What Really Happens to a Car When Its Gasoline Sits? | Garage Rescue Miata

Know what happens with gasoline sits? Bad things. 

We have written about this very subject before. If you don’t want to read the entire article, here’s the take-home point:

Gasoline is made up of aliphatic and aromatic hydrocarbons—let’s call them open chains and closed rings. The light ends, technically known as short-chain aliphatic hydrocarbons, evaporate first, leaving behind the long-chain aliphatic and aromatic hydrocarbons. You don’t need to own a lab coat to understand the results: A fuel that’s light on short-chain hydrocarbons is harder to ignite because it needs a higher temperature to vaporize enough fuel to support combustion, explains Zachary J. Santner, technical specialist at Sunoco Race Fuels

Another issue: As that gasoline evaporates, it leaves behind a varnish called gum.

Wait, one more problem: Gasoline that sits can absorb moisture from the air that we breathe, adding another potential wrinkle to the situation.

Our 1992 Mazda Miata had been sitting for about eight years. Yeah, we screwed up. 

Several friends/experts in the field said not to worry. Oh, sure, we'd let the car sit too long, but it would be fine.

But how did they know? And how much fuel was in the tank? Turns out we left it about three-quarters full. 

Yeah, we screwed up.

We had the car towed over to Miata race prep shop BSI Racing. We figured if things looked ugly, they could handle it while we continued to make magazines and websites. 

We were there for the moment of the big draining. The plug was pulled, and gasoline poured out. At least it didn’t resemble a giant gelatinous blob, right?

More good news: What came out smelled and looked like gasoline.

Maybe we really dodged a bullet here.

They also replaced the fuel filter and, since we had it on the shelf, installed a high-flow fuel pump. Mainly they handled some deferred maintenance:

  • Clutch slave cylinder with braided steel line: The puddle on the floor said it was time. We went with the same Exedy slave that BSI Racing uses in its race cars.
  • Timing belt, pulleys, water pump: All were last changed some 20 years ago.
  • Cam seal: BSI’s technician noticed that ours was leaking but it's just a few bucks to replace.
  • Battery: For a hundred bucks, no worries about getting stranded. 
  • Tires: New Falkens and Koseis to replace 15-year-old rubber. 
  • Heater core hoses: They were pretty hard. Figure about $30 to nip that problem. 
  • Fuel pump, filter and strainer: Definitely needed to replace the filters, and we already had the pump. 
  • Plug wires: Ours looked a bit cracked and tired. About $45 for replacements. 
  • Shifter base bushing: Ours was done, and a new one’s like $6.
  • Oil, transmission fluid, hydraulic fluids: Again, it was time. 

So although the car had stagnated, it didn’t really need that much work—about a full day's worth for one tech. The thought of draining a full tank of fuel on jack stands, though, didn’t thrill us. 

Back to the draining of the fuel tank.

The tank was drained, a few gallons of fresh fuel were poured down the filler, and then the key was turned.

And the car came back to life. 

There was a little smoke at first, but the engine quickly settled into a quiet, rhythmic idle—quieter than ever, seemed to be. 

And that was all the drama. We went into this envisioning havingto drop a fuel tank–or more. 

Our first stop after fetching the Miata? A local Sunoco station that carries 91-octane, ethanol-free gasoline. 

Why is ethanol in our fuel in the first place? There are some good reasons, and you can read about them here.

We also poured in some Chevron Techron fuel cleaner: 1 ounce per gallon as recommended by a trusted friend in the industry. 

Once we put a few miles on the car, we’ll send off the injectors to Marren Fuel Injection. We also have more to clean and several projects to tackle. Much more to come. 

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Comments
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Keith Tanner
Keith Tanner MegaDork
1/22/21 9:13 a.m.

A suggestion - the guys at BSI probably know what they're working with, but some high flow pumps can overwhelm the stock regulator. Might be worth doing a quick fuel pressure check if the car exhibits any unusual running behavior.

Later Miatas didn't have the nice drain plug on the bottom of the tank, you chose the right one to neglect ;)

David S. Wallens
David S. Wallens Editorial Director
1/22/21 9:31 a.m.

Thanks for the tip. The drained fuel, by the way, seemed to pass the sniff test for use in the pressure washer. 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
1/22/21 10:13 a.m.

The flip side is, with an intact modern evaporative emissions system, fuel can last an amazingly long time.

I pulled fuel from a car that had been sitting for seven years and drove on it just fine.

 

crankwalk (Forum Supporter)
crankwalk (Forum Supporter) SuperDork
1/24/21 12:42 p.m.

