look at that S car go

December 7, 2007 5:54 PM

My friend says the fuel injection in my 2002 Jetta wagon (non-diesel) suffers if I let the tank get less than 1/4 full. She also believes that "el cheapo" gasoline, from locally-owned non-chain stations, isn't as good, or is actively bad, for the car. Is there any truth to either of these claims?
posted by luriete to travel & transportation (29 comments total)

!/4 tank? No truth at all. There is nothing at all wrong with running tanks completely empty, even (assuming the tank hasn't corroded inside - this will NOT have happened on a 2002 car).

El cheapo petrol is not as good, often, as main stream petrol as it may have impurities that a less costly refining technique would have removed. However, it will not damage your car at all (assuming a basic level of quality being as you are in the States). It will damage performance slightly though, but it is unlikely to be particularly noticeable. Modern cars are designed to run on a range of petrol grades.
posted by Brockles at 5:59 PM on December 7, 2007

Although, to add a proviso, sometimes Locally owned petrol stations buy exactly the same fuel as the chain ones.
posted by Brockles at 6:00 PM on December 7, 2007

It's okay to let the fuel tank get low
posted by aubilenon at 6:10 PM on December 7, 2007

Locally owned petrol stations buy exactly the same fuel as the chain ones.

From what I understand, this is true. The only difference between cheapo and major brands is that the majors add various additives to their gas.
posted by puritycontrol at 6:12 PM on December 7, 2007

I presume the 1/4 tank thing comes from thinking that all the garbage and impurities and such sinks to the bottom and therefore you don't get as much if you don't use it. However, your car has fuel filters and the fuel intake, obviously, feeds from the bottom of the tank anyways. And yes, fuel does have differences and car magazines occasionally test the stuff. From reading these tests buying at cheapo stations does make it slightly more likely for you to get poor quality gas, however this is also possible at regular priced stations. And like Brockles said, it's almost certainly not going to harm your engine (long-term wear issues are a bit murkier, since that would take a lot of testing, but even here the differences are not going to be dramatic.)
posted by Authorized User at 6:14 PM on December 7, 2007

Also note that these tests were made in Finland, so your mileage in the US may vary.
posted by Authorized User at 6:16 PM on December 7, 2007

Brockles, with all due respect, I'm going to have to disagree with you on your second point.

All gasoline, from the refinery, goes into a pipeline. Exxon, Amoco, you name it, they all go through the same pipeline. I have seen this in person.

The pipelines go to distribution stations where the tanker trucks are loaded. At the distribution station, each company adds their own 'special sauce' additives, but in reality, all gasoline for sale in the US must meet the same basic standards. The only way 'off brand' gas will damage your car is if the station has low stock turnover, and has degraded tanks. I say low turnover because if the tank is allowing water to get inside, if the gas doesn't hang around long, it's gone before it has time to absorb enough water to cause problems.

At least in my neck of the woods (Northeast), most, if not all of the underground gasoline storage tanks have been replaced in the last 15 years. I have not had a bad tank of gas ever, and I go to whatever station's the cheapest. You may live in an area that has more lax environmental laws, and tanks may leak more leading to more bad gas. This sort of thing can happen to any filling station, 'name brand' or not.
posted by Geckwoistmeinauto at 6:18 PM on December 7, 2007 [1 favorite]

I wouldn't let the tank get much below a 1/4 tank myself, but for a completely different reason.

Running on E all the time it the fastest and easiest way of burning out a fuel pump. Most fuel tanks have a little "tray" in the bottom of the tank to keep the last of the fuel around the pump intake, but what if you stop on a hill? The fuel pump depends on the fuel to lubricate it and cool it. If it's not getting a full siphon it suffers.

Your injectors are completely fine, they dispense fuel already pressurized, and even then air in the fuel line won't hurt them because your car will have died by then.

