The Evaporative Emissions Control system (EEC), what most folks call “the charcoal canister” is one of the simplest and nearly-universal emissions control systems, that is there for several clear-cut purposes. This system has been mandated on every US market gasoline automobile since the mid-70’s. Emissions due to gasoline evaporation can account for up to 20% of total harmful vehicle emissions if not controlled. No small shakes. Thankfully, they can be effectively controlled by a system that is mostly passive, requires no regular maintenance other than occasional checks to be sure lines are intact and the vent lines and carbon canister can breathe, and costs absolutely nothing in performance or efficiency.

A forum commenter wrote:

One function of the van’s breather system hoses is to allow atmospheric air to flow passively from the outside through the charcoal canister and expansion tanks and into the fuel tank to prevent any kind of vacuum building up in the tank.

That is one of the purposes, balancing tank internal pressure to atmosphere due to the reduction in liquid volume as fuel is used up. But that could be done without the carbon canister on the end of the line, or merely by using a vented filler cap, as was typically done before the advent of Evaporative Emissions Control.

Here’s how EEC works: the fuel tank has a closed venting network. The fuel filler cap is sealed, so thru the vent network is the only way air and gases can enter or leave the system. Gasoline stored in an enclosed tank develops a certain vapor pressure. When ambient temperature increases, pressure within the tank rises due to vapor pressure and the vapor is routed to atmosphere, but only thru the canister. Gasoline evaporate consists of a variety of light hydrocarbon molecules. The canister contains activated carbon, which is an attractor/adsorber of many kinds of molecules (yes, that’s aDsorb, not aBsorb, look it up).  As the vapor flows thru the canister, the fuel vapor molecules bond lightly to the highly-porous surface of the carbon granules. They’re literally grabbing the HC molecules by chemical attraction, and holding them temporarily. The EEC keeps the hydrocarbons from escaping into the atmosphere where they would combine with other pollutants to form one type of smog. 

Then when the engine runs, a vacuum signal that only occurs when the throttle is opened above idle opens a control valve that connects a vent on top of the canister to the intake plenum. The engine’s intake vacuum draws a small flow of fresh air up thru the canister, the fresh air picks up the fuel vapor molecules by diffusion, and they get routed into the engine intake to be burned in the normal combustion process.

So the carbon canister just serves as a temporary buffer for fuel vapors until the engine can dispose of them. The small addition of air is not large enough to affect the engine running mixture, except possibly at idle when total air volume is small but manifold vacuum is high, which is why the canister venting is controlled so as not to happen at idle.

It seems insignificant, but some millions of gallons of gasoline are lost to evaporation annually. Of course the highest concentration of losses are in urban areas, so this actually turns out to be a major pollutant. If you’ve been to the populous parts of California in the last 30 years or so, they added a boot around every gas filler nozzle that draws the evaporate away from the nozzle when it’s being used to keep that source under better control. I don’t know where the vapors in that system go, but my guess would be that they are routed back into the fuel storage tank somehow, possibly trapped by an activated carbon buffer like the cars’ EEC systems, or maybe recondensed somehow back to liquid. 

The 2WD Vanagon tank venting system differs slightly from most cars due to the unusual shape of the fuel tank, flat with a saddle down the middle. Liquid gasoline expands a lot when warmed so there must be room for it to expand into without leaking out of the system. Many car fuel tanks can’t be filled to the top by design, which leaves some volume for fuel expansion due to temp changes, while others have an expansion/vapor separator tank or tanks above the main tank, like the 2WD Vanagon does. You can watch them work if you don’t believe the need for expansion space; fill your tank on a summer morning, and park the van until the heat of the day. I have seen the liquid gasoline expand enough to nearly half-fill both expansion tanks on a hot day. That is why the venting hoses should be intact and well-sealed, or this expansion will just result in the expanded fuel finding leaks that are now below the liquid level.

