Wrong fluid in fuel tank

An interesting question came up in the Ask a Mechanic forums recently. Trudie accidentally poured brake fluid into her fuel tank and wanted to know whether she needed to drain the tank or if it was OK to drive it with the brake fluid in it.

This happens once in a while - someone pours the wrong stuff into the tank, usually because they bought fuel additive (carb/injector cleaner most often) and accidentally grabbed the wrong bottle. In most parts stores, these fluids are often all on the same shelves.

I've used Sea Foam products to clean the injectors in my vehicles for years and have accidentally purchased the one meant for hydraulics cleaning rather than fule additive.

The rule of thumb is that if the product is meant for your vehicle (i.e. it's petroleum based), if it's in small quantities (less than a pint), it can probably be safely allowed to burn through your gasoline fuel system. With diesel, it will depend more on the water content of the additive and the type of engine your vehicle has.

With gasoline, however, it will likely burn through without a problem if you take a few precautions.

Fill the tank to full to obtain maximum dilution for the accidental additive. Then keep the tank at least half full (i.e. don't let it get below the half-tank mark on the gauge) for a couple of hundred miles or so.

It's also advised that you use Premium gasoline (high-octane, usually 92) rather than the cheaper regular unleaded as this fuel burns "hotter" and will more easily burn off the additive.

There are exceptions to these rules. Two notable ones are: motor/engine oil and transmission fluid. These are thick, viscous petroleum products that can cause problems or add a lot of gunky buildup to your engine if burnt. If these are introduced to your gasoline tank, it's advised that you drain the tank rather than attempt to burn it through. Use common sense, though. A small amount may not be a problem, but a quart or gallon will be.

Be sure to properly dispose of the drained contents. Your local automotive repair shop and some auto parts stores may be able to help.