Hissing under passenger floorboard

[IBrokeDadsCar]

BACKGROUND
Hi everyone. I had a hissing problem from under my glove box about a year ago every time I turned the car on and off, and they found it to be a cracked vacuum reservoir (under the glove box as well). Upon replacement, all seemed pretty much fine.

Later on I noticed that my car was blowing air out of the defrost vents whenever the gas was pressed and the vehicle was under load. Taking it back for repair, they found it to be a loose vacuum line from the initial repair, which they reconnected. All seemed fine again.

CURRENT ISSUE
Lately, I seem to be hearing this hissing again but I can't tell if it's normal, so I'm asking the forum. I hear it every time I turn my HVAC on or off. It does not happen when I press the gas (like others on here) and I no longer have problems with the vents going to defrost under load (like even more on here). Do others with 2006-2010 Explorers have similar issues to this? It only seems to hiss, which almost sounds like the vacuum chamber going into a vacuum state, when the AC is turned on or off, no other times.

 

[JPGator]

I have the same exact problem. Problem started showing up around 42K miles. I get hissing under the dash anytime I accelerate and ac defaults to defrost on a load (going up a hill, etc). Sounds like when you would normally turn the AC on/off. Have noticed a slight hit to gas mileage since this issue showed up.

 

[IBrokeDadsCar]

It's not a hard fix for a DIY guy. Mine was all under warranty before. Just check your vacuum lines under and behind your glove compartment. If you have manual HVAC controls (knobs), vac lines go to those, as well. In an Auto HVAC set-up, no vac lines go to the back of that unit. Check the check valve in the engine compartment just outside the passenger firewall (black and white valve) in the vacuum line. It should only be allowing air one way.

My question still stand though, do other owners with properly functioning HVAC systems have this same hissing sound when their HVAC kicks on or off?

 

[JPGator]

Yup, turned out to be the check valve as I found out from other posters. $15 and 5 minutes and it was fixed. No more hissing sound. Sure beats the $89 the Ford guys wanted to charge me just to diagnose the problem. Can anyone reply to if this will be a recurring issue?

 

[IBrokeDadsCar]

Yup, turned out to be the check valve as I found out from other posters. $15 and 5 minutes and it was fixed. No more hissing sound. Sure beats the $89 the Ford guys wanted to charge me just to diagnose the problem. Can anyone reply to if this will be a recurring issue?

So then, now that your hissing is fixed, does it hiss each time you turn your HVAC on and off? It should hiss only for a second. Interested to see what other working systems sound like or if I have a problem.

 

[JPGator]

Yeah, I can hear it hiss/change pressure for the few seconds only when I turn the HVAC knob on/off or between settings (max AC, dash, defrost, etc). I haven't heard it hiss/change pressure otherwise since I installed the new valve so it seems that was the issue. I tested the old valve and it was allowing air to flow both ways. When I tested the new valve it wouldn't let me blow air back through the direction of the incoming vacuum line so it seems that was the issue for me.

 

[IBrokeDadsCar]

Great, assuming yours is working correctly, that means mine is, as well. Hopefully this thread will help others in diagnosing their issues, too. I can't speak as to whether the check valve will be a recurring issue, but considering it is a simple part and easy to access, I would take that as a repeat problem over something more costly and inaccessible. Thanks again for your help!