Ran hot. thermostat stuck closed. replaced thermostat, started engine and now have knock. is engine salvagable or shot?

Sounds like a bad thermostat, REPLACE IT! And make sure coolant is at proper level. A thermostat should be closed when cold, and wide open at hot engine. if it fails, it will not stay closed at cold engine, or its stuck. When stuck water flows through by pass at thermostat. and does not heatup because thermostat does not hold coolant back until it gets hot. God-Bless!

Darlene, the cooling system could now have air in it now, which may have to be bled out for cooling system to operate correctly, When you said it was leaking, ensure the leak was not internal to engine or you will have a lot bigger issue.

Need to discover why it is running hot to start with but not drive if it is causing the motor to get noisy. Overheating can be due to a thermostat stuck closed, cooling fan not turning on, low coolant level etc. Need to get to the bottom of the overheating before engine is damage badly.

How many miles on engine? Was oil changed after it ran hot? Install a good mechanical oil pressure gauge to see what is really going on. Also might use length of hose put one end in ear and listen with other end to locate exactly where noise is coming from. BE CAREFUL NOT TO STICK HOSE INTO SPINNING PARTS. If noise is from bottom of engine bearings may be shot. However, it could be mechanic left some part loose and that would be an easy fix.

It sounds like the thermostat has stuck in the closed position, it may just need to be replaced. If the truck ran hot it may have caused more serious problems. I would change the thermostat and run it, If the problem then occurs, check for a blown head gasket or a cracked head. but change the thermostat first. Hope this helps....

Replace the thermostat it is stuck shut. Bleed the system before driving away.

If you ran low on oil and ran hot the news is probably not good. If the knock is there when you tap off but goes when you accellerate that is a big end bearing. If that lets go you will be looking at an engine replacement so it needs urgent attention. If it is a lighter knock coming from higher up in the engine it could be a gudgeon pin bearing and whilst it will need replacing it is not as urgent as the big end. For the next time the way to get out of a slippery mess is to let the clutch out slowly and give the engine as few revs as possible to allow the tyres to grip. The moment they start spinning you have lost the plot and you need to start over again.You can even try turning the steering a little to get grip on the sides of the gully.

First you need to see if the engine is overheating or if the engine is running cool. The job of the thermostat is to maintain a set operating temp, usually 190-200 deg f. If it overheats everytime it is ran then your thermostat may be stuck closed. If you start it up and it heats up to like 220 deg and then goes back down to 190 that tells you that the thermostat is sticking closed and poping open when its really hot. If you cant get to 190 deg f that means that the thermostat is stuck open.

The thermostat could be stuck closed.
The worst case is you over heated the engine to the point you could have internal damage to the engine . I would change the thermostat first. IF that does not work , I would take to a shop to have checked out.

There are two primary symptoms of a faulty thermostat (assuming you mean the coolant thermostat in the engine not the A/C Heater Thermostat).  The thermostat can fail in basically two ways:
1)  Stuck open - Engine runs colder, heater takes forever to heat up. 2)  Stuck closed - Engine runs very hot, then will suddenly drop in temperature, then run very hot again.  Or engine overheats constantly.  When the engine is hot, feel the upper radiator hose.  If it is not hot, you can bet the thermostat is stuck closed.  If it is hot, the thermostat may not be stuck close, but not opening at the proper temperature.
Stuck open is annoying, but not dangerous.  Stuck close is dangerous and can cause overheating-related damage to your engine.
If you are handy, you can replace it yourself.  Otherwise it is an easy job for a mechanic.