FORD 2003 Explorer v.5 - Manual (Page 202)

Do not use fuel lines, engine rocker covers or the intake manifold as
grounding points.
Do not connect the end of the second cable to the negative (-)
terminal of the battery to be jumped. A spark may cause an
explosion of the gases that surround the battery.
4.0L Engine
2003 Explorer (exp)
Owners Guide (post-2002-fmt)
USA English (fus)
Roadside Emergencies
202