I've been re-assembling my motor and the old owner gutted most of the emissions equipment in the car. Since I've had to piece together almost every missing piece, I thought I would write up what every part does and why it's missing. This car failed emissions testing almost all the time, and during the rebuild, I now understood why. Here's what I learned from Understanding automotive emissions control by Larry Carley and Bob Freudenberger and from the 1988 Camaro shop manual.

The charcoal canister is part of the Evaporative emission control system. Basically, 20% of the total emission of the car can be produced by a parked car just by having the hydrocarbons in the gas evaporate out of the car.

This system consists of the gas tank, the gas cap, the liquid vapor separator, and the charcoal canister.

All fuel tanks are designed to allow for fuel expansion of up to 10-12%. There's a hump at the top of the tank (sort of) if you've ever taken the tank down to put a new fuel pump in (or high flow fuel pump) you'll notice it sort of has two humps at the top. Basically, those are air pockets to absorb the expansion of gas.

The Gas cap in modern actually venting gas caps or solid (and venting the fuel tank is done by other means.) Venting gas caps will vent tank pressure if it exceeds 1 psi, and allow air to enter the tank if a vacuum exists within the tank. All without allowing gasoline vapors to pollute the environment. That's why the gas cap is always tested at emissions testing stations.

The Liquid Vapor separator. Here's what it's supposed to do, although I never saw this in the gas lines going into my tank. Basically, it sits on top of the tank and prevents liquid gasoline from entering the vent line to the charcoal canister (because the canister is supposed to store fuel vapor, not liquid gas.)

If this part is not working, symptoms could include fuel starvation or a collapsed fuel tank on vehicles with solid type gas caps. The collapsing gas tank is because without venting, the fuel pump creates a vacuum that causes an implosion. (Think of pouring gas out of a jerry can. There's a hole to fill the jerry can as gas is poured out. If you put your thumb on that hole, gas flows really slow. Now instead of gravity, imagine putting a mechanical fuel pump. It's sucking up fuel and everything else in the tank and without enough air, the Jerry can collapses in on itself.)

The Charcoal canister. I only wrote this because the whomever owned this car before got rid of the vapor solenoid on the charcoal canister and I had to get a new one. Basically, it reduces HC by storing the gasoline vapors from the fuel tank. It's a closed system, no vapors go into the atmosphere. When you start the car, those fumes go into the throttle body to be burned kinda like what happens with the PCV. The vapor solenoid will open up the canister to purge the system and send the vapors into the throttle body to be burned up.

Here is why you should not delete the charcoal canister.

Some early Chrysler evaporative control systems did not use a charcoal canister. Instead, the fuel vapors were routed into the engines crankcase for storage. When the engine was started, the PCV system would draw the fumes from the crankcase and into the intake manifold. But the gasoline vapors tended to dilute the crankcase oil which caused a breakdown of oil and sludge. Also, the vapors formed an explosive mixture that literally blew off the valve covers. Because of this, Chrysler went back to using a charcoal canister. Also, keeping the charcoal canister and allowing it to send the gas vapors to the throttle body to be burned up actually saves you gas. With gas prices the way they are, every drop counts instead of letting it evaporate into the air to create more smog. It does not rob horsepower as like the PCV system, it's just a bunch of lines for the gas to be burned up again. There would be minimal gains from sticking on another air cleaner on the charcoal side of the car compared to using one big *** air cleaner where the battery box was like the old SLP system.

 

Yep, you got it. That's it exactly.

The main thing about "purging" the charcoal canister is that it should never occur at idle or on a cold engine.

Even pre-ECM charocal canister systems worked similarly but they used a thermal vacuum switch to qualify when the engine was warm enough to tolerate purge vapors into the engine. On EFI cars, obviously, the ECM can just trigger the purge valve directly when appropriate via an electronic valve (the one you replaced).

 

The surface area of a modern charcoal canister, if it could be flattened out, is close to the size of a football field. When some one keeps adding gas after it clicks-off, it will get saturated with fuel and when it will purge itself, it can cause drive-ability problems. If it is saturated and the tank is overfilled, on a hot day, when it re-leaves itself the fumes will be smelt near the rear end.