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[NinjaBoiX]

I’m in the UK and this ridiculous panic buying of fuel has got me thinking something I’ve never really considered before; is it bad for your engine to keep your tank topped up?

Like most substances petrol and diesel gradually deteriorate over time, the common school of thought is that fuel is only really at its highest octane level for 6 months or so. So:

Let’s say for arguments sake, you use about half a tank of fuel a month, then brim the tank. Over time you’ll have a sizeable percentage of old fuel sloshing about in your tank that is basically just useless liquid diluting the rest of it after a couple of years. This must be bad for your engine surely?

I would always keep my car pretty topped up, but now I’m thinking it’s probably sensible to run your tank pretty much dry every couple of months to refresh the contents.

Anyway, thoughts?

 

[*Nightwing]

“Running water never goes stale”

if you top up regularly it isn’t an issue, if you leave it months in between without using you have to condition the fuel system periodically due to those facts you point out.

 

[NinjaBoiX]

“Running water never goes stale”

if you top up regularly it isn’t an issue, if you leave it months in between without using you have to condition the fuel system periodically due to those facts you point out.

That’s a good analogy about water, but it’s my understanding that the “active ingredient” (octane particles?) is the thing that gradually evaporates. Like alcohol from a bottle of vodka, it won’t magically regain it’s evaporated alcohol by mixing it with fresh vodka.

Anyway, I think by the nature of humans being unintentional fuckwits from time to time, a tank will run itself almost dry without a conscious effort at least a couple of times a year, so as you say it’s probably not an issue.

But yeah, it’s just something that’s never occurred to me before. I think I’ll make a point of running my tank right down every few months, I’ve just got myself a nice new car with a lively engine and I’m always precious about my nice new stuff, haha!

 

[Rest]

Fuel is pumped from the bottom of the tank, fuel is added from the top of the tank. Keeping your tank filled doesn't harm anything, and in my experience makes the fuel pump's job easier. On the other hand, letting your fuel get low increases the chance of pulling silt from the bottom of the tank into the fuel system, which can plug the filter, the pump, or your injection heads. Keep your tank above 1/4 or 1/2 and you will pull much less silt into the active parts of the fuel system.

 

[Rest]

That’s a good analogy about water, but it’s my understanding that the “active ingredient” (octane particles?) is the thing that gradually evaporates. Like alcohol from a bottle of vodka, it won’t magically regain it’s evaporated alcohol by mixing it with fresh vodka.

Gasoline is made of oil and water, two things known not to like to mix. Nothing evaporates out of your fuel tank per se, but after a while the stabilizers in the fuel can't keep the oil and water mixed, and they separate. Pulling water into your fuel system means you have to flush the whole thing.

 

[T8SC]

Yes, over a long period the octane will drop, but cars have knock sensors which will help and either flag a warning light and/or adjust/retard ignition timing etc. (Unless it's tuned to specifically run on a higher octane and no less). This is very basic/brief explanation.

It is worse to run a car close to empty and pull debris and other shit through your fuel pump etc.

 

[ManaByte]

Fuel is pumped from the bottom of the tank, fuel is added from the top of the tank. Keeping your tank filled doesn't harm anything, and in my experience makes the fuel pump's job easier. On the other hand, letting your fuel get low increases the chance of pulling silt from the bottom of the tank into the fuel system, which can plug the filter, the pump, or your injection heads. Keep your tank above 1/4 or 1/2 and you will pull much less silt into the active parts of the fuel system.

This. You don't want to run under half a tank for too long. It's harder on the fuel pump.

 

[Old Retro]

Keeping the tank full is 1000x better than letting it run dry, unless you're gonna let it sit for a few years. Ask me how I know it's bad to test how many miles per tank your car can get and let it get to E?

Answer: you burn out your fucking fuel pump. At least I did. It was a good learning experience back in the day to learn how to install a new one.

 

[BigBooper]

Ain't a problem until it knocks. Here's an interesting video on fuel and octane I watched this year. It's worth the 16 minutes.

