It could very well be that your block heater is on its way out. I have had this happen before (not on a Ford). We switched to a non gfci outlet, then about a month later the heater up and died. Put in a new heater, and all was fine, we went back to the gfci outlet with no problems.

Ben

 

yes I just plug into a non GFI outlet
But GFI is a Great saftey if you get shocked sometimes you cant let go you cant move your paralized basicaly

and the best part of it on the GFI is when it snows all the moisture drips down onto the plug at the truck and trips or even the snow on the ground trips it

One of these days I want to relocate it from the front bumper or that was the plan untill I heard you guys was using the marinco plug thing

at first I was going to relocate to the cab or something wasnt to shure but if there was a place like the fuel filler door area or some thing you could plug it in and close the door and seal it off thats great

IDK I dont like the front bumper area at all
1 time the wife jumped in and took off and it was still plugged in snow on ground just hard to see and misswd it her car broke just a Fluke
I want it by the driver door area then it would never get missed driver would see it upon getting in the treuck

the marinco mod maybe worth it IDK havent ever seen one


you guys that have them marinco plugs does it seal the moisture out when its actully plugged into a cord

 

I would agree with Ben here. If I followed your post correctly, you have;
1. Replaced the GFCI outlet. (still failed)
2. Tried a non-GFCI outlet (and it worked fine)
3. Tried a different extension cord. (still failed)
4. Replaced the Block heater cord. (still failed)
But the GFCI outlet works for everything else.
You have isolated current bleeding through your truck back through the ground wire. If you were to put a 2 prong to 3 prong converter plug inline then I would bet that the GFCI would not trip. However this would only hide the problem rather than fix it and could be potently dangerous as the voltage “could” be on the body of the truck. That would be bad!! Sounds like the common side of the block heater is shorted out to the chassis. Also, resistance is different than impedance. When dealing with AC voltage impedance is what is important. So just because a meter shows the resistance as good does not mean the impedance is. I know this sounds contradictive but what is your resistance between the wide flat prong to the ground prong on your truck? It could be used as an indicator.

 

I'm not saying it can't happen..just it's rare for the element it's self to fail. I'm drawing a blank toward the number of posts I've read in the last 5 years of failed elements. For the record, the element has no electrical interaction with the truck...they may "share a ground" since the element is screwed into the side of the block...but I'm willing to bet if you took both batteries out of the truck the heater would still function just fine.

If you do replace the element...please take lots of photos. It will be interesting to see what it looks like...if you can see anything that has failed.

 

I just checked the ground, and unless I did what you suggested wrong, I have no resistance.
As for the heater core itself and the heater wire/plug, I checked that all out, no grounds, shorts or anything. But again, I did use a backup extension cord and a brand new heater cord - that tripped the circuit as well. I tried to clean the heater plug area as good as could, not the easiest place on the planet to get to.
Sorry, thankfully (for me), there won't be a thread on a heater core replacement.