If you're going to store a car for a long time, I'd rather store it with a near full tank to reduce condensation and the tank rusting so I dont think that was a mistake. I go a year or two or more between start ups on my Z and just keep it topped off with Stabil and ethanol free fuel and it fires up everytime.

 

GCrites80s
GCrites80s HalfDork
1/24/21 8:14 p.m.

If you leave it empty the fuel pump will rust. If you leave it full the pump will gum up. Leave it halfway and both will happen. No way to win.

aircooled
aircooled MegaDork
1/25/21 1:11 a.m.

What if you emptied it and sprayed a bunch of fogging oil in it?

BTW - I stored a car for many years (10+) only running it once a year, gas tank low, adding gas and Stabil each time.  When I finally drained the gas, it looked and smelled nothing like gas!  I did not have any rust  issues though.

David S. Wallens
David S. Wallens Editorial Director
1/27/21 4:50 p.m.

In reply to aircooled :

The  SDS for Sta-Bil will tell you what's in the bottle. Looks to be about 95% petroleum and 5% alcohol. 

93EXCivic
93EXCivic MegaDork
1/27/21 4:57 p.m.

How long does it take for fuel to go off? 

David S. Wallens
David S. Wallens Editorial Director
1/27/21 7:26 p.m.
93EXCivic said:

How long does it take for fuel to go off? 

Good question. Zach at Sunoco and I have discussed this several times, and it's one of those "How big is a rock?" questions. The answer depends on a few things:

The fuel in question: low-grade pump fuel, high-octane race fuel, ethanol or not?

Storage situation: sealed barrel, open container, modern vehicle with a sealed system, older vehicle with a vented system?

Location: humid, arid?

And how "off" is "off"? Like, will you still eat cheese with some mold on it? (Just cut off that part, right?) Fuel can be "stale" and still combust, but is that fuel performing at 100%?

I'll post some links that should help, but the TL;DR sounds like three or so years in ideal conditions--ideal fuel, ideal storage. And ideal here means non-ethanol race fuel stored in a tightly sealed drum. Optima, for example, is designed specifically for storage. At the other end of the spectrum, low-octane pump fuel can degrade in a few months--but, remember, this fuel is designed to meet a price point for consumers who are likely going to use it up rather quickly. And it also sounds like this is a good topic for future editorial. laugh

Here's some more reading about fuel and its aging process: 

What happens to gasoline as it ages?

Fuel for lawn and garden equipment

Why specific gravity matters

The science of high-octane fuel

Race fuel vs. street fuel

Why you should care about vapor pressure

 

rslifkin
rslifkin UberDork
1/28/21 7:20 a.m.

Stabilized fuel can last a pretty long time, especially if it's stored in a fairly well sealed tank or container. 

My Jeep has been doing just fine on year-old stabilized fuel with how little it's been driven lately.  And that's got E10 in it.  With both E10 and E0, the boat has regularly gone 9 - 12 months on a load of stabilized fuel and those tanks are vented.  The fuel (E0) in my snowblower right now is about 15 months old, as it snowed so little last winter that I never used up what I bought in the fall.  And I've used it so little this year that I haven't even refilled the tank yet. 

Generally, I stabilize any fuel that I'm not sure I'll burn within a month right when I buy it.  Equipment all gets stored for its off-season with a topped off tank and fogged.  The fogging oil seems to keep the carbs from gumming up in storage (I don't drain them).  So far, I've never had anything show an issue when it's ready to go back into use. 

maj75 (Forum Supporter)
maj75 (Forum Supporter) HalfDork
1/28/21 7:34 a.m.
crankwalk (Forum Supporter) said:

If you're going to store a car for a long time, I'd rather store it with a near full tank to reduce condensation and the tank rusting so I dont think that was a mistake. I go a year or two or more between start ups on my Z and just keep it topped off with Stabil and ethanol free fuel and it fires up everytime.

 

Let me show you what happens to a Z that doesn't get driven much and gets topped up with Stabil:


 

That's the "gas" that came out of the 280z I bought.  Old guy did exactly what you describe for years.  Drove it once a month and then filled it up and added stabil.  
 

On the positive side, the tank was totally rust free inside.

David S. Wallens
David S. Wallens Editorial Director
1/28/21 8:20 a.m.

In reply to maj75 (Forum Supporter) :

Yup, it's science. Read the Sta-bil Safety Data Sheet, and you'll see what's inside the bottle. 

David S. Wallens
David S. Wallens Editorial Director
1/28/21 9:01 a.m.

