Dear Car Talk:
Sometimes when I go into my garage late at night, I can hear a humming noise coming from my 2005 Tacoma. It sounds like a small electric motor running. What is it, and will it eventually run down the battery? — Randall
What’s it humming, Randall? If it was a German car, it would probably be “Ride of the Valkyries.” I’d be curious to know what Japanese trucks hum.
What you’re hearing is the evaporative emissions pump. In the old days, fuel systems were vented directly to the outside air. Every gas cap had a hole in it to let the gasoline vapors escape. But that was a huge cause of smog. So fuel systems were redesigned and sealed to trap gasoline vapors. Inside a sealed system, we now capture gas vapors in a charcoal canister and then reintroduce and combust them when the car starts up again.
What you’re hearing is a pump that pressurizes your fuel storage system (the tank, filler neck, gas cap, evaporative canister) in order to test it, and make sure it’s sealed. As long as the system holds its pressure, the computer concludes that everything is fine, and it stands down.
But if the system can’t hold pressure — if the pump has to come on too often or for too long — then the computer assumes there’s a leak from which gasoline vapors are escaping, and it will turn on your check engine light.
So if your check engine light hasn’t come on, there’s nothing to worry about yet, Randall. If it HAS come on, first check to be sure your gas cap is on tightly and correctly. That’s the single most common source of a fuel system pressure leak. And if that doesn’t turn the light off in short order, you’ve bought yourself a trip to the mechanic to figure it out.
Either way, stop sneaking up on your truck like that in the garage, Randall. Give it a little privacy at night, eh?
Got a question about cars? Write to Car Talk write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.