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OT But Machinery Related: Why Lower Vacuum When Accelerate?

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  • OT But Machinery Related: Why Lower Vacuum When Accelerate?

    I know the source of the vacuum under the hood is the carburetor. Well, the engine actually. The engine is sucking air (and fuel) through the carburetor. And we bleed off a bit of this vacuum at the carburetor and use it to activate various devices.

    So, when the car is accelerated, the engine runs faster and it should be sucking more air. That should equate to more or a stronger vacuum, not weaker. But when I accelerate my Volvo, it apparently looses the vacuum and the heater/AC system starts to admit outside air. This seems contradictory to me. Even more mysterious is the fact that when I reach a higher speed or the top of the hill that caused me to accelerate, then the vacuum and proper operation is restored. The engine is still running faster, so it is not just a simple matter of engine speed. The vacuum system is capable of operating properly at all engine speeds. Something other than just RPMs is at work here.

    I know I have at least three possible solutions here: find and fix the leak, add a larger reservoir, and/or add an electric vacuum pump. What I want to know is why does acceleration cause this behavior? Perhaps with a better understand as to why this is happening, I can better choose an alternative. And I just want to know.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  • #2
    Part of the reason that vacuum is lower under acceleration is that the throttle plates are open.
    When they're closed, the engine is trying to pull in a certain volume of air, and the throttle plates restrict airflow, so there's a deficit and hence vacuum.
    When the throttle plates are open, the engine can pull as much air as it needs so vacuum falls to 0.

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    • #3
      Originally posted by spongerich View Post
      Part of the reason that vacuum is lower under acceleration is that the throttle plates are open.
      When they're closed, the engine is trying to pull in a certain volume of air, and the throttle plates restrict airflow, so there's a deficit and hence vacuum.
      When the throttle plates are open, the engine can pull as much air as it needs so vacuum falls to 0.
      Bingo. As spongerich has correctly identified, the variable you're missing is the restriction caused by the throttle. The level of vacuum is dependent on the flow rate of the air (a function of engine displacement and RPM) and the restriction it is forced through (a function of throttle position). Stepping on the gas opens the throttle, reducing the restriction, hence reducing the vacuum developed.

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      • #4
        When you accelerate, your throttle goes to wide open and vacuum drops. When you reach cruising speed, the throttle closes and vacuum returns to previous level. Those of us old enough remember vacuum windshield wipers and the need to lift your foot to clear the windshield.
        Jim H.

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        • #5
          Otto cycle engines (gasoline powered 4 stroke) have the problem of sucking air as the power control. The higher the power required the more efficient they are because it is much easier to draw air. At very low power setting with the throttle plate(s) almost closed the engine uses significant power trying to draw air through the nearly closed plate(s). That creates a high vacuum and that slows down the engine because of the high pumping load. That is why idle still uses a lot of gas.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            do you have a rotted hose maybe ? your not mentioning warranty so I assume its more than three years old. grab a spray can of diesel starting fluid from your favorite auto parts store and start spraying at individual connectors with the engine running. listen for increase in rpm. that'll be where your problem is.........................
            you need to put a little tube on the spray nozzle so you can get at what you need to get at, not the whole engine bay.
            and yes I still have a couple cars with vacuum wipers, one has a fuel pump that is dual purpose as a vacuum assist pump. then there's the Mercedes diesel that has NO throttle plate, no vacuum for the brakes. but they did add a actual vac. pump to the engine. . . . gotta love um.

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            • #7
              It's like the difference between current and voltage. The vacuum is similar to the voltage drop across a variable resistor. When the variable resistor (throttle plate) is 0 (wide open) the voltage drop across it is lower as well.

              I had a 740 Volvo - IIRC there is supposed to be a check valve in between the throttle body and the HVAC. If that goes bad you will see the problem you are seeing.

              My 740 was a turbo, so "vacuum" was a relative thing.

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              • #8
                As already mentioned your engine's pistons descending in the cylinder's bore is the source of the depression that creates the vacuum. Without some sort of restriction in the engine's intake tract the vacuum would not be able to build. The throttle plate is this restriction, open it and the strength of the vacuum diminishes, close it and the vacuum intensifies.

                I've owned cars with vacuum wipers and even with a dual diaphragm fuel pump (one diaphragm for fuel the other as an additional source of vacuum when the throttle is opened) these systems left much to be desired. Just what you need...no wipers when passing!
                My diesel powered pickup also has a vacuum pump as the engine does not produced an adequate source of vacuum for the brake booster.