 

[StreetsofBeige]

Just watch out for overflowing the tank. Some people like to get in an extra bit of gas after it's full. Can lead to repair issues. Had that happen to me. Fixed for free under warranty thank god. Dealership told me to never add more gas when it clicks to stops.

 

[AJUMP23]

If you are using the vehicle no, if you are letting it sit for extended periods then yes.

 

[NinjaBoiX]

Thanks for the replies guys, I’m a perennial worrier about my stuff but you’re all right on reflection, the VERY marginal octane loss is far offset by clogging up the fuel filter with crap from the bottom of the tank.

Also, I appreciate your point Rest about filling from the top and drawing from the bottom, but fuel has no prejudice and will inevitably mix in the tank going round corners and such. But yeah, it’s more of a conversation topic than a genuine concern, haha!

I’ve been fastidious with my fuel gauge up until now, letting it run down until 1/3 full then topping up £20-30 to bring it back up to 2/3 full. Again it’s me way overthinking things, but I don’t see any value in carrying around extra litres of fuel “just in case”.

Ain't a problem until it knocks. Here's an interesting video on fuel and octane I watched this year. It's worth the 16 minutes.

Thanks, I’ll give this a watch!

 

[Trogdor1123]

Gasoline is made of oil and water, two things known not to like to mix. Nothing evaporates out of your fuel tank per se, but after a while the stabilizers in the fuel can't keep the oil and water mixed, and they separate. Pulling water into your fuel system means you have to flush the whole thing.

What? There isn't water in gasoline...

 

[T8SC]

Ain't a problem until it knocks. Here's an interesting video on fuel and octane I watched this year. It's worth the 16 minutes.

87, 89, 91 & 93. I remember seeing these octanes when I was in the US and my mind was blown.

Both my cars run on 99 octane. Even our "regular" is 95 octane.

 

[catvonpee]

What? There isn't water in gasoline...

Yeah there is not water in gasoline and you can actually add water to gasoline that has ethanol in it to remove the ethanol. 2 parts gas to 1 parts water, shake vigorously, let the water settle to the bottom then use a separatory funnel/pipet/turkey baster to get all the ethanol free gas out. I used to do this for my chainsaws... But I just started buying engineered fuel that is pre-mixed 50:1 with oil.

 

[catvonpee]

To OP: just top your tank off if you want. The gasoline will mix together when you drive. The agitation from driving will mix the old gas with the new. Gas will go stale after sitting about six months. But probably not long enough to be any concern.

 

[BigBooper]

87, 89, 91 & 93. I remember seeing these octanes when I was in the US and my mind was blown.

Both my cars run on 99 octane. Even our "regular" is 95 octane.

Your 99 is measuring a different thing or a different way than our 93 according to the video.

 

[NinjaBoiX]

To OP: just top your tank off if you want. The gasoline will mix together when you drive. The agitation from driving will mix the old gas with the new. Gas will go stale after sitting about six months. But probably not long enough to be any concern.

Yup, this is exactly where I’m at.

But as a pedantically minded person, it just occurs to me that this fuel will be gradually getting more and more diluted of its useful octane with old “spent” fuel, as I keep topping up/mixing it with “fresh” fuel.

But yeah, this is definitely just me over thinking stuff again, the car is already 10 years old (new to me), any obsessive fuel monitoring ship i could’ve attached my OCD flag to his long since sailed.

But I’m still glad this thread has opened a dialogue, I find this stuff fascinating!

 

[T8SC]

Your 99 is measuring a different thing or a different way than our 93 according to the video.

Research Octane Number (RON) , used in Europe and most of the world
Motor Octane Number (MON)

In the United States gas stations describe the types of gasoline based on the Pump Octane Number (PON). That number is the average between RON and MON. This causes many problems.

If a fuel is 98 RON then it will be 93 PON -> 93 PUMP
If a fuel is 95 RON and 87 MON then it will be 91 PON -> 91 PUMP


America, just because you can be different, doesn't mean you need to be.

