How to Troubleshoot a Truck that Won't Start: 5 Simple Steps

In theory, my truck should have started after replacing the head, I had put everything back together and all I did was replace a few parts, but it wasn't starting. What now? Now comes the process of troubleshooting. Starting from scratch with nothing but my mechanical knowledge and the clues I had from my attempts to start the truck, I have to figure out why it's not starting and how I can fix it.
This is the second video of a 2-part episode. In the first part, I provided footage of me trying to start the truck, without the commentary. I challenged you all to try to figure out what was wrong with the truck based only on the footage (the same information I had at the time. If you have not watched part one, go ahead and check it out and make your prediction. If you have watched part one, were you right about the reasons the truck wouldn't start? Let me know in the comments below.
The truck is powered by a REO 331 CI Gasoline inline 6 cylinder. An overview of the truck can be found here: • REO M-35 Deuce and a Half
I started the truck for the first time of that year in April 2020, and it was driving as good as ever. • REO First Drive 2020
• M-35 Turnaround On Nar...
I rotated the tires and did some other basic maintenance, oil change, fluid check, brake adjustment, cleaning. • M-35 Tire Rotation
I then did a lot of work around the farm with it, hauling brush, and it did not give me any trouble. • REO M-35 Working On Th...
I drove it a couple miles down the road to pull another truck out of a field, and it handled it well, this was July 2020. • Towing My GMC M-211 Wi...
Then, the problems first started on the drive home. • REO Road Trip
It would cold start fine, and idle to warm up, but after that it would not idle when it was warm, and sometimes (as you see in the second part of the "REO road trip" video) it did not want to start while it was warm. My first thought, since problems only occurred when the engine was warm, was that it was a cooling issue. However, I ruled this out in August 2020. After thinking about it, and reviewing the footage, it sounded like one or more of the cylinders wasn't firing. The first logical step was to change the spark plugs, since their age was unknown and the truck had been burning a bit of oil. • REO Spark Plug Changing
Working on the truck in December in Vermont turned into a saga of its own, but I did find out that the head gasket was blown. We replaced the head gasket, but one thing lead to another and now we were going to replace the entire head. • REO M-35 Head Gasket R...
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0:00 Day 1 starting attempt
0:30 Day 1 thoughts
12:00 Day 2 starting
12:37 Day 2 thoughts
19:16 What else is wrong with the truck?

[@tomprovo6204]

Im glad that you got it started. Good troubleshooting technique.

[@calvinsgarage]

Thanks. It seems something always happens to where I'm scrambling to finish work on the truck just before I'm set to go on a trip.

[@psl54rsniper]

All my trucks are GASSERS to, two things to keep in mind, the new gasoline drys out all the old parts in your fuel system and the new gas goes bad fast if you leave it in the tank to long I use a one gallon can and bypss the tank just to run them once in a while. my m35 fuel pump is bad to, with batterys I have found the best thing is to buy two real good 31SEREIS truck batts. with 900 - 1000 amps cold cranking amps then keep good care of them a they last a long time, all those military trucks are high mantanince the gov. did not care they put new batterys in every time they used them.

[@calvinsgarage]

Thank you for the comment. I use ethanol free gas to avoid some of those issues. I also find it keeps better than regular gas, but I'll still use some fuel stabilizer and add in fresh fuel each year, and haven't had a problem.