My Vandura runs perfectly when first started running in Open Loop and doesn't miss at idle or at high RPM. Once the van reaches operating temp and goes into closed loop, it hesitates and backfires when pressing more on the pedal. A very slow increase in RPM will not cause it to backfire or miss. This is all in park position and when it is in drive, I can't get it out of the driveway without backfiring or stalling under a load.
I added Techron to it last year and the problem went away after 2 hours of running in the drive with some idling and accelarating. This problem din't occur again for the whole summer until last fall and Techron didn't work this time.
There is no engine light or any indication there is a problem. What is causing this?