Diagosing a vacuum leak isn't unlike diagnosing a heart condition, Adam.

Dear Car Talk

Dear Car Talk | Jun 01, 2002

Dear Tom and Ray:

I'm a cardiology fellow at the University of Virginia. A graduate-student friend of mine has a 1995 Ford F150 that he loves more than a child. He also listens to his truck with deep intensity. Because I am a loyal fan of your column and because he thinks you are a couple of pedantic car quacks, I've decided to get your advice on his car to prove your skills. In the past month, he has noticed a hissing sound near the intake manifold during the peak of hard acceleration. It's there when the engine is cold, and it gets louder when he releases the gas pedal. He thinks it is a manifold gasket, but I don't know what that is. Could you please give me some advice that I can give to him and that would make a layperson sound intelligent? -- Adam

RAY: Of course we can, Adam. The first thing you have to do is establish that you "speak the language." You can do this by "clarifying" a few of the facts you already know.

TOM: It would sound something like this: "It's an F150, huh? And the noise gets louder when you take your foot off the gas?" You're establishing here that A) you know a great many types of cars; and 2) you know that when a hissing noise gets louder at idle (when the engine is producing the most vacuum), it's a textbook case of a vacuum leak.

RAY: Then you want to break a little new ground. So you ask, "Is it a 4.6?" That's a reference to the size of the engine in liters. No matter what he says back to you, you just nod your head in knowing recognition, as if that answer has significant meaning for you, too, and is leading you unwaveringly toward the answer.

TOM: Then you take your swing: "I think it's a vacuum leak, but I don't think it's the manifold gasket. I think it's the gasket between the throttle body and the intake manifold."

RAY: He'll probably say something like "Think so?"

TOM: You say, "Well, you've checked the vacuum hoses that come off the manifold, right? I mean, I assume you've checked for cracked hoses already."

RAY: And he'll probably say, "Oh yeah," whether he has or not, while making a mental note to check the hoses for cracks. And then, before you can get yourself in any more trouble, turn and walk off with the line "Let me know what you find."

TOM: And if it turns out that you're right, send him a bill for $300 for the diagnosis ... just like cardiology, Adam.

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