by QLD

Given that diesels are reknown for their eco benefits and extremely good fuel efficiency.. seems the greater majority of folks believe (where this has been discussed) that it is a engine management restriction/feature.

Car recognises that it has less than half a tank of juice so tries to deliver even greater fuel saving performance by somehow 'governing' diesel flow... making it less responsive to normal throttle pressing.

With a full tank (or near full tank)... the car ecu focuses on delivering maximum power and performance..

Seems this is unique to diesels and NOT to BMW petrol vehicles.

I doubt the ECU adapts the power level according to fuel level.

More likely indicates an issue with your fuel pump/filter. The pump is partially cooled by fuel I believe, so it maybe running more inefficiently at very low fuel levels.

Eco benefits of Diesel? Other than economy what are they then?

More energy is used to produce a barrel of Diesel than Petrol.
More particulates in Diesel.

[dxb335d]

Originally Posted by QLD

Given that diesels are reknown for their eco benefits and extremely good fuel efficiency.. seems the greater majority of folks believe (where this has been discussed) that it is a engine management restriction/feature.

Car recognises that it has less than half a tank of juice so tries to deliver even greater fuel saving performance by somehow 'governing' diesel flow... making it less responsive to normal throttle pressing.

With a full tank (or near full tank)... the car ecu focuses on delivering maximum power and performance..

Seems this is unique to diesels and NOT to BMW petrol vehicles.

you have a vivid imagination. It's all in your head

There is no such feature!!

[QLD]

Can you post a link to that fact?

Thats what we are all after - definitive proof, sadly I or others who discuss this cant find any, yet we all have noticed the exact same behaviour.

Luckly for us you are very certain - please post your link to the info that confirms this is not an ecu feature on diesel BMWs.

Thx

[Imran@Evolve]

In my experience on the dyno the BMW diesels put the same range of power down regardless on how much fuel is in the car.

Haven't done any scientific test - just talking from experience of dyno's lots of BMW diesels.

I'm with Carl on this one!

[QLD]

Nice, thx I appreciate the reply

For the record though, if you reread my original post I said the issue seems to be 'responsiveness'.... not reduced power.

On a dyno I would expect everything to be equal - a car with the throttle slammed to the floor, screaming to the redline - will produce the same power stats whether it is full or near empty on juice.

My point was (and from other diesel owners) - that the car seems 'less responsive'.... the same throttle pressing at normal driving doesnt seem to produce the same 'sharp responsiveness' as when the tank is full.

Drive agressively... punch the pedal to the floor and there is no issue... but for normal driving, with the same throttle amount of throttle pressing around town - with less fuel it seems "less sharp"... less 'toey'.

I know its hard to comprehend... it is almost impossible to describe - you have to own one and experience it to know what I mean... but it is real - and reported by other diesel owners.

Very curious indeed..

[dape90]

I was not going to get drawn into this but seeing as I only had a quarter of a tank left on my way home last night I thought "let’s try it out and see".

Now here comes the surprising bit.

There is a difference between a full tank and half a tank in regards to feel or responsiveness. However it is only noticeable (which is slight to say the least) when on half throttle. Give it the beans and there is no difference but I could feel more responsiveness with a full tank compared to a quarter tank of fuel.

Had I not read this thread would I have noticed it?
Maybe, only had the car 5 months and still getting to know its querks.

Could it be down to the fresh fuel affect?
Possibly! If petrol can degrade in a tank then I am sure diesel can. It stands to reason that diesels do more MPG than a petrol requiring less fill ups which also means you have older fuel sitting in the tank for longer!
If I had to contribute this to anything then I would say it is fuel degradation and very unlikely to be some kind of BMW economy map on the car.

Anyway, that’s my thoughts.

Cheers

D

[shane55]

I remember I had my car on 4 miles range and really thought I was going to run out of diesel...but I didnt, What I will say is I noticed that the shift up points of the auto box were very low and on the same stretch of road that I travel on every day the auto box was in 5th...and it usually stays in 4th on that 50 kph stretch, there are speed ramps on it pretty close together and I noticed how quickly the car wanted to be in a higher gear? bizzare?

[QLD]

Originally Posted by dape90

There is a difference between a full tank and half a tank in regards to feel or responsiveness.

Just an update on this... when the car begins to be less responsive like several people now have confirmed - do this...

Unscrew Fuel Cap... and remove..... now put the cap back on. Problem fixed!!

Car is back to being responsive again




.

[Dr Dave]

This is also widely known as "The placebo effect"

[HighlandPete]

Originally Posted by QLD

Just an update on this... when the car begins to be less responsive like several people now have confirmed - do this...

Unscrew Fuel Cap... and remove..... now put the cap back on. Problem fixed!!.

Was there a partial vacuum when you unscrewed the cap?

HighlandPete