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Go Back   ClassicBroncos.com Forums > 66-77 Ford Bronco > Bronco FAQ > Engine FAQ's

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Plastic Bottle Under Panel behind Driver's Seat Plastic Bottle Under Panel behind Driver's Seat
Explanation of the Vapor Separator bottle
Many Bronco owner who are working on the interior of their vehicles soon discover a odd-shaped plastic bottle beneath a metal cover on the inside of the drivers side quarter panel. Several tubes exit the plastic bottle from the bottom that disappear into the floor, and one comes up through the floor, goes over the top of the bottle, through a small connection, and then goes back down through the floor. None of this is visible if there are interior quarter panel covers installed. But for those without interior quarter panel trim, it is covered by an obvious sheet metal cover held in place with a few Phillips head screws just behind the door frame on the drivers side. It is part of the evaporative emission system and the bottle is a vapor separator. Beginning in 1970 for California and in 1971 for all other states, evaporative emissions control devices were installed to prevent raw gasoline fumes from escaping into the atmosphere.

The basic theory of evaporative emission controls is to collect gasoline fumes from the top of the gas tanks and send them to the engine where they are burned instead of evaporated into the air. Most vehicles had just the vapor control system, but on Broncos there was also a bottle that separate liquid gasoline from vapors. Hence the bottle is technically known as a vapor separator. Besides helping control smog, containing the vapors is a good thing inside a hot garage since the gasoline vapors are not collecting inside the enclosed garage space and contributing to the general stink of hot fluids. So evaporative emissions systems are a good thing, they dont harm performance, and they make life with an old Bronco a little nicer for all concerned. Despite the general distain for anything labeled emission control device, this one is probably one that does no harm, actually does some good, and you will want to keep in working order.

If you trace the tubes you will find that one comes out of the top of the main tank and goes to the top of the plastic bottle where it passes through a control value which limits the introduction of liquid gasoline. Any that does get through collects in the bottom of the bottle and is returned to the tank via two lines coming out the bottom of the bottle that return the liquid to the main tank. Vapors continue through the control valve directly into a line that exits over the top of the bottle and down through the floor and follows a hard line over to the passenger side frame rail behind the right front wheel well where it enters a charcoal canister that stores the vapor. The vapor is drawn out of the charcoal canister through a 1 inch diameter hose, up into the engine compartment near the back of the inner fender and then goes to the clean-air side of the original air cleaner. Another 1 inch hose coming out of the top of the canister and sticking up into the engine compartment next to the other hose has a small plastic cap. Presumably this is an air inlet for the canister. On start up the high vacuum draws the vapors into the carburetor where the vapor is burned normally. A similar line comes out of the top of the auxiliary tank and goes through the same system.

One final note, that someone might confirm: I was told by one of the engineers that helped develop the concept, that the charcoal is re-newable by baking the canister for a few hours.

Here is a little addendum that I shamelessly swiped and herein plagiarize from another automotive forum (Fiat, actually). I believe it further lays out some of the reasons for keeping this system active:
"The charcoal canister is a simple and clever device. It absorbs evaporated fuel from the gas tank, which is stored on the surface of the charcoal inside in the form of HC molecules. When the car is running, fresh warm air is drawn through the charcoal and it releases the HC molecules which are then drawn into the intake mixture to be burned as part of the combustion process.

Unless you have a bad vacuum line or two, the system works well and reduces evaporative emissions, especially in the hot summer months. Evaporative emissions account for a large part of smog and further contribute to climate change. Smog and air pollution in general is getting worse in many areas, witness rising asthma rates and related infant mortality. Additionally, keeping evaporative emissions in control contains gasoline additives, reduces your exposure to gas and fun stuff like benzene and generally smells better.

Because the system also allows a certain amount of pressure to build before releasing the evaporated fuel to the canister, it actually prevents some evaporation and over the long haul results in more gas actually doing what you want it to do when you put it in there, that is make the car go, thereby increasing mileage.

In regards to emissions laws, being exempt from testing doesn't actually make it legal to remove or tamper with factory installed emissions devices, and doing so may technically be a federal offense."  

Last edited by MarsChariot; 04/14/08 at 08:27 PM..
Glenn on 04/27/07, 04:44 PM
I disconnected mine and I hope it doesn't come back to haunt me. I unhooked and completely pulled my aux. tank and that is when I found the mysterious plastic bottle. I had asked quite a few people what that charcoal canister was and nobody seemed to know? I never saw it on any other broncos in the mags and didn't know where the one end of the garden hose hooked to so out it came.
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1975stroppebaja on 04/27/07, 05:17 PM
Nice writeup! I'll have to remember this thread for when it comes time to hook mine back up. Thanks!
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stock1970 on 08/21/08, 12:51 AM
I have a 1970 stock bronco. I sent in to ford for a build sheet and it stated that it was actually built in 1969. The bronco is a 70 but it does not have this plastic bottle or the tubes out of the tops of the gas tanks. So the question is if it never had it would it be legal in cali?
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Bronk on 09/06/08, 11:32 AM
As far as I know The vehicle only is required to have emission equipment for the year of manufacture.

On topic I just picked up a 74 bronco ranger and was looking at the plastic bottle and trying to figure it out. I was guessing vacume can (for what I don't Know) thanks for info
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aomastu on 01/23/10, 06:11 PM
Going to Fuel Injection

When I go to fuel injection, I will not need this anymore. Correct?
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matt w on 11/16/11, 04:43 PM
How does one decide if the Charcol Canister needs to be recharged?

And if so how does one "RECHARGE" the charcol canister?
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Bronk on 11/17/11, 07:36 AM
Originally Posted by aomastu View Post
When I go to fuel injection, I will not need this anymore. Correct?
They were used on fuel injected engines also but it is not needed for the engine to run.
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Bronk on 11/17/11, 07:42 AM
Originally Posted by matt w View Post
How does one decide if the Charcol Canister needs to be recharged?

And if so how does one "RECHARGE" the charcol canister?
One final note, that someone might confirm: I was told by one of the engineers that helped develop the concept, that the charcoal is re-newable by baking the canister for a few hours.

I guess you bake the can ???? i have never heard of that before but it might be crazy enuff to just work. lol.

How would we know if it isn't working?
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RobertD on 04/07/21, 07:41 PM
I would assume a raw gas smell would be a good indicator it’s not working correctly.

I have also read that “packing” the fuel tank (ie filling more after the pump handle has clicked off) is the cause of most charcoal canister failures. Not just in Broncos obviously but in all vehicles with the evaporative emissions system installed. To the best of my knowledge there is absolutely no performance benefit to removing it, and I’m not a Green Nut by any stretch.
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