Join Date: Nov 2009
Location: The southern armpit of California
The emissions canister is not supposed to vent while at idle. If the valve that controls the venting gos bad, you'll end up with a nice vacuum leak.
I've diagnosed a faulty canister doing this simple test: Hook up all the lines as they should be connected. Try pinching off the purge line that feeds the canister vapors to the carb - if the idle smooths out, the canister is likely bad (that is, the valve that controls the venting is bad).
They don't make the OEM style canisters for the 258 anymore, but a canister from a 4.0L can be adapted to work.
BTW, if you are still running a stock fuel pump, it is located on the passenger side of the engine block.