On my 2007 V-Max when I turn it on the coolant is at a good level, then I drive around running errands and I stop check the truck and it seems like the coolant level is almost empty. Then while the truck is standing the coolant level goes back to normal. Is this normal? Never noticed this in my older GMs.

 

You may need to bleed the system, sounds like when the t-stat is opening your motor is taking all the fluid..... Turn the truck on and have your cap off, when t-stat opens start dumping 50-50 mix or water in untill its full!

 

Could this harm my engine? How can I check if my thermostat is working?

 

You probably have a 180 degree t-stat, is the truck running hot by any chance? Pull the cap keep note of your engine temperature, if the coolant is sucking down right after 180 then your low on coolant, fill it and make sure you dont have any leaks..... If you run the motor hot yes it will damage it

 

Yeah it has the stock thermostat, it runs the same like always at 210 doesn't go over.