My 2000 F150 with 120,000 has developed a quirk that it just won’t start, but only occasionally. Starter spins fine, but just won’t fire. I had it at a mechanic for several days, and of course it started just fine every time for them! I suggested new spark plugs, but they said it was not likely that. They said they can’t pinpoint the issue unless it fails to start while they have it. I am just living with the problem now: I just keep doing short bursts of the starter and eventually it will go, but I am concerned about being stranded with this.
You Don’t Say Whether Or Not The Mechanic Checked The Vehicle For Any DTCs (Diagnostic Trouble Codes) That Could Hint At The Culprit. Did He/She Do That ?
Has The “Check engine” Light Illuminated ?
DTCs can help find problems that are intermittent, like the one you describe.
Can you hear the fuel pump run on the start it decides not to work on? Maybe something like an intermittent connection at the fuel pump relay, ground at the tank or something could cause a no fuel pressure on that start.
I don’t see much of this any more, but how old are the plugs and wires?