A/C posessed!

Hi, I have a 78 Caprice, I changed the intake for an edelbrock, When I turn on the AC it works great until I push the pedal, the doors close and air go out throgh the defroster ones, When I return to normal acceleration the air retun to the doors in the front, I know that doors work with vacuum and somebody told me that to avoid that I have to install a vacuum check valve soemwhere, to avoid this changes, Does anybody knows this or another way to keep the air flowing ok?

the reason the doors are closing is that they are kept open by vacuum pressure. when you accelerate, there is less vacuum generated by the engine, and the 'doors' are closing. you have a vacuum leak somewhere in the system. i would check all your hoses and connections for leaks. its possible that the new eidelbrock has an extra vacuum port that you need to plug.

Your '78 may also be equipped with the "Orb of Power", a.k.a. "Sphere of Suction" or "Globe of Nothingness" - Sometimes even called a vacuum reservoir. It your's is cracked, split, damaged, or leaking for some other reason not listed, the symptoms would be very similar. There is just enough vacuum at idle to hold the damper positions, but under load the reservoir or check valve fails to hold vacuum, and the dampers revert to the default positions (full defrost).

Yes it has the "sphere" maybe is connected backwards, because is nor broken or anything, do you know how is connected?

if you can find another vacuum resivoir, you can just hook it up to the other one and see if that helps. if it does, then you have a leak. otherwise something is just broken.

Hi,

Hopefully this helps, I am not sure about a 78 Caprice, but I have a 73 and 91 Caprice. Both developed similar problems. There is a small check valve on one small vacuum line that leads to manifold vacuum. This vacuum line supplies the vents with vacuum selected by the HVAC control on the dash. I have found the rubber dries out in this small inline check valve and leaks. Resulting in what you are experiencing. I went to Napa, and in the "Help" carded section, they have a generic replacement that works great.

Thanks I will check that I have bought the valve you say, but as my car does not have one at the beginning, do you know how is connected and what side?
Thanks.

Hi,

You can simply add the check valve to the existing line, leaving the old checkvalve in the system, if you can't locate it. Start off by finding the manifold vacuum port on the engine, in 1978 if I remember correctly there are a myriad of vacuum hoses strung around the engine to thermal port switches and such mostly for emission controls. The hose you'll be searching for leads back to the firewall, it will be of similar size to that of the vacuum hose for the transmission modulator. Once you found that hose observe the flow arrow on your new checkvalve (might have to refer to packaging for diagram) if not clearly labeled. The one I bought had two different colors denoting the flow direction.

You'll want to place the checkvalve in line with this vacuum hose with the flow or arrow pointing toward the engine.

That should cure that, if not, then you have a vacuum leak in the HVAC control hoses, reservoir (if the car has one), dash selector, or door diaphram servos. You'll need a small hand vacuum pump and gauge (cheap one sells at autozone, or napa) then isolate individual components and test.


Hope this helps,

Keith

I just bought the valve I will install it this weekend and let you know.
Regards.