Does anybody know if the idle on a 1991 p/u
5.0 engine is idling low would effect the e4od trans running properly.

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I don't know how applicable this is to your question. Many years ago, GM cars with the 350 and 400 THM trans's wouldn't drive if the idle was too low. When idle speed got up into the 550 range they would kick in and pull. Had a 350 Chev that would slip off the high idle in cold weather, idle rough maybe around 475 or so in gear, trans would grab only when I got on the gas and rev's came up high enough. Suspect not enough pressure out of the main pump at low rev's to do the job.