Looks like this time around it was a water leak in the intercooler or water ingestion problem.
Pulled the head, hard brown water scale on the underside and up inside the intake valves, up to 1/8" thick. Water scale thick enough to catch your fingernail on the seating area of the valve discs.
They were running under 3/4 load and 75% nat gas, suddenly started hearing a popping that said unburnt nat gas was getting into the muffler. They checked cylinder temps, and a couple were up to 1020 F, then suddenly the cylinder in question dropped to 250 F under load - same symptoms we had last year when one head not only burnt the valve, but also stuck open.
Cheap rebuilt parts may be to play as well. They've been going with the lowest bidder the last 3 years, swapping out the original 30 year old heads with rebuilt ones. We've had to send 2 back right out of the shipping box in the past year I've been here - one had a crack in the jacket water passages (apparently they didn't magnaflux the head), the other had both inlet valve seats machined over 1/8" off center!
Bloody thing needs a good overhaul and modernization - half the parts for the control system are listed as obsolete - but try convincing UTSA that it's cheaper to spend 200K now for an overhaul, vice waiting for it to completely fail and spending 3 mil for a complete replacement.
