E10 Petrol and Cars That Sit: What Ethanol Does in Six Months

E10 petrol became the standard grade at UK pumps in September 2021, and it changed what happens to a car that sits still.

Fuel that used to be tired after a year is now a real problem at six months, and on some older cars it is a problem at three.

Here is what the ethanol actually does, which cars care, and what to put in the tank before a lay-up.

What E10 Petrol Is

E10 contains up to 10% bioethanol. The previous standard grade, E5, contained up to 5%. Super unleaded remains E5 at most forecourts, and the government sets out the detail in its E10 guidance.

Ethanol is an alcohol, and it behaves differently from petrol in three ways that matter to a stored car: it attracts water, it dissolves things petrol leaves alone, and it evaporates at a different rate.

None of that matters in a car driven weekly. All of it matters in a car that is not.

Phase Separation Is the Main Event

Ethanol is hygroscopic. It pulls water vapour out of the air in the tank, and it keeps doing so through the vent for as long as the car sits.

Up to a point the water stays mixed in. Past that point the mixture separates: a layer of water and ethanol drops to the bottom of the tank, and the petrol above it is left with a lower octane than it started with.

The bottom of the tank is exactly where the pickup sits. So the first thing the engine gets on start-up is the water layer, which is why cars that ran perfectly in October will not fire in March.

In a steel tank that layer also sits against bare metal for months, and rust inside a fuel tank is an expensive fault to chase.

What Ethanol Attacks

Ethanol is a solvent, and older fuel systems were designed around a fuel that was not.

• Rubber fuel lines. Pre-1990s hose softens, swells and eventually perishes. The debris then travels forward into the carburettor or the injectors.
• Cork and fibre gaskets. Common on older carburettors, and they shrink or disintegrate.
• Some plastics and fibreglass. Certain older tanks and float chambers are affected, and fibreglass tanks in particular can be dissolved from the inside.
• Zinc, brass and aluminium. Corrosion in carburettor bodies and fuel taps, especially where water has separated out.
• Old varnish and deposits. Ethanol lifts decades of gum off the inside of a tank and carries it into the fuel system in one go.

Which Cars Actually Care

Anything built before roughly 2000 sits in the caution list, and anything with carburettors sits firmly in it. Classics, older motorcycles, and many small-capacity engines are affected.

Modern injected cars are compatible with E10 and cope well with a season, though the water attraction still applies to the fuel itself. A modern car will start after a winter on E10 petrol. It may not run its best for the first few miles.

Bikes are the worst case of all, because tanks are small, air spaces are large in proportion, and float bowls are tiny. Our guide to storing a motorcycle for more than one season covers that side.

What to Put in the Tank

For a classic or anything carburetted, fill with E5 super unleaded before the lay-up. Halving the ethanol content roughly halves the problem, and the price difference over one tank is trivial next to a carburettor rebuild.

Fill the tank rather than leaving it part full. A full tank has almost no air space, and no air space means very little water for the ethanol to find.

Add a fuel stabiliser and then run the engine long enough to draw treated fuel through the whole system, including the float bowls. Stabiliser sitting in the tank while untreated fuel sits in the carburettor achieves nothing.

For stays over a season on a carburetted car, drain the float bowls afterwards. Varnish in a float bowl is the single most common reason a classic will not start in spring.

How Long Is Too Long

Untreated E10 petrol is noticeably tired at three months and unreliable at six. Treated and in a full tank, it will generally see out a winter without drama.

Beyond eighteen months to two years, plan to drain and refresh rather than hope. Fuel is cheap relative to a fuel system, and a stored car coming out after two years has plenty of other things that deserve the attention instead.

What else changes over that sort of period is covered in what changes after the first twelve months.

What It Costs

A tank of super unleaded and a bottle of stabiliser is maybe forty pounds more than doing nothing. A carburettor rebuild, a set of fuel lines or a tank clean runs into the hundreds.

Storage here is £150 a month including VAT for the first three months, then £165, with the humidity-controlled bay at £200, then £225. Figures sit on the locations and pricing page.

How Pitstop 66 Can Help

Fuel and fluids are checked as part of the eight-stage intake, and we will tell you if a car arriving for a long stay should go in on E5 rather than what is in the tank.

Mechanics are on site, so draining, refreshing and float bowl work happen in the same building rather than as a separate errand. Classics are handled through classic car storage and longer stays through long-term car storage.

Owners reach us from Cheshire and Oldham, and collection runs to 65 miles. To arrange storage, enquire here or call 0161 464 7494.

Also worth reading: storing a motorcycle for more than one season, what a 911 needs over a winter and storing a car mid-restoration.

Key Takeaways

• E10 petrol pulls water out of the air and eventually drops a water layer to the bottom of the tank.
• The pickup sits at the bottom, which is why a car that ran in October will not fire in March.
• Ethanol attacks old rubber, cork, some plastics and brass. Pre-2000 cars are the caution list.
• Fill with E5 super, brim the tank, add stabiliser, and run it through the whole system.
• Untreated fuel is tired at three months. Past eighteen months, drain and refresh.