Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
By Daniel J. Stern
Most of us think of the tailpipe when auto emissions are mentioned, but toxic emissions come from other parts of the car, too. The open fuel systems used through the late 1960s freely vented gasoline vapors, which reacted with sunlight to form toxic smog.
The fuel system had two external vents: one at the fuel tank (sometimes through the cap), and one at the carburetor float bowl. Starting with the 1971 model year (except in California, which started in 1970), cars had to contain gasoline vapors, which meant redesigned tanks, filler necks, gas caps, fuel lines, and carburetors. Chrysler’s version was dubbed “Vapor Saver” or “ECS” (Evaporation Control System).
The 1970-71 ECS used a complicated fuel tank, which had a small inner tank at the top connected to the main tank chamber by a very small passageway to prevent overfilling. When gasoline was pumped in, the main tank chamber filled up; the secondary chamber was still mostly empty. After the driver hung up the nozzle and drove off, fuel flowed into the secondary tank from the main chamber, dropping the fuel level in the main chamber and preventing fuel being forced out of the tank when things warmed up and expanded. (This also helped to prevent fuel stains from fuel sloshing out of the tank.)
These 1970-71 ECS fuel tanks had four vent fittings, one at each corner of the top of the tank, brought together in the trunk to a more-or-less vertical metal cylinder leaning against the left wheelhouse. This cylinder was a vapor/liquid separator, and it had five hose fittings: one for the hose from each corner of the tank and one for a hose connecting to a line that ran up to the right-rear corner of the engine bay. Inside the separator, each of the four fittings from the corners of the tank continued vertically, each to a different height. The fitting to the engine bay continued inside the separator, almost to the top.
The idea with the four-corner venting and the different-height pipes inside the separator was that no matter what angle the car was at, in what direction, there would always be at least one vent fitting above the liquid-fuel level, so there would always be a vapor vent, thus preventing pressure buildup that would force fuel vapors out. The lines to the four tank-corner vents were relatively short inside the separator, so any liquid fuel that got into the separator (or condensed in it) could eventually flow back to the tank. The line to the engine bay extended the highest inside the separator so that only vapor, not liquid fuel, would enter that line.
The 1970-71 ECS was unique; it used the engine crankcase to store fuel vapors, both from the cylinder in the trunk, and from the carburetor bowl. A line from the trunk cylinder was connected to an extra nipple on the valve cover’s crankcase breather cap. The carburetor bowl vent was routed via another nipple on the breather cap on V8 engines, or via a nipple on the fuel pump body on Slant Six engines. When the engine was stopped, fuel vapors traveled into the crankcase and remained there until the engine was started. The collected vapors were then drawn off out of the crankcase and into the intake tract via the PCV valve.
This system worked, but could cause hot start/hot idle problems, because there was no control over when the vapors were pulled into the intake tract; it started as soon as the engine is cranked, and it continued as the engine was running, including at idle, when the mixture was easily upset. Thus, most of the unburned hydrocarbons wound up out in the atmosphere anyhow, only they were first put through the engine and then emitted from the tailpipe as black smoke!
For 1972, the ECS was redesigned so well that systems of nearly the same design are still used. The inner fuel tank chamber was replaced by a cleverly designed air pocket in the tank’s dome. These tanks had only one vent fitting instead of the previous four, and the trunk cylinder was no longer needed; the single tank vent ran directly to the engine bay.
Up front, rather than using the crankcase to store fuel vapors, the company added a black plastic tank filled with activated charcoal. It was placed at the front corner of the engine bay, with lines to the carb bowl and fuel tank vent; it had a purge valve mounted on top, with a small-diameter vacuum line teed into the distributor vacuum advance line, and a large-diameter line from the underside of the purge valve teed into the PCV valve hose.
Because there is only vacuum in the distributor vacuum line above idle, the purge valve only opened above idle, ending hot start/hot idle problems. When the purge valve opened, air was drawn through the bed of charcoal granules, which released and diluted the stored fuel vapor.