 

[jason10mm]

So

Yeah there is not water in gasoline and you can actually add water to gasoline that has ethanol in it to remove the ethanol. 2 parts gas to 1 parts water, shake vigorously, let the water settle to the bottom then use a separatory funnel/pipet/turkey baster to get all the ethanol free gas out. I used to do this for my chainsaws... But I just started buying engineered fuel that is pre-mixed 50:1 with oil.

So if I pour out my gas can and drain my mower tank at the end of summer and let it settle, it is like half gas, half water, usually won't run the mower if it is mixed. So I can siphon off the TOP liquid and run that in my mower?

I habe a small yard now and am just gonna go electric next summer, but dealing with that last bit of summer gas is always a pain.

 

[NinjaBoiX]

Haha, this thread has raised all the points I hoped it would, so thank you for that!

But even factoring in my potential scenario of topping up half a tank once a month for 6 months, that would still only leave 1.5% of “spent” fuel in the tank after half a year.

You’d imagine it that fuel manufacturers have already factored this in, along with the aforementioned very likely scenario that most drivers would’ve let their tank run well below this level more than twice a year anyway.

*shrugs*

 

[catvonpee]

So

So if I pour out my gas can and drain my mower tank at the end of summer and let it settle, it is like half gas, half water, usually won't run the mower if it is mixed. So I can siphon off the TOP liquid and run that in my mower?

I habe a small yard now and am just gonna go electric next summer, but dealing with that last bit of summer gas is always a pain.

Yeah buddy, I wouldn't shit you guys because you're my favorite turds.

 

[Liljagare]

Just avoid trying to put more in after the fuel pump cuts off to "top off", this will damage the carbon fume recycler in the long run in most modern cars.

 

[Yamisan]

Why in the world would you ever think so?

 

[NinjaBoiX]

Why in the world would you ever think so?

Because science and stuff? I don’t understand your confusion.

I’m fully admitting that it’s a non issue really, and the benefits of keeping your tank topped up far outweigh the cons.

But physics support my argument, even though I refute their inevitable conclusions in this very thread.

 

[Yamisan]

Because science and stuff? I don’t understand your confusion.

I’m fully admitting that it’s a non issue really, and the benefits of keeping your tank topped up far outweigh the cons.

But physics support my argument, even though I refute their inevitable conclusions in this very thread.

Science and stuff proves your wrong, physics does to. You just don't understand what you preach. Ignorant.

 

[Rest]

What? There isn't water in gasoline...

Looks like I had bad information before, you're right.

 

[Rest]

Also, I appreciate your point Rest Rest about filling from the top and drawing from the bottom, but fuel has no prejudice and will inevitably mix in the tank going round corners and such. But yeah, it’s more of a conversation topic than a genuine concern, haha!

Most gas tanks have baffles in them to reduce mixing, likely due to sloshing mixing sediment into fuel (and allegedly to reduce explosion hazards, but I find that doubtful.)

 

[Ozzy Onya A2Z]

Better to keep things topped up, especially if you have a diesel engine. Running diesel to empty can mean not getting it started again and paying a high cost for someone with the right experience or tools to get your diesel engine running again. Generally petrol engines will restart with refuelling but it's bad if you do run out of fuel often.

 

[NinjaBoiX]

Science and stuff proves your wrong, physics does to. You just don't understand what you preach. Ignorant.

What? I’m not preaching anything.

Chill.

 

[Fools idol]

Engines are temperamental beasts. I own a couple naturally aspirated v8's (an e92m3 and a 458 italia) and two modern engines as wel (the new supra and a rs6 w/ turbos). Let me tell you, the difference between using high octane fuel on the older engines is huge. The two 2013 cars run very differently, and cold starts in the 458 are horribly clunky if the tank is low or anything other than v-power fuel is used.

Modern cars are infinitely more abusable, so if you have one, I wouldn't be worried about it.