Poured blue devil in a Ford explorer to fix a head gasket. Heater core was said to be out but we found all hoses led back to the motor. After hooking them correctly, we had heat but now wont start?

Not completely following you here... What do you mean "jumped time in front and back"? And what is this about the heater hoses - they were installed incorrectly, and you switched them around to where they should be? And now you have heat from your heater core again, but the truck won't start/run? And you had no antifreeze in the radiator, only water? First, antifreeze does more than provide coolant, it also provides much needed rust inhibitors inside the motor's water jacket. It also doesn't boil as easily as water. It doesn't surprise me that, based on your description so far, this truck had some serious coolant related issues. All I can add is: 1) get all the hoses routed and connected properly 2) do a flush on the entire coolant system including heater core 3) add antifreeze to the proper level 4) make sure the thermostat is working 5) make sure the water pump is working. Then asses how the engine in running. If the head gasket is blown, gasket repair additives like Blue Devil can only do so much. Good luck.

Ok. The time jump thing was from my sisters bf and his so called mechanic friend who was in fact the guy who ran this vehicle with no coolant in the first place. The heat wouldn't work, so these bozos unplugged the hose that went to the heater core and connected it back into the motor somehow to make a complete circle, with no outlet. It was a complete cloud of smoke and was stalling out, it looked like steam, the boyfriend told her the head gasket was blown, we drained the radiator, put 2 full bottles of antifreeze in it, changed the thermostat and hooked the hoses up correctly, then put the blue devil in and followed the directions to a T, she drove it to town, it had heat, it stalled out a couple times but was ok for a day or two later, then wouldn't start at all. The idiot boyfriend is trying to tell us it jumped time in the front and back, I've never heard of front and back myself but didn't argue with him. We did notice the mass air flow sensor looking very black and was hoping it could be the reason. I just wanna help her fix it. I hope this was a better explanation. I hope u can helpand if so step by step directions on what to do and buy. Thank you, for your time.

Makes more sense. I think your mechanic friend was referring to the front and rear timing chain cassettes, which is a major repair. Did he indicate how he got to that conclusion? Obviously, if that problem isn't fixed then it's not worth bothering with the other problems. As to the overheating, from what I've read so far, it seems you have a blown head gasket, so no matter what you do with the coolant system, it will continue to overheat/boil over as combustion gases are leaking into the water jacket, and over-pressuring the coolant system. (Disconnecting the heater hoses to bypass the heater core is hokey but that didn't cause or fix the issue so it's a non-factor.) As to the MAF, if it looks bad then cleaning it can only help, you have to use MAF cleaner, as other cleaning products can damage it. In fact if the MAF is that bad then the rest of the throttle body is probably in bad shape too, so clean it as well. To be honest, until the timing chain is fixed these other problems are moot. That HAS to be fixed first and ASAP, or the engine will destroy itself if you run it with bad timing. Between that and the possibly-bad head gasket, it seems this engine is due for a tear-down and rebuild.

Thank you for your help. I very much appreciate it.