Running rough

Hi - I have a '91 Safari / Astro... When I start the van it will run great for about 2 minutes while the ECM is in bypass mode (when it's cold). Then it picks the idle udle up to somewhere between 1000-2000 rpm and starts to miss and run very rich. I have changed the oxygen sensor and am at a loss... I was wondering has anyone else had this problem??? Thanks

I've never had the problem but it sounds to me like your fuel injection system is leaking underneath the intake plenum.since engines require more fuel while running cold but as it warms up the leaking fuel is causing your richness. that's my guess and I think it's a pretty good one . Good Luck Art