It is possible for the engine to run if the return and feed lines are reversed. It probably won't run well, but it will run.

Diagnosis should be relatively simple. Disconnect the "normal" return line from the cannister. Put the open line into a suitable container. Turn the switch to the ON position. No need to start. This will activate the fuel pump and should pressurize the fuel system if the lines are connected correctly. If fuel starts to fill your container, you have identified the problem.

Jim Paget
88 YJ with a few changes

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