For 1973 (that’s after a single year), the purge valve was replaced by a purge port added above the carburetor’s throttle plate; the throttle plate itself was the purge valve. This was much less precise, and some of the hot start/hot idle problems returned, but it was cheaper to build, so Chrysler stuck with it.
These systems all used what a “pressure-vacuum” fuel cap; while an old vented cap had a passageway for air to flow in and fuel vapors to flow out freely, the pressure-vacuum cap added two spring-loaded check valves.
One of those check valves had very low tension, to easily allow air to flow into the tank, so there would be no vacuum as fuel was used. The high-tension check valve was there for safety, in case vapor volume was much higher than usual and could not be vented through the charcoal can and purge lines. Under some circumstances, such as rapidly driving up a mountain on a hot day with a tank of especially volatile gasoline, the vapor volume could be higher than normal, and the cap would bleed off excess pressure.
The 1970-71 two-nipple (Slant Six) or three-nipple (V8) breather caps are hard to find, and therefore expensive. A standard 1972-up Mopar breather can be adapted by drilling holes and adding brass or nylon hose nipples, but a much better option is to upgrade to the 1972-type system with charcoal can and purge valve.
The canister bracket is easy: every ’72-and-up Mopar has it at the right-front corner of the engine bay, or you can make your own. The canister must be kept upright, with the ports at the top.
Canisters for older GM and Ford vehicles are easy to find, but not those for carbureted Mopars. Fortunately, these canisters are easy to adapt, because they are functionally highly interchangeable, as long as they have the right ports.
Get a new charcoal can; when they get a couple of decades on them they can start to break down internally and throw carbon granules into your carburetor via the purge line – no fun. If you’re required by budget or time constraints to use an old canister, or you’re interested in preventing a messy carb rebuild to get all the carbon sand out, put an ordinary fuel filter in the purge line.
Chrysler #4723 551 or Standard/HyGrade #CP3167 is a service replacement for many ’80-90s Mopar applications and can readily be adapted to earlier cars. It has all the right ports. It has three sturdy threaded studs on top for easy bracket fabrication if you need to, and it has a built-in purge valve perched atop the canister.
The existing part numbers have been updated. Try this canister and this purge valve.
If you’re using a different canister, or have installed a non-original carburetor without a purge port on your 1973-up car, you’ll need a canister purge valve that only opens when the engine is above idle. Fortunately, standalone purge valves are readily available (e.g. NAPA Echlin #2-28011).
The purge valve’s big fittings go inline with the “PURGE” hose from the canister, which gets teed into the PCV hose using a metal ¼" x 3/8" emissions tee such as Dorman #47009.
On a carbureted engine, the small fitting on the purge valve gets teed into the vacuum advance hose, which if everything is set up and adjusted correctly should, on most cars, have no vacuum at curb idle speed and plenty of vacuum above idle.
There is usually no suitable above-idle-only vacuum hookup on an EFI setup, but most such systems have a closed-throttle switch of one sort or another – some throttle position sensors have a binary closed-throttle switch function built in. If you don’t have such a switch, add one. A lever- or roller-type momentary-break (normally closed) switch is easy to find and install. Put it on a simple bracket with slotted holes for adjustment and set it so that it closes at a throttle position corresponding to about 1,000 rpm. Then power a vacuum solenoid through this switch. The solenoid will open above 1,000 rpm and close below that. Plumb the system with a fuel-resistant vacuum hose from a source of manifold vacuum to the solenoid and from the solenoid to the purge valve.
Whichever version of ECS your car has, it’s well worth keeping. It’s even worth adding to a car that didn’t originally have it. Aside from (curable) hot start/idle nuisance issues, this system works for you, not against you. It cuts air pollution, which is not just good for society but also nice for all of us individually: when ECS replaced open fuel systems, cars stopped stinking up the garage (and house) with gasoline fumes.
But that’s not all; one of the best things ECS does for you is keep hold of gasoline that would otherwise float off into the open air, so you get to actually use it to run your engine.
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