EGR Valve and Charcoal Canister
by Bill Abbott
The EGR should have a control hose that goes to the intake manifold and allows the EGR valve to open at wide-open-throttle, allowing some exhaust gas to mix with the intake stream. The exhaust gas into and out-of conections to the EGR valve are metal tubes in a '77 Rabbit, I expect the same in a 74 bus. One goes to the exhaust manifold and the other to the intake manifold...
To test the EGR operation, start the engine and then disconnect the control hose. The EGR should open and engine should slow/hesitate/ act like it's getting some exhaust gasses mixed into the intake stream. If disconnecting the hose makes no difference, either the hose is leaky/ blocked or the egr exhaust gas pipe(s) are blocked.
The basic failure of an EGR is the valve diaphram that senses intake maifold vacuum and closes the valve. With a clean hose on the control input, suck on the hose and put your tongue against the open end of the hose. It should hold the lower pressure, stick to your tongue, for at least a minute. You can do the same test with a vacuum gauge of course, if you have one. If the valve won't hold low pressure, then it won't close completely at idle and your bus will ildle poorly.
Bentley SHOULD cover this, but I've never looked.
On our '70 single cab, there's a hose from the fuel-tank to the charcoal canister, which is what allows it to do its job, venting the fuel system to ambient pressure. This might be a small diameter (1/4"/6mm) hose but I think ours is large (10mm?) and connects to an appropriately sized metal hose that runs back to the engine comparment from the vicinity of the fuel-filler, high on the right side of the front of the engine comparment.
A second, large diameter (10mm?), hose connects the charcoal canister to a fitting on the side of the aircleaner (Broken when I got the bus, JB Welded by me 4 years ago and still solid).
The third, large diameter (10mm?) hose on the charcoal canister connects to the opposite side from the one that goes to the aircleaner and comes from a fitting on the cooling shroud. This one should probably be on the opposite side of the charcoal canister from the hose that conects to the tank.
The theory of operation on the charcoal canister is that it absorbs gas fumes from the tank as it vents out of the tank during filling /expansion of the contents and also allows air to flow through into the tank as the gas is consumed. The hose from the cooling shroud blows air into the canister and that air vents to the aircleaner, where is the slightly 'enriched' airflow mixes with regular air sucked into the aircleaner and goes through the engine, burning up the gas fumes.
The charcoal canister actually becomes ineffective eventually, and is intended to be replaced at 30,000 mile intervals I think. I don't know anyone who does that but that's what's intended.
The other venting connection on our '70 runs from the oil filler pipe to a second fitting on the aircleaner, venting the crankcase to the aircleaner. Again, the theory is that the oil-laden smelly air from the crankcase vents to the aircleaner where it gets passed through the enging and the hydrocarbons burned, or at least given a good scare :).
The point of the exercise is to vent the gas
tank and crankcase to ambient pressure without allowing smelly or
hazardous effects.
