vacuume leak problem '91 300e

Hi,
Here's the symptom: the dial for controlling the temp no longer works. If it's on AC it stays there even if I put it all the way to hottest. Then I
noticed, if I rev the engine, it switches to the heater and works fine.
I'm guessing it's a vacuum leak somewhere. Is this a back yard mechanic job, or something that requires intelligence to fix?
Thanks,
al
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Speaking for backyard mechanics everywhere, I take offense to that.
If I were to charge 4 times as much and get a labotomy, I could then work at the dealership.
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Ha ha. Lobotomy, good one.
I was referring to my expertise as a back yard mechanic.
I don't even have a back yard. I own a hammer, though.
No offence intended as it was self-referential.
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You are on the right track... check the vacuum hose that leads from engine to your cabin... in the hood area only.
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Hi Tiger, OK, will check. Can you give me a clue about where one end of it might be? Under the hood is a fairly large place if you don't really know what you're looking for. Thanks al

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The clue is that yellow checkvalve... one in/two out... located by the brake reservoir area.
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Thanks for the info. Now, as soon as it stops raining...
Oh yeah, I looked up vacuume and discovered I spelled it wrong. Looks like the e isn't needed.
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What would a vacuum leak have to do with the temperature calibration?
Note: "vacuum" is spelled vacuum.
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Ya know... heater flap control?
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