Ok- i wrote a response before my “embrace the suck” comment that didn’t post for some reason.
That sensor is your engine coolant temperature sensor that sends a signal up to your injection pump to tell it to advance for cold timing and to operate the Little solenoid that holds the throttle open slightly more at idle. The purpose of these working is to help it start easier in the cold. You can un thread that sensor and screw in the new one. Some coolant will leak out while you’re doing it so just have the new one right next to you as you remove the old one, and start the threads on it as fast as possible. You won’t lose a ton of coolant, but you will lose some, just top the engine off after it starts with coolant and be sure to bleed the air out.
Well, this might be a small contributing factor to the harder starting in the cold, it is not gonna be the only problem.
That crazy hard starting you’re having, and the glow plug cycling on and off so fast is probably because of that one glow Plug you could not change. Try unplugging that one glow plug and see if it allows you to have the glow plugs on for a longer period of time as you cycle them with the key. The 7 new glow plugs are self regulating, but your bad sensor can also be telling the glow plug controller to turn off way too soon. So that sensor can be impacting it in all three of those ways.
Next, that engine is cranking over really slow, and in the last video before you got the jumpstart, it was cranking over so slow that with perfect glow plugs, cycled as long as they can it still wouldn’t have started. The batteries are hurting.
A bad battery can cause the low reading, even with a good alternator. As well as a bad alternator can keep the battery charged too low and give that low reading. Also bad battery, cables, or battery cable connections can cause it.
So:
Disconnect all four battery cable terminals, clean the batteries and the cables with a wire brush, baking soda mixed with water to make a paste. Then rinse it all with water.
Now you need to load test the two batteries individually. If you don’t have a load tester or have someone you can borrow one from, harbor freight sells a really good one for $17, Centech 100amp. Long-term, it’s a tool worth owning, had mine for 35 years and used it professionally for 20 of that. Short term auto parts. Stores will test it for free, but you have to have the truck or the batteries at their store.
If you have the tool yourself, you can disconnect the other end of the battery cables (clean them like the other end was done on the cables and where the cables attach) then use the load tester to the battery cables by hooking up the other hand back to the battery and the tester to the opposite end of the cables. Now test the battery again through the cables, there should be almost no noticeable difference of width or without the battery cables. If there is a drop in the reading, then you probably need new battery cables.
The factory battery cables are a smaller diameter cable than what is really desired. This is GM being cheapskates. The battery cables you buy at the auto parts store are going to be junk. Buying the cable and the ends, then making your own cables is not too hard of a DIY project, and you will end up with a far better set of cables that will last many, many years and do a better job helping the truck start easier.
Obviously when testing the batteries you might find one or both are toast. If you have one old battery and one new battery, the old battery will draw down the new battery to its level as soon as you hook them up. This dramatically shorten the life of the new battery. Because of this, the smart movies to buy both batteries at the same time.
At some point in time repacing the starter is going to come up. At a minimum get (AC Delco preferably) from a store with lifetime warranty if money is wicked tight. Otherwise the best option and something that will dramatically help the truck with starting better is to buy the Powermaster 9052 starter. It is more expensive and only has a one year warranty. But outside of me smashing one off road ina way that would have ruined any starter- I don’t know anyone that ran a 9052 for less than 7 years or less than 100,000 miles. They spin the engine over much faster and that helps with better compression and builds more heat. There is videos we can show - if you wanna see, just ask.
If you were full of cash, I would say do it all- batteries, cables & starter. And of corse the temp sensor.
As I see it the temp sensor is a must do. Testing alternator is a must do- but you have to have good batteries to test it in the truck or you have to remove it from the truck to take inside autoparts store for testing. Testing batteries is a must do. If batteries test good then cleaning cables is must do and really should test them.
More to come
