Jeeper  Adventures Under the Hood  Jeeper

Evaporation Canister

The evaporation canister can be found mounted on the firewall just in front of the driver's feet. It's the size of a coffee can and has four hoses attached. From left to right in the photo they are:

1. Purge signal (ported vacuum)
2. Purge line (manifold vacuum)
3. Line from fuel tank
4. Line from carburetor bowl
Evaporation Canister

- Details -

The object here is to prevent the escape of gas vapor into the atmosphere while the engine is not running. A functioning gas cap has a two-way relief valve in it that forces evaporating gas up a vapor tube into the engine compartment and into the evaporation canister, where it is trapped in charcoal. Gas evaporating from the carburetor is routed here also.

When the engine is started and warmed up, ported vacuum at the "purge signal" cap opens a valve and the trapped vapor is pulled into the carburetor by way of the purge line and burned.

Canister purging should not take place at idle or when the engine is cold, when the CTO should prevent ported vacuum from appearing at the canister.

This canister can make trouble if the purge diaphragm under the little cap is leaking, which forces the engine lean, resulting in a rough and unsteady idle.

A simple test for this is to use a pair of long-nose pliers to squeeze the main purge line shut while the engine is idling. There should be no change in the idle. If there is, the canister should be replaced.
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