Some more reading:

What's in that fuel additive? 

crankwalk (Forum Supporter)
crankwalk (Forum Supporter) SuperDork
1/28/21 9:07 a.m.
maj75 (Forum Supporter) said:
crankwalk (Forum Supporter) said:

If you're going to store a car for a long time, I'd rather store it with a near full tank to reduce condensation and the tank rusting so I dont think that was a mistake. I go a year or two or more between start ups on my Z and just keep it topped off with Stabil and ethanol free fuel and it fires up everytime.

 

Let me show you what happens to a Z that doesn't get driven much and gets topped up with Stabil:


 

That's the "gas" that came out of the 280z I bought.  Old guy did exactly what you describe for years.  Drove it once a month and then filled it up and added stabil.  
 

On the positive side, the tank was totally rust free inside.

I've been doing it for almost 8 years with a build thread on this forum and always first start happens instantly. Maybe he started with terrible fuel? I use ethanol free marine premium from a marina with marine stabil. Maybe I burn more between intervals and replace it with more new fuel than he did? Maybe your early EFI is more sensitive than my triple Weber's? . I'd rather store wet with treated fuel than deal with repeatedly draining a tank, blowing it out, rubber seals, floats and pumps being dry for a year or more at a time. I also suspect your previous owner wasn't as obsessive about fogging, sealing up intake and exhaust and  filler necks.
 

I always hear it can't be done  but I've been practicing it for a long time now and  can only share what has worked for me. I'm not saying the fuel stays perfect but I don't think twice about letting it sit for a year or two, burning it off, then business as usual on the next tank.

Tyler H (Forum Supporter)
Tyler H (Forum Supporter) UberDork
1/28/21 9:16 a.m.
crankwalk (Forum Supporter) said:

If you're going to store a car for a long time, I'd rather store it with a near full tank to reduce condensation and the tank rusting so I dont think that was a mistake. I go a year or two or more between start ups on my Z and just keep it topped off with Stabil and ethanol free fuel and it fires up everytime.

 

This is a good tip.  I drained and pulled the tank on an MR2 Turbo that had been sitting for 4-5 years.  Replaced the fuel pump while the tank was out (a PITA job.)  It looked like new inside.  Flash forward about 2 months to the ceremonial first drive after doing a V6 swap. After a mile or so the fuel pump got loud and the car started bogging down.  Much troubleshooting later, I pulled the tank again and the fuel pickup sock was completely clogged with rust.  

I wound up having to use the Eastwood gas tank sealer kit to get the rust under control after that.  Keep those tanks full, folks!

 

David S. Wallens
David S. Wallens Editorial Director
1/28/21 2:42 p.m.

In reply to Tyler H (Forum Supporter) :

Yup, we replaced that sock as well. So far, though, so good. 

rslifkin
rslifkin UberDork
1/28/21 2:50 p.m.

In reply to Tyler H (Forum Supporter) :

Just another reason why steel is a terrible material to make a fuel tank out of.  Unfortunately, I have 2 of them in my fleet...  Tank breakdown of stuff I've got is as follows:

Plastic fuel tank: Jeep, Prius, Mower (Honda with Honda engine)

Steel: BMW, Snowblower (Ariens with Briggs engine)

Aluminum: Boat (2 tanks)

crankwalk (Forum Supporter)
crankwalk (Forum Supporter) SuperDork
1/28/21 3:10 p.m.
rslifkin said:

In reply to Tyler H (Forum Supporter) :

Just another reason why steel is a terrible material to make a fuel tank out of.  Unfortunately, I have 2 of them in my fleet...  Tank breakdown of stuff I've got is as follows:

Plastic fuel tank: Jeep, Prius, Mower (Honda with Honda engine)

Steel: BMW, Snowblower (Ariens with Briggs engine)

Aluminum: Boat (2 tanks)

Didn't Ducati have plastic fuel tanks that the ethanol would swell them up ? I'm thinking aluminum fuel cells sans the foam are probably the safest bet but are also the easiest to drain anyway.

Benjamin_Casto
Benjamin_Casto New Reader
1/31/21 11:24 p.m.

How many miles has this car racked up? 

rslifkin
rslifkin UberDork
2/1/21 7:18 a.m.

In reply to crankwalk (Forum Supporter) :

Possibly.  And in boats, some people had issues with resin incompatibility with ethanol in fiberglass tanks. But if they use an alcohol safe plastic or fiberglass, either makes a good tank material.  The lack of corrosion risk and plastic not being fatigue prone has some significant benefits. 

David S. Wallens
David S. Wallens Editorial Director
2/1/21 2:11 p.m.

I had the Garage Rescue Miata at the track Saturday and Sunday. It revs out so nicely. Still going to send out the injectors for cleaning with Marren as it's been 20 years since they have been serviced, but the engine pulls all the way to red line. 

 

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