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I have a 1.5 gallon can of 40:1 fuel mix for my single 2-stroke tool, a chainsaw. I rarely use it, like maybe once every 2-3 years, so despite using a fuel stabilizer, the can of mix can get really old.

Would 1.5 gallons of 40:1 mix do any harm to a non-performance vehicle (particularly the fuel injectors) when mixed with a full 22-gallon tank of fresh gas?

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If you don't have an oil analysis of the 2-stroke oil in the 40:1 mix, it's risky. Zinc, phosphorus and molybdenum additives are used in some oils to help protect valves. These are all poison to platinum and palladium catalysts used in catalytic converters, and they can also ruin oxygen sensors. In addition, ash residue from the 2-stroke oil itself can clog the catalyst bed in a car's catalytic converter.

I'm not saying that a single 1.5 gallon dose will break your car -- it won't -- but anything that will shorten the life of expensive emission control parts should be avoided whenever possible.

Better to use up the 40:1 mix by using it a bit at a time in a non-automotive gasoline engine such as a lawn mower, tractor, roto-tiller, pressure washer, snow blower, generator, etc. That's how I've been using up old 40:1 mix for years with no issues of any kind. When I'm looking to get rid of some, I typically use 1/4 to 1/3 tank of premix in a full tank of fresh gasoline for any 4-stroke outdoor equipment.

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  • +1 for the last bit. Much better place to be using the 2-stroke gas. Jul 8, 2023 at 2:11
  • @MTA, as you suggested, using this mix in other small 2-stroke engines should help. If not, donate the fuel to town maintenance departments using 2-stroke fuel. Neighbors with 2-stroke engines to donate this fuel? Lastly, can this 40:1 mix be further diluted by, say a pint of it poured into a full tank of gasoline? A quart/full tank?
    – F Dryer
    Jul 13, 2023 at 22:22
  • @FDryer The 1.5 gallons in question contains X grams of Zn, P and Mo plus Y grams of oil that will leave Z grams of ash when it is burned. Those X, Y and Z grams will not be diminished at all by diluting the 40:1 mix in large amounts of gasoline. The concentration will change, the total amount will not. So no, spacing out the contamination of the catalytic converter over a long period of time will contaminate it just the same. Best to use none of it in a vehicle with a cat or an oxygen sensor. Much better to use it up in other small engines.
    – MTA
    Jul 13, 2023 at 22:37
  • @MTA, thank you. Spitballing and needed insight with regard to preventing catcon damage.
    – F Dryer
    Jul 13, 2023 at 23:07

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