So when i am under load, or rev it up in park, i hear what sounds like a vacuum leak or air leaking behind the dash, cant hear it in the motor, but right after it shifts gears it stops for a second and comes back, im not sure what it is since the hoses going to the heater core are filled with coolant? Any help please

 

What are you working on?

 

97 f150, not sure what engine...

 

4.6 v8, sorry did not mention

 

Check your climate control lines and your brake booster.

 

May be a dumb question but i thought only the square bodies had brake boosters, and the vacuum is coming from almoat under the windshield in the middle of the dash when at about 2500 rpms

 

Yes, your truck has a brake booster...

In any event, from what you are mentioning, it sounds like it is the vacuum lines that control the hvac.

 

yes that is what i am thinking but i have taken the dash apart and cannot seem to find a line or anything the noise is coming from

 

Then it might be on the other side of the firewall in the engine bay, behind the engine.

Is the service engine soon light lit when running?

 

Theres no engine light, and the noise cant be heard from outside the truck. Even when revved up, ive checked both heater core hoses and dont see any leaks

 

Does the HVAC operate as it is suppose to?? meaning when you switch to defrost, air then is directed to the front windshield & the same goes for the other options...floor, panel...etc. without much hesitation or clicking?

If that works....then it might be some debris hitting the HVAC fan?? does it change tone with a touch of the throttle or when changing HVAC fan speed?

Is you transmission fluid at correct level?? ( presuming automatic)

 

Everything works as intended, and the leak noise changes with acceleration and tranny fluid is great

 

How quickly does the noise fade when you turn the truck off?

 

The cabin ventilation system is vacuum driven. There is vacuum drive to the flap that directs the air from the floor to the vent to the windshield positions. When the vacuum goes away, the flap defaults to the windshield vents, you might be inclined to call this the defrost, but it isn't technically defrosting at all.

In any case, open the hood and loom in the corner by the battery tray. You do not _have_ to take the battery out, but it helps if you do. There is a bundle of vacuum hoses that goes into the right side fender, and one of these hoses goes into the firewall behind the glove box. The one that goes through the firewall has a small check valve on it, the hose with the check valve is broken. Among the hoses that goes into the fender is one that goes out to the EGR Solenoid. This one has nothing to do with this problem, but if you ever have an EGR issue, you will need to remember to check this hose when you are diagnosing the problem. It escapes my memory at the moment, but I recall that the EGR hose is white. The hose you are looking for that goes to the Life Support System will be a very small hose, I'm going to describe its diameter as being about the size of a USB cable. It will be tucked between the battery tray and the firewall, on the right side of the truck. The check valve is blue and white as I recall.

 

I know smoke testing your lines can detect leaks in the engine bay, but will it do the same for lines in the cab and behind dash, if so how do i do it