As far as grade of gas, check your owners manual. Most cars are made to run on 87 octane, but high compression engines, which I would guess the Jetta is, need higher octane, probably minimum 89.
posted by sanka at 6:18 PM on December 7, 2007

little to no truth, at least not worth worrying about, and at least as far as I can tell in a bunch of researching these questions over the years
posted by caddis at 6:23 PM on December 7, 2007

If your car's not knocking, it doesn't need the higher grade fuel.
posted by klangklangston at 6:47 PM on December 7, 2007

I wouldn't let the tank get much below a 1/4 tank myself, but for a completely different reason.

Running on E all the time it the fastest and easiest way of burning out a fuel pump. Most fuel tanks have a little "tray" in the bottom of the tank to keep the last of the fuel around the pump intake, but what if you stop on a hill? The fuel pump depends on the fuel to lubricate it and cool it. If it's not getting a full siphon it suffers.

posted by sanka at 9:18 PM on December 7 [+] [!]

i popped into this thread to say exactly that. it bears repeating though.
posted by Ziggy Zaga at 6:52 PM on December 7, 2007

who said anything about a higher grade (octane) of fuel? also, some cars, like the honda accord 6mt, have a knock sensor and programming in the computer to eek a few extra horsepower (10 more than the standard 240) out of premium fuel. worth it? probably not unless you are racing or something.
posted by caddis at 6:53 PM on December 7, 2007

Purely as a practical matter, running on E all the time is also a sure way to end up needing to get somewhere quick exactly when you can't (whether it's a zombie attack in your neighborhood or merely needing to run to the store for some contraceptives or some such thing; ideally not both, however). But 1/4 tank is well within range of a fill-up in most parts of the US, I'd imagine.
posted by socratic at 6:56 PM on December 7, 2007

"who said anything about a higher grade (octane) of fuel?"

Sorry about that, misread the question—I thought the friend was telling him to avoid the cheapest gas.
posted by klangklangston at 7:16 PM on December 7, 2007

I'll poke my head in here--we've got local "convenience store brands" that literally buy leftover fuel from tankers from multiple brands in the area. In a given week they might get overflow sheetz/bp/chevron/amoco/getty---whatever, all dumped into their tanks. They also buy stale gas from these suppliers on the mega cheap. I personally have multiple friends whose cars have suffered tremendously because those friends wanted to save 10 cents a gallon on gas. Every mechanic I know in this area will tell you---avoid "XXXXX Mart Gas".
posted by TomMelee at 7:26 PM on December 7, 2007

Running on E all the time it the fastest and easiest way of burning out a fuel pump.

While your fuel pump can still create enough pressure in the rail of the fuel system (ie not causing a misfire through low pressure at the injectors) it is flowing enough fuel to cool it. I have never seen a 'burnt out' fuel pump in my life, and every single time I have sent a (high performance, high g inducing) car out, it returns with around 3-4 litres (just less than one US gallon) in the tank. Vibration is the only thing that has killed a fuel pump in all the time (20 years) that I have been doing this.

I suspect that this is urban myth. The only time your pump will start to starve and not flow enough fuel is in the 20 seconds (maximum) or so before the car actually runs out of fuel. They simply don't create that much heat to need all that much cooling, and while there is any at all in the tank, they are fine.

Brockles, with all due respect, I'm going to have to disagree with you on your second point.

Under a lot of circumstances, as my proviso states, they do buy the same stuff. But they have to make their savings somehow, and they do this by buying fuel as TomMelee points out. A lot of the time you can get lucky. It is not the supply of the fuel that the inconsistency comes in, but in the subsequent storage and distribution.

Still, the difference is extremely unlikely to damage the car, so the point is pretty much moot.
posted by Brockles at 7:39 PM on December 7, 2007

All gasoline, from the refinery, goes into a pipeline. Exxon, Amoco, you name it, they all go through the same pipeline. I have seen this in person.

