[tobas]

The backfiring occurs after the engine has been turned off and then when restarted. This can happen immediately after shutting it off or after 10-15 minutes. Van would still be somewhat hot, not entirely cold engine. Doesn't seem to do it when engine is cold. In terms of any other symptoms, engine does surge on occasion? ?MAP sensor was replaced to combat this issue per mechanic's recommendation when scanned but did not solve the problem. Could a vacuum leak be causing the surge and perhaps the lean fuel mixture (as you suggest) which may be resulting in the backfire?
The coolant temp. sensor was replaced approx 6 months ago but the van was doing the backfiring even then.