newwwwbie!

Whenever a motor runs hot, pinging is common.

My vote is a intake manifold problem. This would explain the difficut re-start (vacuum leak), the external cooling leak, and the lack of oil in the water.

Also possible that the heater lines are leaking. The difficult re-start could be from just plain over heating.

Very common for oil and cooling to cross mix when ever a head gasket blows.

Suggesting performing a compression test so that you will know what you are dealing with.

Note, it's important to remember that the V6 will trap air in the motor if the coolant level gets too low. When re-filling a V6 it is vital that the coolant re-fill proedures be followed.

Difficult re-starts could also be fuel pressure leak down. Try cycling the key on/pause/off several times before starting. Listen for the fuel pump to run. Does this improve starting?

Metal plug on the heads? Are you sure you aren't talking about the block? Is it an indented piece of metal like a cap upside down? Or does it look like it has a square notch in it? It might be a freeze plug in the block. I'm not familiar with anything like that on the heads.

You just bought this from a used car dealer? They should have some sort of 30 day period where they should fix anything on the car for free. Otherwise I'm calling lemon on this one. The dealer should have told you there were problems ( of course some idiot dealers don't) . You should fix this mess... not you. And it shouldn't cost you a penny.

Aight.. first I wouldn't say it's a motor mount. The car don't go from running smooth to shaking bad and be a motor mount that quickly. Those things wear out slowly. The way it's shaking sounds to me like it's missing... meaning it's mis-firing on a cylinder or 2. I'm also betting you that the Check Engine light is flashing on and off as it runs ... ? Yes? That would be the computer telling you it's detecting a continuous misfire when it flashes like that. Is your exhaust smoking? White smoke maybe... if it is? Did you refill the coolant after you replaced the intake manifold plug? Also, after you refill the coolant, you must also bleed the air out of the cooling system. There is a special plug with a white cap on top in front of the intake that is used for bleeding the air. It's basically a bolt plugging up a hole and it has a white piece of plastic attached to it with red writing on it. It's right above one of the coolant sensors. To bleed the air out, while the engine is cool remove that bolt. now remove the radiator cap. Add coolant if necessary. I like to put the cap back on the radiator and squeeze the upper radiator hose slowly. That can push air bubbles out of that plug hole. Do that a few times... kind of wiggle that upper hose even then replace the plug. Start the engine, let run for a minute, shut off , wait a few minutes then remove that plug carefully and the radiator cap. Repeat once or twice to get the air out. This air bleeder port was added on this engine specifically because this engine has a flaw where it can trap air at key locations near the heads , and that creates HOT spots. So this is why you need to bleed the air out.

As for why the engine is mis-firing< that could be a number of things. When did it start misfiring? Check your spark plug wires and anything else and make sure you didn't accidentally disconnect something. Only part of the lower intake should have any coolant in it. It may have sucked up some from earlier so it could still be trying to burn that off which is why you may have a hard start. Check to make sure white smoke isn't coming out of your exhaust. Also check your coolant and make sure no rainbow like colors show up (oil) , and check your oil, pull the dipstick and make sure no milky whitish colors show ( coolant) up on it. This would be signs that the head gasket may have blown.