Vacuum Leak - Under the Dash?
#18
10-18-2011, 09:47 AM
Posting Legend
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.
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.
If that eliminates the vacuum leak sound and the change in idle then go looking for the leak by following the vacuum line. As said before, the first connection you'll find is where the line goes through the heater box - at least it is on A/C trucks and I think it is on others.
From there the vacuum goes to the HVAC controls. You can pull and plug the connections along the way, and as 1986F150six says you can see them with the glove box and radio out. From the controls, which certainly can leak as I said before, the vacuum fans out to the various "motors". You can test them, assuming the tests to that point haven't turned up the leak, by moving the controls to the various positions and listening for the leak and watching for movement.
Edit: I didn't realize there aren't vacuum controls for the non-A/C units. Disregard my post.
#19
10-18-2011, 09:57 AM
Lead Driver
#20
10-18-2011, 10:00 AM
Posting Legend
#21
10-18-2011, 10:25 AM
Lead Driver
#22
10-18-2011, 11:19 AM
Post Fiend
L85, was your truck factory A/C or not? As ct pointed out, non-a/c heater/vent systems are totally cable controlled and do not use a lick of vacuum.
#24
10-18-2011, 12:40 PM
Originally Posted by Rogue_Wulff
L85, was your truck factory A/C or not? As ct pointed out, non-a/c heater/vent systems are totally cable controlled and do not use a lick of vacuum.
I think I may have found something...
I put my hand over the carburetor as the engine was running and it did not stall. So I guess that means there is a vacuum leak somewhere. I went messing around, and discovered a "hissing" sound coming from around the PCV valve. I pulled the PCV valve off the valve cover and the idle did not change much at all. I would have thought this would be a MAJOR vacuum leak? I shook the PCV valve and it rattles, and there is a rather strong suction on my finger. But, I can turn it around in the grommet and the "hissing" sound changes. And sometimes when I remove the PCV valve, I hear a LOUD "sucking" sound coming from the valve itself, and at other times there is no sound at all. In both cases, the PCV valve still has strong suction on my finger.
Does that mean anything?
#25
10-18-2011, 05:56 PM
The PCV is ALWAYS a vacuum leak.
Not an issue there...
The fact that it flows both ways is a problem however.
Cheap enough to replace.
Do you mean to say that there is suction coming from the valve cover?
If you can't find an external leak with propane or starting fluid you might have an internal leak between the lifter valley and an intake port.
Are any one of your plugs unusually oily?
Not an issue there...
The fact that it flows both ways is a problem however.
Cheap enough to replace.
Originally Posted by LARIAT 85
And sometimes when I remove the PCV valve, I hear a LOUD "sucking" sound coming from the valve itself,
If you can't find an external leak with propane or starting fluid you might have an internal leak between the lifter valley and an intake port.
Are any one of your plugs unusually oily?
#26
10-18-2011, 09:51 PM
Originally Posted by ArdWrknTrk
The PCV is ALWAYS a vacuum leak.
Not an issue there...
Not an issue there...
Originally Posted by ArdWrknTrk
Do you mean to say that there is suction coming from the valve cover?
#27
10-18-2011, 09:57 PM
Posting Legend
Right. What I am saying is, being that it is a vacuum leak, shouldn't I have noticed a BIG idle change when I removed the PCV valve from the valve cover? I did not notice much of any idle change when I removed the PCV valve from my engine.
No. When I remove the PCV valve and put the end of the valve on my finger, there is a pretty good suction. And that is good. But sometimes, when I pull the PCV valve out, the valve itself has a pretty loud "hissing" or suction noise coming from it, and other times it doesn't make any noise at all.
No. When I remove the PCV valve and put the end of the valve on my finger, there is a pretty good suction. And that is good. But sometimes, when I pull the PCV valve out, the valve itself has a pretty loud "hissing" or suction noise coming from it, and other times it doesn't make any noise at all.
When you pull the valve out do you always hold it in the same plane? IOW, is it always vertical or horizontal? If not that probably explains why it hisses some times and doesn't others as the element in the valve can close if you turn it sideways.
#28
10-19-2011, 12:31 PM
Post Fiend
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As for the PCV valve, I'd replace it, and the grommet it pushes into.
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