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The user's manual of my car contains the following warning:

Do not add washer fluid when the hybrid system is hot or operating as washer fluid contains alcohol and may catch fire if spilled on the engine etc.

Is this really the case? I think the scenario might be possible if you spill a lot of washer fluid on a hot engine, as alcohol in the mixture evaporates faster than water, so if you happen to have a static electricity spark, it could theoretically ignite the evaporated alcohol from the washer fluid.

However, I find it hard to believe someone could spill so much washer fluid on the engine that this scenario is even a remote danger.

If the interpretation of the warning is taken to the extreme, it means you cannot add more washer fluid if you run out of washer fluid on a long trip!

From Wikipedia, you can see the flash points of alcohol-water mixture.

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A quick search for "washer fluid MSDS" yields a material safety datasheet that lists the flammability limits for methanol as follows:

Lower Explosive Limit (LEL): 6 % (by volume)
Upper Explosive Limit (UEL): 36 % (by volume)
Flash point: 90 °F
Autoignition temperature: 878 °F

So methanol will readily vaporize (flash off) if it comes in contact with a hot surface (90+°F). It will readily ignite without spark above 878 °F. Exhaust manifolds can reach that temperature.

To ignite in the presence of a spark, methanol needs to be within the lower and upper flammability limits, which will be governed by its concentration levels.

  • I completely failed to consider the exhaust manifold and autoignition. I know that exhaust manifolds can get hot, as I accidentally touched one a long time ago. Perhaps there's some sense in the warning if you're careless, but then again exhaust manifold is quite far away from the washer fluid filler. – juhist Jul 29 '18 at 15:37
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Highly unlikely to catch fire. Alcohol level is low , maybe 20 % max. I have added 50 % to water for washer fluid and you get a MUCH stronger alcohol smell in the car than any commercial mix. So there is not enough alcohol vapor to reach the explosive limits in the commercial mix. Dripping hydro carbon onto a hot metal surface has a low risk of ignition. We did a lab test dripping gasoline on a 800 ( +/- ) F steel surface and had no ignition. A spark will ignite it but this is a low risk with modern engines and individual coils ( no bunch of high voltage wires from a distributor leaking sparks ). Warnings such as this are often caused by a single exceptionally unusual case where there was a problem and some lawyer sued somebody. Before your grandfather was born , alcohol water mixes were used in car radiators , this caused fires with large leaks hit the old distributors ( 1940's).

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