A customer's 1987 diesel truck taught me something about patience

Had a guy come in last month with an old F-250 that would barely start when cold. He was about 70, had owned the truck since new. I quoted him a glow plug replacement and he just stood there shaking his head. He said, "Son, that truck has outlived two marriages and three mortgages. She ain't the problem." I was set to argue but he asked if I could just watch him start it one morning instead. We met at 6 AM, he pumped the pedal twice, waited ten seconds, and it fired right up. Turned out he had a ritual that worked around a weak starter. I learned more about diesel cold starts from that old guy in one morning than I did in two years of trade school. Sometimes the fix isn't parts, it's listening. Any of you had a customer who knew their vehicle better than the manual suggests?