vac line?
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Joined: Oct 2007
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vac line?
1985 ext cab longbed f150 IL6 4wd 4 sp std
while the truck is running driving etc theres a hissing noise, when I fully depress the brake pedal it stops. brakes work fine but its driving me nuts, what is it? where is it? how do I fix it?
thanks ahead of time
Shane
while the truck is running driving etc theres a hissing noise, when I fully depress the brake pedal it stops. brakes work fine but its driving me nuts, what is it? where is it? how do I fix it?
thanks ahead of time
Shane
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
You're going to have to look around the engine bay for an unconnected hose. Several things connect to manifold vacuum on these trucks: PCV valve, power brake booster, AC\heater controls, air cleaner warm-air duct controls, auto trans modulator (not applicable since you have a 4 speed), vacuum reservoirs. Several things run to ported vacuum - distributor vacuum advance, evap purge and EGR. You can check for vacuum leaks by spraying a can of carb cleaner at suspected areas while the truck is idling and the idle jumps. Sometimes it helps to lean out the idle really far for this check so that the jump in idle speed is noticeable, then just tune it back when you're done. Since you said the hissing changes when you get on the brakes, I'd suspect the issue is with the brake booster or the line running to it.
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