There's your problem right there. It didn't SUDDENLY overheat. Overheating takes a little time, as the temperature creeps up slowly, and if you don't IMMEDIATELY shut the engine off you're risking engine damage, as you've found out.
Well, a typical car has an “idiot gauge” for a temperature gauge. It is designed to basically sit in the middle unless something abnormal happens, so that owners don’t “freak out” when the needle wiggles a bit in normal use. I could see this sort of gauge giving a centered reading as it heats up, only to hop up to “H” very quickly. (Reason #107 why I like real gauges…)
The chain of damage is often not known until a repair is made allowing the system to function again....until the next linked problem occur
Absolutely! I had a bad radiator cap on my truck. I replaced it, allowing the system to pressurize until the “next weakest link” rears its ugly head. (Let’s say it’s the upper rad hose.) Then you fix that, and the system pressurizes further, until the radiator starts leaking around the transmission cooler, or what have you. Aging cooling systems are NOTORIOUS for this.
One thing to consider: if you will not pay for the head gasket, what will happen. You may not get your car back. But if you don’t want it back because you won’t fix the head gasket, you might not have to pay the $800 you owe him. A lot depends on whether the shop wants the $800 no matter what happens to the car. You might try to barter the car for the $800 you owe. You should discuss this possibility with the shop owner if it appeals to you. I suggest saying that you don’t want to pay for the head gasket and ask for suggestions for alternatives from him. He might just offer that he take the car in exchange.
+1 to busted’s post. A water pump can hemorrhage (or seize) suddenly and the engine’s temp spike quickly. Trouble is, by the time you suddenly realize it damage could have already begun.
Unless there’s obvious evidence of damage other than the water pump on the now-disabled engine, the mechanic would have no cause to go beyond replacing the water pump, testing the system integrity (for leaks), and, if he has integrity which this guy apparently has, test-driving the car. It’s at that point that additional problems caused by the overheating like the T-stat and the headgasket can become evident. At that point he should and apparently did notify you for your decision on whether to proceed.
It sounds to me like he does good work. The problem isn’t the mechanic. The problem is that the engine is shot. Huppmobiles will do that. Or is that another make? You never did tell us the year, make and model of the car.