Is there really such a thing as a muffler "weep hole"?

Dear Car Talk

Dear Car Talk | Jun 01, 2000

Dear Tom and Ray:

A friend of mine has a '96 Jeep Cherokee two-wheel drive that he took to the dealer recently because of an ever-so-slight exhaust noise coming from beneath the vehicle. The service department told him that the noise was coming from the muffler's "weep hole." This hole is supposed to allow moisture to drain from the muffler. I thought I knew quite a bit about automobiles, but this is a new one to me. I have a hunch that the local service department people are yanking him around. Can you shed some light on the legitimacy of the muffler weep hole? -- Greg

TOM: It's completely legit, Greg. Believe it or not, almost every muffler has a weep hole. And the purpose of the weep hole is to allow water -- a byproduct of the engine's combustion -- to escape. That way it won't sit there and rot the muffler from the inside.

RAY: They're usually inaudible, but sometimes you can hear them if you have a highly developed ear for exhaust noises. It's also possible that the guy with the awl who banged the hole in your friend's Jeep might have been hitting them a little too hard with the hammer that day.

TOM: The simplest test is to compare the car's sound to another '96 Cherokee at the dealership. If it sounds the same, the dealer is probably right.

RAY: Or, you can help your friend by plugging up the tailpipe with a rag. When you plug up the end of the tailpipe, the exhaust will be forced through that little weep hole. And it should make a noise almost like a low whistle.

TOM: Then, if you also plug up the weep hole (do this before the exhaust system gets hot if you value the skin on your fingers), you'll be able to tell if there's a leak anyplace else.

RAY: If there is a leak, you'll be able to hear it and even feel it immediately. And if there's no leak, the car will stall because the engine can't run if there's no place for the exhaust to go.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One