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Unread 08-16-2009, 03:08 PM   #1
Finaltheorem47
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Vacuum Reservoir

What does the vacuum reservoir do? I removed it and plugged the line when I put a new bumper on because it wouldn't mount where it used to.

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Unread 08-16-2009, 03:22 PM   #2
xjfever
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It conserves vacuum to run your cruise control and vent selection for the heat/air when you are at higher rpms and lose manifold vacuum. If your vents go to the defrost selection when at higer rpms, or your cruise control doesn't work anymore you can relocate it to the engine compartment.
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Unread 08-16-2009, 03:23 PM   #3
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It essentially stores vacuum so that there is always some available to drive the accessories on that line (AC vent control, heater control valve, and cruise control). Think about the way engines work here--pistons create vacuum when they suck air through the intake manifold. Usually this air comes in through the throttle body. However when the throttle blades are closed the engine is not able to suck in as much air, and the vacuum pressure in the manifold goes up. Conversely when the throttle blades are open (acceleration) the engine is able to suck in lots of air through the throttle body, so there is less overall vacuum pressure in the manifold. In turn, accessories that need vacuum to operate are starved out during acceleration (do a quick acceleration and step on brakes, they will be really weak). This results in things like air conditioning flopping around the different HVAC vents because not enough vacuum to keep the selected outlet open, or having the cruise control cut itself off because there is not enough pressure to maintain the valve.

As for the canister, when the vacuum is being pulled through the manifold and lines, the air that is naturally present in the canister gets sucked out, resulting in negative pressure in the bottle. When the manifold vacuum dies out during acceleration, the air pressure in the canister tries to normalize with ambient air pressure, which means that the canister is now sucking air out the accessory lines, which means that it becomes the vacuum source for the accessories on that line.
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Unread 08-16-2009, 04:06 PM   #4
Magus2727
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can't add much to what has already been said but it also holds a a vacuum for quite some time after the motor is off. that's the rushing of air sound that you get when the vehicle is off but you adjust the climate control settings.
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Unread 08-16-2009, 04:39 PM   #5
Finaltheorem47
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Thank you for your replies. My HVAC has functions fine, but the cruise control doesn't work. I don't believe the cruise control worked before I took it off though.

I am unsure because when I got the Jeep, I tore it apart before I started driving it, so I am just going off of what the previous owner told me.

I am pulling about 16 inches of Mercury though at idle. Will the reservoir effect braking at all? I am already planning to re-install it now, but I'd just like to know. Will some fuel line hose work well enough to use when I relocate it?
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Unread 08-16-2009, 07:58 PM   #6
ehall
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Quote:
Originally Posted by Finaltheorem47 View Post
I am pulling about 16 inches of Mercury though at idle.
That's a good number for idle.

Quote:
Will the reservoir effect braking at all?
Not by design. There is supposed to be a check valve on the accessory line at the firewall, which will prevent vacuum pressure from going back into the manifold. If the check valve is broken or missing then yes the bottle will be a vacuum source for everything else that is attached to the manifold. .

Quote:
Will some fuel line hose work well enough to use when I relocate it?
Fuel and heater hose are hard to use since they are not as flexible and pliable. You end up getting too large a size, and then it does not seal as well and you lose vacuum pressure. Better to get real vacuum hose that is just a little too small, and use silicone dielectric grease to lubricate and seal the connectors.
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Unread 08-16-2009, 11:24 PM   #7
Finaltheorem47
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Okay, thanks.
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Unread 08-17-2009, 09:40 AM   #8
XJohnnyC
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On my 92 MJ, I got a reservoir from a 97+ XJ. They're round and smaller than the pre-97 oblong-shaped ones. So it's easier to find a nice spot to relocate it.
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Unread 08-17-2009, 02:57 PM   #9
Matt Gertsch
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I've got a cone intake, so I was able to put mine where the air box used to be. I was able to reroute the hard lines and use only limited vacuum hose. Make sure you keep the check valve in place on the manifold and the one near the firewall.
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Unread 06-03-2013, 09:23 PM   #10
JeepJohn92
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Im having issues with my vacuum lines and less than satisfactory idle. Would a bad canister kill my idle speeds? Ive unplugged the lead from the cannister to my manifold and the RPM's at idle and in gear are much better. Im lost on this one.
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