Yes, you're right. Everything goes through the same pipeline, at different times. Generally they flush plugs of pressurized water (or nitrogen or another substance) through to separate things. In fact, natural gas and crude oil also get pushed through the same pipelines, but you wouldn't want to put those in your car.
posted by SpecialK at 10:37 PM on December 7, 2007

Since 2004, six automakers (BMW, General Motors, Honda, Toyota, Volkswagen and Audi) have promulgated a tighter standard, called Top Tier Gasoline, for gasoline additive performance than the current 1995 EPA minimum standard. Gasoline that meets the Top Tier standard generally has more effective cleaning and anti-fouling additives, and higher levels of ethanol than EPA specification. Economy and mid grades of Shell and Chevron Techron gasolines meeting Top Tier standard will have as much as 2 times the cleaning additive as EPA requires, and top grade Shell V Power and Chevron gasolines can have as much as 5 times the minimum cleaning additive EPA mandates.

The high additive high octane grades of Chevron and Shell gasolines are capable of removing significant wax, gum and dirt deposits from fuel injectors, intake valves, and other fuel system parts, and reducing combustion chamber fouling deposits, when used regularly, and at generally lower cost than consumer tank added additives.
posted by paulsc at 10:49 PM on December 7, 2007

The guys at Car Talk discuss the difference between Tier 2 and generic (EPA) gas (from their gasoline myths page):

Is there a difference in the gas between the big names, and the El-Cheapo gasoline at my local Costco or Stupey-Mart?

There is. A number of years ago, some of the major auto manufacturers were ticked off with the deposits from gas that met EPA detergent standards. In other words, they were afraid they'd take the heat for problems, when their customer's engines clogged up from the twigs and dirt that was getting deposited on their fine handiwork.

So, they set their own guidelines, which they call "Tier 2." All of the gas from the major gas companies meets the Tier 2 standards. The gas at your local Quickie Mart probably does not. The difference? In some tests, after about 10,000 miles, there was a minor amount of crud built up on engine components that ran exclusively on Quickie-Mart gas. Do we think this is an issue? Not really. But, would we advise running your car for 100,000 miles only on Quickie-Mart gas? Not if you're concerned about the longevity of your vehicle. We recommend switching back and forth between Tier 1 and Tier 2 — maybe every other tank full — to keep your engine running clean.

It sounds like as long as you use it periodically, you're probably fine. (I'd love to see some hard data on how often, exactly, you need to use the Tier 2 gasoline to get the benefits.)

Frankly, I think it's more trouble than it's worth to constantly look for "brand name" gas stations. I think it's a lot less trouble to just buy gas wherever and then deal with the additive issue separately if the engine is running dirty; regardless of whether it might be slightly cheaper to get the additives built into the gas. I'd rather deal with it less often than constantly be going out of my way to buy gas at a name-brand station rather than the local generic one.
posted by Kadin2048 at 12:27 AM on December 8, 2007 [1 favorite]

The version I've heard is that you should keep the tank at least half-full in the winter to avoid condensation of water in the fuel tank. I guess that would be applicable, if at all, in cold and humid climates, particularly where there are large variations in air temperature.

I never did figure out just how likely it is to matter, but it sounds reasonable. Google turns up some evidence that it's not a bad idea, including an airplane crash: Thus, water contamination present on the occurrence flight may have originated primarily from condensation in the fuel tanks when the aircraft tanks were less than half full for several days.
posted by sfenders at 6:40 AM on December 8, 2007

The version I've heard is that you should keep the tank at least half-full in the winter to avoid condensation of water in the fuel tank.

Again, I've never heard of an issue in a car. Using a plane crash as an example is possibly a little over-dramatic... Where, if enough water got into the fuel system to cause an issue, a plane's engine will stop (worst case is on take off or climb out, as in your example) then the engine will stop (very little inertia to a plane engine - no flywheel and only a propellor to turn) in a car, it most likely will just lurch a bit and carry on.

