Cadillac: 99 concours overheating. Head gasket?, combustion chambers, head gasket

Question
I just bought a 1999 cadillac concours. When I took it for a test drive, it performed fine, no problems other than a slight knock in the engine when idled. About four days ago it started overheating majorly, to the point where the car told me to shut off the engine. So I took it to a friend and had the cooling system checked, he put 40 lbs of presure on it and couldn't find any leaks. So we figured just flush and fill the coolant and see what happens, did that twice...still overheated (I did not know about the GM pellets at the time, so they weren't added). He told me it might be the thermostat, so I had that replaced. Now it still runs hot, but hasn't gone in the red since the thermo. was replaced. The mechanic that replaced the thermostat said it might be the head gasket and to keep an eye on it. Long story short, how can I tell if my head gasket is blown? Didn't notice anything off in the oil (none of that milky brown look I've heard about), and no water from the tailpipe. Also, if the temperature gauge isnt in the red, could my engine still be damaged by running the car?

Thanks for your time


Answer
Normal engine temp during moderate driving is 220-230. Under stress 240. You need to put the pellets in. You need to put more air pressure 80 lbs minimum into the combustion chambers and look for bubbles in the radiator.

make SURE that you have the IDENTICAL type and heat range thermostat in the vehicle as was installed by the manufacturer.

Usually when the engine temp reaches critical high temp you get a warning light and or the engine automatically shuts off.