I usually leave both of mine on all night (2x 600watt in-block frost plug heaters - 1200 watts total) in the winter(-40* sometimes). If it's close to freezing temp (32*F) - i only run one (600 watts). Electricity is relatively cheap here ($0.06/KWH), and I need the truck. You should not be able to "overheat" your engine with a 600W heater.. It's an awful big chunk of metal with 4 gallons of water in it, and once the water heats up real hot, it should open the t-stat and convection would run the water through the rad (keeping it around operating temp). Sounds like something is very wrong. Unless you're block heating in the summer or something (I did that once when my glowplugs fried. Got hot enough to start, but nowhere near overheating, with 1200W worth of heat).