The auto cut-off works on back pressure doens’t it? I wonder if the intake could be designed to create more back pressure with petrol than diesel, and hence cut out the petrol pump?

Did it last week, and I mean really did it – £20-odd quids worth in before i noticed. The different sized nozzle thing works to an extent – but its only a one way thing, people are much less likely to put diesel into a petrol than the other way around.

Its the second time I’ve done it, just been tired and pre-occupied on both occations. The first time the the pump handle was green, but the advert on it was for mars bars, so to look at it was mostly black. The diesel nozzle next to it had a green advert on the handle.

This time round I’d been running around in a hire car the previous week, so had this little mantra in my head still reminding me not to use the diesel.

This would be a be a useful application for Murphy’s Law – His real law, not the one that people mis-quote. The story behind that is facinating incidentally and involves Murphy accidently putting a test pilot in a coma, but i digress. His law is “If there are more than one ways of doing something and one of those ways ends in catastophy then design that catastrophy out. In his case it wasn’t the fact that he nearly killed someone that was the catastrophy (although it was an accident), his job was to measure the effects of G-Force, and his sensors had been mounted back to front and had recorded no data. So although he’d nearly killed a man, he hadn’t found out how much force would nearly kill a man, the test pilot was strained in vain. But Murphy realised that it shouldn’t have been possible too fit the sensors back to front in the first place.

Anyway the three pin plug is a application of that law, you physically can’t get the plug in the socket the wrong way round, and you shouldn’t be able to physically get petrol into a diesel or vice versa, no matter how dumb you are. Most people getting it right most of the time isn’t good enough.

My BMW has a black filler cap and when you take it off the underside of it and the inside of the filler pipe is green

The car is diesel

Topped it up with diesal and no worries at all, I think if its t’other way round, youre fubar!

Denzils usually cope with a small amount of petrol no problems, but a large amount will totally trash a modern one. However petrols are more tolerant to the mistake – they may not run (almost definitely wont if its a heavy dose) but flush them out and they’ll work fine again without any costly replacements.

To be honest im not really sure how it works, i think i have read that there are two catches on the inside of the filler, because a petrol nozzle is smaller it can’t physically push both these catches at the same time, and until these catches are pushed in a flap is left in the way not allowing petrol into the tank.

Comes as standard on all bmws after 2007 afaik.

However petrols are more tolerant to the mistake

Seems a shame then that the current arrangement of small nozzles for petrol and larger for diesel protects the wrong cars

Putting diesel in a petrol car isn’t a problem – she’ll run, but it won’t sound very nice.
Putting petrol in a diesel and you’re in trouble…

Seems a shame then that the current arrangement of small nozzles for petrol and larger for diesel protects the wrong cars

I guess it’s just historic. Most cars were petrol, then diesels came along and became more popular in passenger cars so they had to produce a different noozle and couldn’t make a smaller diameter nozzle??

On a different note, my skipper helpfully put a 10 litre jerrican full of two stroke in my 205 GTi at the end of the racing season a few years ago. I took the risk and topped it up with half a tank of unleaded, and it carried on. The plugs even stayed clean. 😳

Seems a shame then that the current arrangement of small nozzles for petrol and larger for diesel protects the wrong cars
I guess it’s just historic. Most cars were petrol, then diesels came along and became more popular in passenger cars so they had to produce a different noozle and couldn’t make a smaller diameter nozzle??

The switch to smaller nozzles for Petrol is pretty recent though

They should switch to a combination of nozzle shapes, but that makes them more expensive to make (car and pump) and would need to also be retro-compatible, which would require narrowing of the nozzles I suspect, which in turn would mean reducing flow rate or increasing foaming in the tank as it fills. None of which is good. Ultimately it’s not their fault people are a bit forgetful.

Running a petrol engine on diesel can very quickly damage the engine, whereas running a diesel on petrol is far less likely to damage the engine (only time the engine would get damaged, is if the injector tips failed, but chances of that happening are slim). Fuel system failure is a risk on diesels, but the risk is greatly overplayed.

The reason for engine damage is diesel burns at a higher temperature than petrol, which the internals of a petrol engine aren’t designed to handle. Typical symptoms are melted/cracked piston crowns, catalyst damage, and cylinder head/valve cracking.

there are two catches on the inside of the filler, because a petrol nozzle is smaller it can’t physically push both these catches at the same time

Ford do the same. On the Focus at least, there’s a ring that has to be expanded by the pump nozzle in order to yield access to the tank; a nozzle that’s too small won’t release the opening (and one that’s too big won’t fit at all, of course). The car comes with a little funnel you can use to override it if you get a non-standard nozzle (ie, you’re overseas).