Vacuum Leak!

[5.0 Mountaineer]

My mounty has a vacuum leak I cannot find. When I have the heat or a/c on, shifting is a little sluggish and I'm losing some performance. I figure its probably a vacuum leak because under acceleration above 2,500rpms the air flow from the vents goes to the defroster. With the the selector in the off position the truck is more responsive and idles better. I was wondering if you guys knew any common vacuum leaks.
Also the switching of the air flow only happens when the truck is in gear. If i rev the engine when it is in park it doesn't happen. Thanks in advance, Steve

 

[Runnin'OnEmpty]

Steve, since the idle is better with the selector off, the vacuum leak must be in the controls or hosing in the dash, or a bad vacuum motor in the plenum. I would try listening real closely with the engine at idle while moving the controls, and see if the leak shows up. You could also look under the dash with a flashlight, to see if any hoses are coming loose.

 

[machman706]

I'm having the same problem with my 99 5.0 Exploder....I've been looking for this vacuum leak forever and have not been able to locate it. Mine seemed to do the same thing in the summer as well when running the air conditioner as well. Whenever you give it the gas to go up hill or if it downshifts then the heater/AC will stop working while the blower motor is still running. I believe we are having the same problem. If you find anything, please let us know.

Ben

 

[Bashman]

You can also try using propane from an unlit torch. With your engine idling, move the nozzle around the vacuum lines and connections and when your engine accelerates, that's where your leak is.