Graham FisherBy Graham Fisher

Regular unleaded gasoline is what most gas-powered pressure washers are designed to run on, as long as it is fresh and has no more than 10% ethanol (E10). Avoid E15 or higher, leaded fuel, and gas that has sat in the tank for more than about 30 days. Manufacturers typically specify these limits in the owner’s manual, and following them prevents most common engine failures.

Why Gasoline Quality Matters More Than Octane

Most small engines on pressure washers are rated for 87-octane regular unleaded. Buying premium rarely helps, because the engine’s compression ratio does not require it. What does matter is freshness and ethanol content. Gasoline begins to degrade within weeks, and the lighter compounds evaporate while the remaining fuel forms gum and varnish. These deposits clog carburetor jets and fuel filters, which is why a pressure washer that ran fine last spring may refuse to start or sputter under load this year.

Shopping for one? See our guide: Karcher K3 Pressure Washer Review (2026): Pros, Cons & Who Should Buy.

How Ethanol Affects Small Engines

Ethanol attracts water from the air. In a fuel tank that sits unused, that moisture can separate from the gasoline and settle at the bottom, causing rust and poor combustion. Ethanol also tends to dissolve the varnish that has built up inside the carburetor, which can loosen debris and send it straight into the jets. Many engine manufacturers warn against E15 and higher blends because they can damage rubber fuel lines, diaphragms, and seals not designed for them. If your local station only offers E15, look for ethanol-free gasoline or add a fuel stabilizer.

Factors That Affect Which Fuel You Should Use

Several details determine the right choice for your machine:

  • Engine manufacturer: The owner’s manual lists the approved fuel type and maximum ethanol content. Follow it first.
  • Storage time: If you use the washer weekly, standard E10 fuel is usually fine. If it sits for a month or longer between uses, stabilizer becomes important.
  • Climate: Humid regions speed up water absorption in ethanol blends, while very hot storage speeds up evaporation and gum formation.
  • Two-stroke or four-stroke design: Some pressure washers use separate oil and gas mixing for two-stroke engines. Mixing ratios must match the manual exactly, and this is one place where guessing causes real damage.

What to Do About Old or Wrong Gasoline

If you discover stale or incorrect fuel in the tank, do not simply top it off with fresh gas. Drain the old fuel into an approved container and take it to a local hazardous waste collection site. Then clean the fuel tank and replace the fuel filter if it looks discolored. Check the carburetor bowl for varnish, and remove and clean the jets if the engine runs rough or will not idle. Dispose of any fuel that smells sour, looks dark, or has visible water or sediment, because it will not burn cleanly.

Practical Steps and Tips

  1. Check the owner’s manual for the approved fuel type and maximum ethanol percentage.
  2. Buy gasoline from a station with high turnover so you are not getting old fuel.
  3. Add a fuel stabilizer to every tank you do not plan to burn within 30 days.
  4. Run the engine until it stalls after storage to empty the carburetor of stabilized fuel.
  5. Store fuel in a labeled, sealed container away from heat sources, and keep it out of living spaces.
  6. Mix fresh fuel for each use rather than leaving partially used tanks sitting around.
  7. Never store the washer with fuel in the tank in an enclosed, poorly ventilated area.

Because gasoline is flammable and carbon monoxide from running engines is dangerous in enclosed spaces, always operate a gas-powered pressure washer outdoors. For complex carburetor repairs or fuel system problems, consult a qualified small-engine technician.

What We’d Use

A fuel stabilizer formulated for small engines is the most practical addition for anyone who stores a pressure washer between seasons. View on Amazon

For a pressure washer that will sit for long periods, a fuel stabilizer that also protects against ethanol-related moisture is a reasonable choice. View on Amazon

A approved red portable fuel container with a spill-resistant spout makes measured refueling easier and safer. View on Amazon

Can I use premium gasoline in my pressure washer?

You can, but it is usually unnecessary. Most pressure washer engines are designed for regular 87-octane fuel, so premium offers no clear benefit unless the manual specifically calls for it.

Is E15 gasoline safe for a pressure washer?

Many small-engine manufacturers advise against E15 and higher ethanol blends. Check your owner’s manual, and if it does not approve E15, use E10 or ethanol-free fuel instead.

How long does gasoline last in a pressure washer tank?

Fuel can start to degrade within about 30 days, especially if it contains ethanol. Adding a stabilizer extends its usable life, but draining the tank before long storage is still the safest approach.

What happens if I put old gas in my pressure washer?

Old gas can form varnish that clogs the carburetor and fuel filter, leading to hard starting, rough running, or a no-start condition. Draining the tank and cleaning the carburetor usually resolves it.

Should I drain the gas before storing my pressure washer?

For storage longer than a month, draining the tank or running the engine dry is the most reliable option. Stabilizer is a good backup, but it does not eliminate every risk.

Ready to choose one?

Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our non-vacuum cleaning gear guides, checking product details against manufacturer specifications and owner feedback.