A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Does adding more gas line antifreeze than recommended hurt or help your car?

February 24, 1984

Dear Cecil:

Does adding more Heet gas-line anti-freeze than the label recommends help? Does it hurt?

Cecil replies:

We must learn, Mike, to resist the notion that if a little is good, a lot is better — a typically American idea that has given us megavitamin therapy, expansion-team baseball, and the Vietnam War. Adding more Heet (or any gas drier) won't help much. If you've got more than an ounce or two of water in your gas tank, which a 12-ounce bottle of Heet will sop up adequately, you'd better take your car to a garage to have the fuel system drained. On the other hand, a little more Heet won't hurt all that much either. It's basically methyl (wood) alcohol, which is heavier than gasoline and consequently sinks to the bottom of the gas tank. There it mingles with any water in the tank, which also sinks to the bottom. The resultant mix is combustible, although just barely, so when it gets pumped through your engine your car may sputter a bit. In the absence of water all Heet does is raise the octane of your gas slightly. A lot of extra methyl alcohol will crud up your fuel system, so my advice is to quit experimenting and do like the label says. Bear in mind that antifreeze is a preventive measure — if your gas line is already frozen, all you can do is wait for a thaw.

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