Vacuum Question on Acceleration

H

Hawkk72

New Member

Dec 13, 2006

47

0

0

Jan 17, 2008

#1

• Jan 17, 2008

• #1

I have a 1975 Mustang ll with a 2.3 L automatic, factory air. It doesn't matter if I'm driving with the heat or air on but when I have the fan blowing and accelerate the dash air switches from either the floor or dash vents to the defroster vents. After letting off the accelerator it switches back to where it should be on either the a/c or heat setting. I had this problem in a Mustang although not to this degree. Any little bit of acceleration sets it off. Any ideas? Faulty vacuum motor or bad vacuum source?

 

Power Surge

Founding Member

Jul 20, 2002

1,516

4

38

Palm Coast, FL

Jan 17, 2008

#2

• Jan 17, 2008

• #2

Hawkk72 said:

I have a 1975 Mustang ll with a 2.3 L automatic, factory air. It doesn't matter if I'm driving with the heat or air on but when I have the fan blowing and accelerate the dash air switches from either the floor or dash vents to the defroster vents. After letting off the accelerator it switches back to where it should be on either the a/c or heat setting. I had this problem in a Mustang although not to this degree. Any little bit of acceleration sets it off. Any ideas? Faulty vacuum motor or bad vacuum source?

Click to expand...

Sounds like your vacuum storage tank isn't hooked up or has a leak and doesn't hold.

 

H

Hawkk72

New Member

Dec 13, 2006

47

0

0

Jan 18, 2008

#3

• Jan 18, 2008

• #3

Is that the one located inside the front left fender?

 

Wart

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

Founding Member

Sep 1, 1998

408

3

58

NE Ohio

Jan 18, 2008

#4

• Jan 18, 2008

• #4

The heater/ac box is spring loaded to move to the defroster position in case of loss of vacuum. It is a safety thing. You roll on the throttle, manifold pressure increases and your box turns to default.

These cars are 30 years old, hard to say what equipment is still installed or plumbed in.

Assuming all factory apparatus is intact you could have a cracked hose or fitting, bad check valve, or failing / failed vac can. Could even be ruptured diaphrams in the heater box motors.

OTOH you wrote Any little bit of acceleration sets it off which leads me to believe the vacuum source is plumbed directly into the H/AC controller. No check valve or anything.

 

H

Hawkk72

New Member

Dec 13, 2006

47

0

0

Jan 18, 2008

#5

• Jan 18, 2008

• #5

Thanks for the info. The car is all original, bought it from the first owner and she said she didn't have any work done to any degree over the life of the car. Hoping I just find a bad vac line.

 

78CobraII

Moderator

Jul 31, 1998

1,808

8

39

Birmingham, AL

Jan 21, 2008

#6

• Jan 21, 2008

• #6

I had to replace the vacuum canister on my '76 hatch back in the late '80's. Even here in the south it had rusted through in several places.

The Ford replacement canister had a 1979 part number!

 

H

Hawkk72

New Member

Dec 13, 2006

47

0

0

Jan 28, 2008

#7

• Jan 28, 2008

• #7

So far I've only had a chance to check under the hood. I found a set of 3 vacuum lines together on a T fitting which was disconnected from something on the driver side fender apron. After I connected it I drove around and it did nothing to correct the problem. Still would like to know if the vacuum can is located behind the front driver side fender. No one has answered that question. If it is, I assume I'll have to remove the fender to gain access?

 

78CobraII

Moderator

Jul 31, 1998

1,808

8

39

Birmingham, AL

Jan 28, 2008

#8

• Jan 28, 2008

• #8

Correct, the vacuum canister is behind the passenger-side fender. If you open the passenger door and look into the fender, you can see it.

However you don't have to remove the fender to access it. Jack up the car on the passenger side, remove the wheel, then remove the fender liner.

The liner is held in with sheetmetal tabs stuck through slots in the liner and bent over, and maybe some screws into the inner fender, depending on the year.

 

H

Hawkk72

New Member

Dec 13, 2006

47

0

0

Jan 28, 2008

#9

• Jan 28, 2008

• #9

Thanks, Cobra. So what is the cannister behind the driver side fender?

 

beenstanged

New Member

Aug 11, 2006

41

0

0

taylor michigan

Jan 29, 2008

#10

• Jan 29, 2008

• #10

i'm going to say its the charcoal can for emissions.I'm not sure but i think it returns back to the fuel tank from the black coffe can looking can