The venting lines to the canister are fitted to gravity valves in the top of the expansion tanks. The gravity valves are normally open to breathe freely both ways, but if the vehicle is overturned they contain a ball that will fall and block the outlet, to prevent fuel running out if you roll the car. Similar gravity or “rollover” valves are used for the same purpose on the top of the Syncro tank, where there is one at either end, and in most other cars’ tank venting systems.

So, the system covers these functions:

1. Allows fuel volume expansion in hot weather.
2.Allows pressure equalisation to atmosphere for changes in fuel volume due to thermal expansion and reduction of fuel volume from usage.
3.Captures, stores, and disposes of fuel vapors rather than venting them directly to atmosphere where they can contribute to ground smog.

There are several simple tests that can be done very quickly to confirm the venting system is intact, reasonably airtight, and clear, that the canister can breathe, and that the control valve works as intended. 

Bentley describes pulling a vacuum on the vent line from the tank at the canister end to confirm the system is closed, but that test doesn’t confirm that the vent line is clear, since a blockage in the vent line between the tank and the canister could also allow a vacuum to be pulled. To test that the vent line is clear all the way to the tank, pull vacuum on the vent line, and then open the fuel filler cap, which should release the vacuum. 

These tests along with a visual inspection would tell you that the venting network is OK, both airtight and clear, although the visual may also tell you it’s time to freshen up some hoses and grommets.

To test for clear venting without a vac gauge/pump, just remove the filler cap, attach a piece of hose to the vent line removed from the canister, and make sure you can blow air thru. This doesn’t, however, confirm that the vent system and tank are reasonably airtight.

There are also simple tests in Bentley for the EEC valve operation. The purple line off the forward side of the throttle body on any year waterboxer Vanagon is the throttle signal line that opens the EEC control valve. There’s no vac on that TB port at idle (unless your TB is badly worn), but vac pulls on that port as soon as the throttle tips off idle. If you remove the purple line from the TB, you shouldn’t be able to suck air thru it because it terminates at the control valve, so that confirms the EEC valve’s internal diaphragm is intact.

The white line coming into the plenum is the canister purge line. When the vac signal on the purple line opens the control valve, the purge line is ported to the top of the canister. So, you won’t be able to suck air thru this line from the plenum end either until vac is applied to the purple line. That opens the valve and air can then be drawn thru the white line. The air that comes thru this line has passed thru the canister and is delivering the stored HC’s to the intake. The white line is narrow so the volume of air drawn thru is very small so it doesn’t throw off the way the engine runs as long as it isn’t coming in at idle. That’s the purpose of the control valve.

There can be problems if the purge line is open at idle, because then the small volume of air coming thru the purge line can be enough to throw off idle speed or mixture. Most times if I test by pulling vac on the purple line at idle, the engine stalls. If the control valve were faulty and stuck open, or the TB is so worn there is enough vac to open the control valve at idle, it could cause idle to be erratic. Since the vac signal would be borderline in strength, as idle speed varies due to the purge air the vac signal could also vary, so the valve could be opening and closing with changes in idle speed, caused by the extra air it is admitting, and you have a surging idle.

I think one of the main problems that crops up with this system happens during hot weather hot restarting, and is also due to worn TB’s. It only takes a little more than 1″Hg. vacuum to open the control valve, they’re very sensitive. When the engine is being cranked a badly worn TB might allow enough vac while cranking to open the control valve. On a hot day the gasoline evaporation rate will be very high and the charcoal can be pretty loaded up with HC’s. The control valve opens during cranking and concentrated fuel vapor is fed into the plenum. Hot engines are very sensitive to being too rich when restarting, so the EEC system could throw off mixture during cranking and make the engine difficult to start.

So, hard hot restarts, and erratic idle surging. Those are the problems that the EEC can cause, but in most cases those problems are caused by a worn TB. The control valve being stuck open by itself can cause the same problems, but failures of the control valves are fairly rare while worn TB’s are the norm.

This throttle vacuum control method is common on pre-OBD era cars, but OBD cars tend to use an electric purge valve that is controlled by the engine computer.

The final but important test not mentioned in Bentley is to blow into the top nipple of the canister and make sure air can flow thru it out the bottom.