                If your vacuum operated HVAC controls have just recently started to malfunction and a vacuum reading of the engine reads normal I would suspect that the system may have a vacuum reservoir that has a leaking check valve which allows retained vacuum to bleed off under situations of low engine vacuum.
                I am not familiar with your Volvo's HVAC system at all and it may or may not incorporate such a system, however such systems are common for vacuum operated accessories.
                Home, down in the valley behind the Red Angus
                Bad Decisions Make Good Stories​

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                • #9
                  Voltage and current? Yikes, I better stop connecting my vacuum gauge for tests. I sure don't want to get shocked.

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                  • #10
                    There is typically a vacuum tank somewhere in the vehicle that is used for AC and other vacuum controls. There is also an anti-bleed down valve between the engine manifold and that tank to prevent the tank from venting back to the engine during low vacuum events such as going up a mountain pass or passing another car.

                    If that bleed down valve is defective you will get the condition you are seeing. Another cause is a leak in the vacuum circuit that connects to that tank.

                    My Dodge pickup has this problem. When going uphill the cruise control, which uses engine vacuum, will not maintain speed, and the AC air will flow out the defrost ports. Cooling still happens but not the way you'd like it. The cruise control actuator is located under the battery and is subject to any acid discharge and it will begin to leak. The factory recommends a plastic battery box to solve the problem. I've also replaced the damaged vacuum tubing which got the cruise control working again.

                    That doesn't explain why the AC doesn't work right though and I've not found that problem. I suspect a leak in the vacuum circuit as the bleed down valve does work correctly. I've yet to locate the vacuum tank on the bastid, but I know it's in there somewhere.

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                    • #11
                      Originally posted by dp View Post
                      That doesn't explain why the AC doesn't work right though and I've not found that problem. I suspect a leak in the vacuum circuit as the bleed down valve does work correctly. I've yet to locate the vacuum tank on the bastid, but I know it's in there somewhere.
                      Not sure what year Dodge you have so this is just a shot in the dark so bare with me.

                      From my large collection of shop manuals and from working on a similar problem on buddy's Dodge this summer I remember that the vacuum reservoir was in the passenger side cowl plenum area. Also, there are two vacuum check valves in the system, that I'm aware of at least.
                      If you like I can post some illustrations from my manuals.
                      I'll check back later.
                      Home, down in the valley behind the Red Angus
                      Bad Decisions Make Good Stories​

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                      • #12
                        I guess that explains it. I guess.

                        I did not know that there was a check valve. I will look for it and as a first step, replace it if it exists. Worth a shot.
                        Paul A.

                        Make it fit.
                        You can't win and there is a penalty for trying!

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                        • #13
                          Test your vacuum diaphragm that operates the HVAC, they are subject to allot of cycles and wear and therefore leaks...

                          In a somewhat related topic - just diagnosed and old toyota truck with a 22r engine in it, carburetted,,,

                          would not idle worth a damn, knew instantly by the sound that it was "pigging out" on fuel,

                          but no help from the idle adjustment screw as it made no difference what-so-ever...

                          so I immediately remove the air cleaner base and start looking around for a vacuum operated diaphragm that's attached to the carb base itself, bingo - find one - pull the hose and fuel comes out, I say "found it - you got a small ball bearing?"
                          buddys Dad says "no but got a BB" I say "that'l work" put BB in hose and plug it back on - put everything back together and it's purring like a kitten even though it's got 1/4 million miles on it and it's 30 years old... total diagnostic time - oh I don't know - maybe 5 minutes...

                          moral of story - vacuum lines can turn into small but fully open fuel lines depending on what their hooked up too... and they are not accustomed to delivering the excessive fuel evenly either - it's simply gets over drawn into the cylinders closest to where the fitting is mounted on the manifold,,, and it's most evident at idle where the most draw is with the least air flow,,, engine would clear it's throat if you reved it due to vacuum dropping off in the manifold (except for inside the carbs venturi) but immediately go right back to "pigging out" when idle resumed - yet another clue as to why I immediately went on my bad diaphragm "witch hunt"

                          diaphragm was hooked to a thermal sensor on the manifold most likely had to due with enrichment or something when cold... screw it, thing runs like a champ and it's irrelevant...

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                          • #14
                            Going back a bit, I remember the vacuum tank fuel delivery system. Mounted on the firewall the low manifold pressure would draw fuel from the tank and store a pint or so. This was then fed by gravity to the carb. Up-draft carb of course. Our Cad V-16 had two of them.
                            Jim

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                            • #15
                              Hm- that's the first I've heard of a vacuum (assisted?) fuel delivery system. Seems counter to the requirement- low vacuum at a time when the fuel need is highest-
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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