88e250 van. Runs great for 10 min, then idle rough, backfire,steady ck engine

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Lite comes on , then shuts off. I have replaced inline fuel filter that was rusted inside. Van continues with same symptoms so, I put Injector, gas treatment in tank. Did not help. Both gas tanks and in tank fuel pumps replaced last year. Any suggestions? Is there any way to troubleshoot steady check engine lite? Does this year van have diagnostic port?????? Any help would be appreciated.