Although the chances of (problem inducing) water forming in a car fuel tank is pretty low. Aircraft tanks are usually much larger (especially your example - they use hideous amounts of fuel) so the volume of air is larger (when half full) than most car tanks are when completely empty. This isn't like for like.
posted by Brockles at 7:40 AM on December 8, 2007

Using a plane crash as an example is possibly a little over-dramatic...

Yes, possibly. Another difference would be that even if your car engine did quit, it would be unlikely to cause the vehicle to fall out of the sky. The point is, it seems like a generally good thing to have less water in the fuel.
posted by sfenders at 8:05 AM on December 8, 2007

My dad gives this advice, too. I think it must go back to the olden days, when cars were less reliable. Most likely before fuel injection. He also advises against buying Nissans because he bought a used one that was a lemon... twenty years ago. So, my guess is that the advice had some validity once upon a time but has been passed down without question ever since, even as automotive technology has improved.
posted by kindall at 9:01 AM on December 8, 2007

I thought the issue with condensation was the possibility of ice forming; e.g., in the fuel pump - maybe more of a problem back when the pumps were not burried in the fuel tank?

Clearly, however, less gas in the tank means more mpgs due to less weight. I would think 1/2 tank (say 70lbs) ought to be worth a good part of one mpg. So I keep it near full in the winter (in the northeast) and around 1/2 full in warmer months.
posted by Kevin S at 9:28 AM on December 8, 2007

possible downside: you increase wear on your 'low fuel' idiot light to the point where it burns out, arguably prematurely.

my girlfriend has successfully done this.
posted by fishfucker at 11:47 AM on December 8, 2007

The only difference between name brand gas and off-brand gas are the additives that are introduced after the refining process. The purity of the gasoline itself is uniform across the board. Each company puts in its own additives (detergents, stabilizing agents, etc. ). The amount and type of detergent that is added can affect performance, especially related to the fuel injectors as the detergents reduce the formation of deposits on the fuel injector nozzles. Fuel injector nozzles are a particular point of interest since nozzle diameter is very small in order to allow for efficient fuel atomization, so therefore any deposits on the nozzles would inhibit proper fuel-air mixing, which would in turn create lower power output, poorer fuel economy and also increased harmful emissions.

That being said, I don't really have an idea to what magnitude using off-brand gas would affect performance. All gasoline over time will leave deposits and I cannot find any information not from the gasoline companies themselves that have any definitive empirical information on the subject. I would say if you really want your car to run properly for a good, long time; stay up to date on your service intervals, and use proper components (oil, fluids, filters, etc.); that stuff is far more important than the gas you put in.
posted by goHermGO at 12:31 PM on December 8, 2007

My father is a retired chief engineer for one of the leading oil company's and he insists that there is no difference between various brand names or brand name versus non-brand name.

As people say above it all runs through the same pipes everyones "special additives" are really the same thing.

As for running your car lower then 1/4 tank...I have always heard that condensation can be created inside the tank which can cause corrosion. I really don't know if thats true or not though.
posted by Black_Umbrella at 2:10 PM on December 9, 2007

Again this is possibly from age old usage - where fuel was not stored as effectively and so water crept in and condensed inside the tanks.

Being as most modern fuel tanks are either galvanized or plastic/composite, then rust is no longer an issue. Nor has it been for some time. This is, I feel, old fashioned advice hanging over technology that has negated it.
posted by Brockles at 2:22 PM on December 9, 2007

I try to avoid going below 1/2 tank in the winter, but not for any of the reasons above. If your vehicle gets stuck in a remote location (crazy weather conditions, accident, etc.) you'd be able to run the engine and provide heat for a longer period of time. I did manage to get my car stuck in a late-night winter storm about 7 years ago, and I had to wait at least 4 hours for a truck to tow me out. Would have been a bit chilly without heat, not to mention potentially dangerous with it being so damn cold out there.
posted by bhayes82 at 1:15 PM on December 10, 2007