this winter the block heater seemed to not really heat alot.
i would plug it in and i could hear it making some noise as it started to heat the fluid, as other diesels i have owned.
it just never seemed to get very hot even after 4hrs or so, temp could have been around 32F.
just seems that other diesels i have had heated up better.

i am getting ready to do a lot of work - ip, injectors, HO water pump, radiator, ect. now would be a good time to replace block heater if necessary.
also, i was told a long time ago - unplug block heater before starting the engine, even a minute before. the movement of the fluid while plugged in has an effect on it while hot, i.e.; damaging to it.
jon

If your truck is a 96 1/2, youīve already got the HO water pump and dual t-stats. Check the part # on the t-stat housing to be sure.

I donīt bother to use the block heater anymore. It starts up every time even if itīs -5 f. If the block is at 32 deg f. it should fire right up. If your injectors and glows are bad, that will make a huge difference with starting.

Iīve never heard that itīs bad to have the heater plugged in while running.

the block heater in my truck still works really well. if i leave it plugged overnight(10pm to ~6:30am) in the temp gauge sits at 160*F.

It may be that since it's not a circulating heater, and the heat is transferred by the coolant it takes some time to get it all up to temp. I too can hear the heater start to work, and i can hear it "hissing" as long as it's plugged in.

Get a multi meter and measure it's resistance ( ohms) V/ A=R ( I can't find divide on keyboard) so 120/ 6.5= 18.5 ohms Pm me if you need more. ( where is coffee icon??)

IMO, 8 1/2 hours of on time is a monumental waste of electricity. A couple hours should do the trick.
Ted

my block heater was dead when I got truck

i always just leave mine plugged in over night as well. I plug it in before i got to bed (10 or so) and unplug when i leave for school ( about 6). its usually just under 160 when it fires. We had a timer hooked up in the garage and it would come on at like 3, but it ended up breaking and we never replaced it.