Heater control / vacuum system problem

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eaglecap

Member

Jul 31, 2000

66

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7

Jun 24, 2005

#1

• Jun 24, 2005

• #1

1975 Ghia with factory air. If heater/AC control is set to Vent air flows OK when the car is idling or running at highway speed. However, when I accelerate the air flow moves to the defroster. After I let off the gas or the car gets up to a steady speed then the air flow goes back to the dash vents. Other control positions have the same type of failure. I figure it is a vacuum pressure problem (since vacuum drops when accelerating) but do not know what is causing it. I just rebulit the car and all the hoses are new. Checked for leaks and found none. Whats keeps vacuum pressure steady on the HVAC controls when the engine vacuum changes??

 

Wart

I'm Mad as HELL and I'M not Gonna Take it ANYMORE!

Founding Member

Sep 1, 1998

408

3

58

NE Ohio

Jun 24, 2005

#2

• Jun 24, 2005

• #2

The heater box defaults to defrost when it loses vacuum.

Check the check valve, and make sure you still have a vac can.

 

78CobraII

Moderator

Jul 31, 1998

1,808

8

39

Birmingham, AL

Jun 24, 2005

#3

• Jun 24, 2005

• #3

Yup...the vacuum can will pretty much rust away to nothing. I kept using sealer on mine until it finally just collapsed in on itself. When I went to Ford for a replacement (nearly 15 years ago!) the service replacement was a Fairmont piece.

Let us know if you find a new replacement can.

In a pinch, you could use a coffee can...the only problem would be cutting a 3/4" or so hole in the top of it without ruining the coffee!

 

E

eaglecap

Member

Jul 31, 2000

66

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Jun 24, 2005

#4

• Jun 24, 2005

• #4

OK, those seem to make sense. Now, do you know where the check valve is located on a '75 V6? I assume by vac can, you mean some type of vacuuum reservoir. Do you know where it is on the '75 V6? If it had one it has to still be there. I am the original and only owner and I pulled and replaced the engine myself when it was rebuilt this year.
The only reference I find in the 74-76 Chilton Manual is a venturi amplifier schematic which shows a reservoir and check valve that has to do with holding vacuum steady for EGR operation during acceleration.

 

T

THE COBRAMAN

pig 'rassler

Founding Member

Mar 11, 1999

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36

Oklahoma

Jun 25, 2005

#5

• Jun 25, 2005

• #5

The reservoir is usually mounted to a bracket screwed to the door hinge pillar on the passenger side. It's near the upper hinge, and is forward of the hinges. You'll probably have to pull the liner to access it. Liner on all but '78s is held in by 2 sheetmetal tabs that protrude through slots and are bent over the liner.

You should see a vacuum hose on top of the cowl pinchweld that disappears under the inner fender. That's the source for the reservoir.

 

E

eaglecap

Member

Jul 31, 2000

66

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7

Jun 25, 2005

#6

• Jun 25, 2005

• #6

I haven't found the can yet but I did find the two vacuum hoses that go into the fender well. I will have to see if I can find these tabs you are talking about. Funny what you forget. Sometime in the last 30 years (the car has been garaged for the last 10, rebuild just completed) I bypassed the can. Do not know why but probably because the leak caused the engine to run bad. I plugged one hose and blew on the other, no resistance, so the can is probably shot. Maybe I can get an aftermarket one that will fit.

Do you know if this system has a check valve as well? The vacuum port out of the intake manifold is a large connector that has 4 ports, two are plugged, one goes to the vacuum can and the other to the brake booster system. I was wondering if that connector on the manifold is suppose to include a check valve.

 

C

cobraii351

Founding Member

Jan 10, 2000

1,104

4

39

Altoona, PA

Jun 26, 2005

#7

• Jun 26, 2005

• #7

Ford rangers use the same diameter can just longer. Like a Hi-C can. The check valve is in the top of the can, making it VERY difficult fo replace. If you find a car in the bone yard that has just 1 line going to the can closer to the engine there should be a check valve with 3 hoses coming out of it. That is the check valve you need if you can only find the 1 hose cans. I have a couple of ranger cans and they work great.

 

Wart

I'm Mad as HELL and I'M not Gonna Take it ANYMORE!

Founding Member

Sep 1, 1998

408

3

58

NE Ohio

Jun 26, 2005

#8

• Jun 26, 2005

• #8

Thsis is the check valve, at least on my '78 V6 Mach:

 

E

eaglecap

Member

Jul 31, 2000

66

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Jun 27, 2005

#9

• Jun 27, 2005

• #9

Thanks guys. Mine has two hoses going to the can in the fender well so the valve must be in the can. Mark's picture shows me an alternative anyway. Now I have to see what I can pick up. Parts place said they have a universal vac can kit for about $40.

 

Wart

I'm Mad as HELL and I'M not Gonna Take it ANYMORE!

Founding Member

Sep 1, 1998

408

3

58

NE Ohio

Jun 27, 2005

#10

• Jun 27, 2005

• #10

Heres the hellofit,

When I got this car about all the hoses and clamps were factory. There is little to suggest this valve wasn't factory or a Ford dealer install.

 

E

eaglecap

Member

Jul 31, 2000

66

0

7

Jun 27, 2005

#11

• Jun 27, 2005

• #11

Got the can out. Had a rust hole in the bottom. Bottom of the can is now a layer of JB Weld. Has a two hose connector on top that seems to be more of a restricter than a check valve, but it could be bad. Will put it back after the JB Weld sets up and test it. If it still fails I will put an inline check valve in. Thanks again for the help.

 

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cobraii351

Founding Member

Jan 10, 2000

1,104

4

39

Altoona, PA

Jun 28, 2005

#12

• Jun 28, 2005

• #12

The 78 I just got rid of had a 1 hose can. I think the later ones must have switched over. The 2 hose insert in the top of your can does the same thing as the 3 port valve that Wart posted a pic of. 1 is vaccume from the engine. the other goes to your heater setup. You should notice a difference in your idle as well not that you fixed the vac leak you had. The inline check valve is not going to help you. The can what you are really going to need. you will only need the 3 port valve if you replace the can with one that only has 1 port.