left gas cap open in rain

Left Gas Cap Open in Rain, Should You be Worried?

We sometimes need to go to a place in a hurry, and during a fill-up, unbeknownst to us, we could let the gas cap open in the rain. Is it something you need to worry about? Let’s get deeper on this before jumping to conclusions.


How Long Has it Been in the rain?

First, check if your outer cap is still closed. If it is, then your car will be just fine. Modern fuel tanks with a gas cap on are designed to be airtight. Also, the fuel tube insertion is angled almost horizontally, so not much water will likely be seeping in the rain.

If your outer cap is opened, check on how long you’ve left it out in the rain. One hour of medium rain is enough to change the water and fuel composition in the tank and require you to drain out the fuel. In addition, water is denser than fuel, and it will settle to the bottom, so it is invisible to the naked eyes. 

Now, let’s say it’s a pretty intense rainstorm, and the rain is coming down diagonally. There’s an increased chance of some water getting past the door or even making it through those two tiny vent holes on either side of the pipe. There’s a chance you’ll get about a tablespoon or two of water in your tank.

How Much is Water in the Gas Tank Safe?

Non-ethanol fuel blends can be up to 0.5% by volume, while pure ethanol fuels are up to 10% by volume. To translate, anything more than natural condensation is terrible for your engine. Up to a quart of water, it will just sit in the bottom of the tank. If your fuel has any ethanol, E10, E85 will blend with the water, carry it to the engine, and burn it.

The real pain begins when water finds its way to the engine. The car won’t likely start.

Most gasoline these days is ethanol-based, which means it will absorb water from the air. If it’s not much water, the water separator on your fuel filter should take care of it.

How to Remove Water from the Gas Tank?

You can check with the mechanic on how much water has contaminated your gas tank. Your light engine should give you a warning first to ensure they have a method to check. They will put a paste on the bottom of a stick to see any water in the tanks.

Up to a quart of water, they will recommend below.

left gas cap open in rain

  • Add a can of B12 Chemtool at your next fill-up. It’ll remove water from the fuel.

  • ISO-Heet is a fuel additive, and you can use this also, add 1-2 oz per gallon of gas, be careful with too much HEET could burn a valve.

More than that, they will recommend draining the tank.

What are the Symptoms of too Much Water in the Gas Tank?

Having water in your tank will cause severe damage to your engine. You might see minor performance issues start, and then it can lead to more severe problems that may require a new engine and fuel. The presence of water can affect the combustibility of gas by hindering the burn rate. It will not only cause your car to shake, but it could also lead you to think your car’s transmission is slipping if you press the accelerator.

  • Examine if your check engine light is on. This is the way your car complained of something wrong with the fuel intake.
  • If you hear a thump or your engine is acting sluggish
  • Your vehicle could be sputtering if you let too much water in the tank.

Fix and Mitigation

You should get your car scanned at an auto parts store and replace the gas cap if it does not seal well. It’s always a good idea to guarantee the gas cap is tight. If you want to drain the fuel yourself, there should be a drain plug on the gas tank’s bottom, or you can pump it out with a power pump.

left gas cap open in rain

Remember to take care of your leftover fuel. Depending on the states, disposing of old gas is a problem you need to be concerned about, more than just leaving the gas cap open in the rain. The repercussions for a gas spill at home without a gas line are enormous. You might cause a fire or explosion that will be investigated by law enforcement, and you may also incur fines of one million dollars.

In addition, some big gas stations have a dump barrel to dispose of oil from car oil changes. Please take advantage of this resource when it is offered, as it will keep your environment clean!

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