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low vacuum

seymouj

NAXLA Member #1223
When my engine is working hard (I mostly notice it when going up a hill on the highway) my ac or heat or whatever I have on switches to defrost on its own and then when I finish climbing the hill and let off the gas, the air switches back to the vents.
If I understand correctly, that switch is vacuum controlled and is in the defrost position when there is no vacuum. It would seem I don't have enough vacuum to operate the switch when climbing a hill.
I looked for vacuum leaks including near the canister under the front bumper and near the battery (because I read somwhere that is a common place for leaks) but didn't find any.
I can live with the loss of AC or heat blowing on me for a couple minutes but I am concerned that the low vacuum might be a symptom of a bigger problem.
Any suggestions on what I should check or if this condition suggests a bigger problem?


87, 4.0, auto, 190,000 miles
 
Keep looking there is most likely a broken vacuum line somewhere. I in fact did have the same problem on my 87, 4.0, Auto, 188K and it was the line by the battery. A very little hole in the line can cause it. Look hard.

Kenny
87 & 00 XJs
 
I guess I am an "internet mechanic" as some may call it (or a complete idiot), but how do you go about testing a vacuum line for a leak assuming it is not visible to the naked eye?
 
Common place to find that leak is either going to be near where the line crosses under the battery, or at the vacuum disconnect on the front axle.

Check the vacuum by shutting the engine off and pressing the brake. As the vacuum is depleted, it should be more difficult to press the pedal. If there's a leak there shouldn't be a difference in pedal pressure
 
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