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Discussion Starter #1
Hi,
(1967 Cougar XR7, carb 302 '87)

Im having a problem with an increase in idle when the engine gets up to operating temperature. When the engine is cold it idles at 750 in park, and it drops about 200 rpm when put in gear. When the car warms up after some driving, i have an idle above 1000 rpm in gear, and about 1200 rpm in neutral/park.

I have a 750cfm holley 4160 on the car, which has an electrical choke that is not connected. Its got a mallory ignition without vacuum advance.

Am i looking at a vacuum leak or something else? I also have a hissing sound during braking, like there is air coming out of somewhere under the dash. Could this be related to the high idle? If not, what could the hissing sound come from?

Thank you
 

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I'm thinking a vacuum leak here, under the hood, and at your headlight switch..
 

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^^Yepper - I'd lay $ on that one....
 

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Discussion Starter #4
Do you really think its the headlight switch when its only hissing during braking?
Need to find a good overall layout of all the vacuum hoses in the car. Will have to dig into the shop manual.
Thanks!
 

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Well, if it's hissing during breaking you may have a bad power booster - there's a diaphram inside it that may be leaking and causing that hissing sound. Can you change the way the car idles by stepping on the brake? Another good way to check for vac leaks is to use a can of carb cleaner and spray around areas prone to vac leaks - like carb base, vac lines, intake, etc....
 

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Discussion Starter #7
This may help 1967 and 1968 Vacuum Diagrams
from the Cougar Club of America web site.
Thank you, all of the vacuum hoses have been cut and there is no control valve in the car. Ive tried to plug everything I've seen been cut but ill check again.

Well, if it's hissing during breaking you may have a bad power booster - there's a diaphram inside it that may be leaking and causing that hissing sound. Can you change the way the car idles by stepping on the brake? Another good way to check for vac leaks is to use a can of carb cleaner and spray around areas prone to vac leaks - like carb base, vac lines, intake, etc....
Yes, i forgot to mention that i can change the idle by stepping on the brake. It will go down a few 100 rpms by doing to, but its still a bit high. This confirms the bad brake booster, right?
Ill get some carb cleaner to spray around prone areas. Im guessing i have a leak at the carb base since it only occurs when the engine is warm .

Thank you all!
 

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Discussion Starter #8
I've been looking for a vacuum leak but cannot find any. Ive sprayed at all the prone areas in the engine bay without finding anything.
Havent changed the brake booster yet, but could that be only reason for this phenomenon? Shouldn't it be affected already when its cold and not only when its up to normal operating temperature?
 

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My guess is that that is at least one of your likely problems....you can test it by capping off all the vac lines and see how it runs that way....then bring them back "online" one by one (starting with booster) - step on the brake - if it stumbles - you found one. Divide and conquer - it will help you find bad lines, etc.....
 
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