heavy-use valve seat wear
I have an issue we have discovered while using a (bone stock) 2000 3.0L Ranger as a long-distance tow vehicle.
Under sustained, heavy use, it's relatively common for the stock valve seats to get pounded down. They are apparently quite soft. Since this engine has no provision to adjust the valve lash, this will eventually result in a held-open valve, which will quickly become a burned valve (typically an exhaust valve).
The first symptom is a occasional single cylinder misfire at idle. This becomes a steady misfire at idle and an occasional misfire at cruise (which will trigger a check-engine light on OBD-II vehicles), and finally a permanent misfire, with loss of compression in the affected cylinder.
We've seen it play out twice. Our truck was purchased used, and put into service towing cars on a flatbed trailer. This was mostly highway cruising over rolling hills with the cruise-control set at 60MPH, punctuated by periodic sustained WOT hillclimbs lasting several minutes in the mountain passes. The factory engine lost compression in one cylinder after about 15,000 miles of this use. At that point the engine had over 100,000 miles on it, so the failure was chalked up to simple old age, and it was exchanged for a name-brand remanufactured long-block. We did zero investigation of the cause before giving up the engine for a core.
The remanufactured engine suffered the same fate after about 20,000 miles. At this point we were curious, so rather than filing a warranty claim on the reman engine, we pulled the intake and head ourselves, sure that we would find some sort of vacuum leak from a bad gasket or cracked manifold, leaning out cylinder #1. We were wrong. We took the head into a machinist to have it rebuilt, and he told us that the Vulcan engine's stock valve seats are prone to sinkage under heavy use, and recommended the we have harder ones installed. We took his advice.