leave it running!!! - Dave M
A friend of mine has told me that a tdi uses an incredible amount of fuel during start up and if you are only planning to leave the car for 10-15 minutes its cheaper to leave it running! Can you tell me if there is any truth in this? the car in question is an R reg Audi A4 1.9 TDI
Thanks in advance Dave
Re: leave it running!!! - Andy
Dave - I don't know whether that's true, but it sounds pretty bad from a pollution point of view.
Diesels have a very high 'particulate' output (far more than petrol) and I was horrified the other day when I went for a walk at a local beauty spot. When I arrived, there was this guy sat in his car, doing some paperwork with the engine running. After my walk about half an hour later, he was still there, engine still running. The car park area was hazy with fumes, and a strong smell of soot filled the air. What a plonker.
Re: leave it running!!! - Stuart B
Andy wrote:
> Diesels have a very high 'particulate' output (far more than
> petrol)

Depends which particulate size range you are measuring, in fact the ones which are thought to be the worst for health ie the very small ones are just as bad with petrol, in fact GDI petrol are the worst.

Do not just believe all the anti diesel propaganda that is pushed out, you have to take note of the facts.
Re: leave it running!!! - Dave M
Thanks everybody i agree this is a long running argument between me and the infamous Dave Lacey if you would all now be so kind as to email him and tell him to switch off his rattly noisey smelly old diesel car it would be appreciated
thanks Dave
Stop vs Leave it running - David Lacey
I tend to leave my car running when stopped (a) The air conditioning will not work when the engine is shut down (b) Starter motor wear (c) If anybody approaches from behind at high speed and cannot stop, I can take some form of evasive action to avoid an impact. (BTW, I have used this to some success in the past)


Re: Stop vs Leave it running - Dave M
i can see that this isnt going to stop this row,
dave ok starter motor wear umm well i cant argue with that
air conditioning: why do you need air conditioning when you are sat in my lounge?in fact if you turned off your old diesel rattler it would be more comfortable for even you because you could hear my telly
Evasive action?? what are you going to do in my sofa if someone careers towards the back of your car also id be more worried about the fate of my house as its much bigger and more likely to get hit by a lunatic on my driveway
back to you dave
Re: leave it running!!! - Cliff Pope
Interesting, but it doesn't answer Dave's question.
Presumably every car has a 'don't turn off before' time.
For some it might only be a matter of seconds, for others 10 minutes or longer. Has anyone any figures?

Cliff Pope
Idling fuel consumption - Stuart B
Its a fair point you make there Cliff.

On my 2.0 Ecotec DTI the instant fuel consumption meter which is seems to be measuring fuel flow from the impulses or whatever from the engine management computer and seems to tally with brim>brim calcs shows at idling either 0.1 or 0.2 gall per hour.

If I switch absolutely every bit of electrical gear on, eg aircon, heated seats/ rear window/ wipers on fast etc, sometimes I can get it to read 0.3 gph.

Now assuming the 0.2 gph figure is typical and accurate, which I admit is a big assumption, then 10 minutes at this is roughly 0.03 recurring gallons or 150 millilitres. This seems a massive amount of fuel to use just on start up and I would guess the "breakeven" figure is well less than a minute.

It must be remembered that this is hot start consumption and not cold start. But even so with diesels you do not get an enrichening of the mixture as with petrol, the timing is sometimes slightly changed.

Plus what are you going to doing in this 10-15 minutes? Not leaving the car with the engine running & keys in surely! Invalidated insurance in case of theft, plus illegal on public road, unattended and engine running.
Re: leave it running!!! - Ian Cook
Hey, guys - as well as considering emissions, what about noise?

I think it's anti social to leave a vehicle running when it's not necessary. I don't know why, but drivers of diesel vehicles seem to do this a lot. I drive diesel and switch off, when stationary, wherever possible.
Re: leave it running!!! - mike harvey
Leave it running. Where do you live, I need a new car? The insurance Co. will not pay out for theft either, as reasonable care has not been taken.
Dodgy Mike
Re: leave it running!!! - Cliff Pope
I wasn't implying that one should leave vehicles running unattended, simply that there is a period of time in which everyone leaves a car running while waiting inside, eg minor hold-ups in traffic, lights, etc. This period must vary from car to car, and it would be useful to know. Should one turn off at red traffic lights, level crossings, etc etc?

There is also the cost of wear and tear on the starter motor to consider. All a starter motor does is start the engine, so in town driving it would be easy to halve its life.
As for noise, a petrol engine is a lot quieter idling than starting up.

Cliff Pope
Re: leave it running!!! - Andrew Smith
The recent batch of hybrid cars switch the engine of the moment you come to a stop. So I would guess there is an advantage almost imediately.
However it is probably better to leave it running over a short period of time for wear and tear reasons.
Re: Stop vs Leave it running - Bob Harris
VW produced a Polo (Formula E?) some 15 years ago. Amongst other economy measures - higher gearing etc - it cut off the engine a couple of seconds after each stop. I believe it was when you selected neutral and restarted when you moved it back into gear.

I seem to remember a motor magazine conducting some back to back tests with a 'normal' polo and concluding that the savings were marginal.
Re: Stop vs Leave it running - Alvin Booth
I cannot see how you would use more fuel to any extent in re-starting a diesel engine when it is hot,
The injectors will only open by pressure for a predetermined time when restarted and within a fraction of a second it would be using the same amount of fuel as if it were ticking over.
A diesel engine uses absolutely no fuel on the overrun so your friends idea Dave would also suggest that coming off the overrun and pressing the accelerator would use 10/15 minutes of fuel immediately, if you get my meaning. no truth in this at all Dave.

Re: Stop vs Leave it running - Dave M
thankyou all who replied it was a serious question dave is pretty hot on his car trivia so i wanted to be sure of my ground before i fell about to much at his never switch his car off logic
thanks again
Re: Stop vs Leave it running - peter todd
what about the chance that one or more injector nozzles may not be atomizing correctly which will wash oil film from the bore/s & wear the rings while it's sat there ticking over- I'd turn it off if it had at turbo as well- letting the shaft bearings rotate at a low rpm with the lowest pressure ain't to clever either
Re: Stop vs Leave it running - John Kenyon
peter todd wrote:

< >
> I'd turn it off if it had at turbo as well- letting the shaft
> bearings rotate at a low rpm with the lowest pressure ain't
> to clever either

Rubbish! For a properly maintained engine (=regular oil & filter
change), oil pressure at idle will be sufficient to maintain lubrication
of the turbo.

However someone who's just given their car a good caning/italian tuneup
who turns off their turbo-engine without a cool down, whenever they stop is going to screw their turbo by coating the bearings with overheated oil.


Value my car