You probably have a glow plug or two out. As for leaving it plugged in, no you will not boil over the coolant. The heating elements in the block heaters only produce enough heat to keep the cooling system at around 120 deg F.

Cheers!

__________________
Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

2001 BMW 740i E38 M62 (past)
1981 300SD w126 OM617 (past)