My truck has been having an erratic idle lately, and I have eliminated everything I can see in the engine bay as the culprit. I seem to hear a vacuum leak "hissing" sound inside the cab. I searched around and found that the noise is coming from somewhere on the passenger's side under the dash, right around where the heater box is located. There is no coolant leaking, but I can definitely hear some sort of vacuum leak under the dash there when the engine is running.

What in the world can this be? And before I tear into my dash, can somebody tell me what I am supposed to be looking for?

 

I didn't think these truck had vacuum operated HVAC systems... Mine doesn't, but it's basic only has defrost and 'heat'. Other than that vacuum shouldn't be coming inside the cab.

 

I have a later bodystyle, and my heater controls are definitely vacuum operated.
There is a line that comes from a three port vacuum manifold high on the firewall near the center of the cowl and runs into the cab, over below the heater hoses.

Perhaps you can disconnect this and see if the hissing stops.
If so, I would think one of those brittle plastic lines has cracked.

 

The vacuum runs all the way to the HVAC control valve, and then back into the engine compartment. I've known the controls to leak in addition to the lines themselves. And, the "motors" could leak, although I've not seen that.

Like Jim says, take the line off in the engine compartment and see if that stops the leak in the cab. If so, then start tracing. On several trucks I've seen, including my "driver", the line was broken where it goes through the heater box almost on the firewall inside the engine compartment. There's a fitting there that connects the hoses on the engine side with hoses on the cab side, and since it is in the heater box it could sound like it is in the cab.

Actually, there are two hoses at that point. IIRC, one is the vacuum supply into the cab for the HVAC controls and the other is a line to control the recirculate motor that sits in the engine compartment on the passenger's side. The little fitting on the box seems to be brittle and easily broken. If it is the vacuum source to the cab then the controls shouldn't work properly and may be in Defrost mode since that is default. If it is the one coming back out then you are probably locked into Fresh Air mode and can't go into Recirculate.

I fixed my broken fitting by drilling it out and putting a piece of very small copper tubing through and reconnecting the inside and outside hoses. And, I dribbled gasket sealer into the recess on the outside to seal it against water or air.

 

These guys have nailed it pretty good. Factory A/C uses vacuum to control almost everything HVAC related aside from the blower. I don't know when this started, by even the 74 listed in my sig is this way.

 

My mom's 1970 Torino was this way, too.

 

I've done this with brass tubing from the hobby shop and windshield sealer.

If it's just the plastic line, it is easy enough to arrange a splice by slipping both ends into a short length of rubber vacuum hose.

 

Yes, after I sent the post it occurred to me that my tubing was brass and that it came from a hobby shop originally. So, I was hoping that if L85 found that was the problem he'd come back and ask questions. Glad you clarified that, Jim.

And, the runny RTV windshield sealer would be maybe even better than gasket sealer as it gets into every nook and cranny then sets up pretty well. I used it on the windshield of Dad's '81, but didn't have any handy when this project came up.

 

I did this exact thing on the 95 B2300 (ford built, mazda branded)

 

Lariat 85,

It has been awhile, but I traced for a leak on a 1984 F150. Under the glove compartment and by the right kick panel, there is a splice/connector which is made of clear flexible plastic. Check it to be sure no one has accidently "put a foot" to it. The vacuum line then goes towards the multi-ported HVAC control valve and then to the various servo motors controlling the blend doors. A quick fix for a broken plastic line is to place a short piece of very small ID rubber hose [7/64"???] between the broken area, as ArdWrknTrk mentioned.

The problem originated when the owner had a new radio installed and one of the tiny vacuum lines was pulled loose from a servo motor. Easy fix!

Is your ventilation system working okay? Often, with a vacuum leak, regardless of control valve setting, the air will be directed to the defroster when under low vacuum engine conditions [i.e. climbing a hill or accelerating].

 

The ventilation controls seem to be working normally.

However, I noticed that if I turn the vent on, my RPMs on my tachometer drop slightly, and then raise back up when the I turn it back off.

I never noticed that before. Is this normal?

 

Check your voltage while you turn the vent on, I'd be suspicious of the fan motor sucking "too much" power.

EDIT: Is it the AC compressor coming on?

 

Or, referring back to the initial post, perhaps there is a leak [after the HVAC control valve] in the line going to the servo motor for vent position of doors. This would cause a lean condition when using vent.

 

No A/C.

Where is the servo motor located?

 

Under the dash... others can provide diagrams, but I will go from memory. There are flaps [doors] controlling the various routes of air flow for the HVAC system. On the right hand side, behind and above the glove compartment, is a door which controls whether the air source is outside or recirculate. From there, the air goes either to the heater core or bypass. This door can be seen when you replace the heater core. It is to the left of the core, as the air flow progresses from right to left. Then, there is a door which now directs either cool or heated air [depending on whether the air has been directed through the heater core] to the dash vents. Finally, there is a door which allows the air to be directed to the floor or defroster.

The doors are controlled by the HVAC control valve, which is behind the dash on the other end of the selector slides. It simply aligns vacuum ports and sends signals to the servo motors. The valve is plastic and can crack due to age. As of 4 years ago, the part was still available through Ford. Before replacing it, though, check the vacuum lines.

After removing the glove compartment and radio, everything can pretty well be seen. I remember the servo motors all having one line except for the one up near where the factory AM only dash speaker would be. That one has 2 ports, since I believe it is the one which allows for defrost/heat mix.

I hope this helps and that my memory is close to being correct.

P.S. The truck I worked on had factory air.

P.P.S. Before going further, why not cap off the vacuum line which passes through the firewall above the glove compartment? Normally, this is supplied by the black coffee can or plastic sphere vacuum reservoir. By doing this, the possible leak under the dash would be completely bypassed and then you can see if the idle situation clears up, before digging into the dash.