June 2006

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Dear Tom and Ray:

You mentioned recently that one could determine the integrity of one's exhaust system by plugging the tailpipe. If the engine stalls, the system is in good condition, and if it keeps running, the exhaust is leaking somewhere. I tried this on my 1991 BMW 535i, and even though I plugged both exhaust outlets and the engine kept running, there were no detectable leaks in the exhaust pipes or components. Does this test still hold true when used on a car, like mine, that has a high-compression engine? -- Don

RAY: Yes, it's true for every car, Don. So either you didn't plug up the tailpipe well enough, or you do have an exhaust leak.

TOM: Cars with dual-tipped exhausts, like your car, are notoriously hard to plug up, though. There's a chrome-tipped cover that goes over the ends of those two pipes. And if you just cover that chrome piece with a rag, exhaust is going to escape around the edges of the chrome cover, even though you don't notice it.

RAY: So you need to block up the tailpipes themselves, which are behind the chrome tips. The easiest way to do that is with a wet rag. Not only does a wet rag conform more easily to the shape of the pipe to block the exhaust better, but it also delays, by at least several seconds, the amount of time it takes for the flesh on your hand to start sizzling.

TOM: If you totally seal the ends of both tailpipes and the car keeps running, then you absolutely have an exhaust leak somewhere.

RAY: And if you can't detect it under the car, it could be under the hood. Remember, the exhaust system is not just the muffler and the tailpipe -- it starts at the cylinder head. If there's a pinhole in a pipe under the hood or a gasket up there that's leaking, that could be enough to allow the engine to keep running. And since the car is a 1991, I wouldn't be shocked if you've got an exhaust leak or three by now.

Back to the June 2006 index

© 2008 by Tom and Ray Magliozzi and Doug Berman. Distributed by King Features Syndicate, Inc.

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