Ok so I'll try and make this right to the point this is for a Eldorado I just bought AS is I got it for parts for my 02 Eldorado but figured I'd give it a whirl. So I had engine code for random misfire I cleared code and replaced all plugs I noticed one had a browish powder on it not sure what that was. Car had dirty oil and NO coolant. I ran a can of GM top engine cleaner through it drive it for 2 days to clean it out also with a can of chevron tech in the gas with shell 93. Of course I filled coolant up as I drove it home started to over heat noticed the coolant wasn't going past water pump so I replaced thermostat that didn't resolve a thing I then replaced the water pump and flushed the system before and after replacement filled up with dex-cool. Also replaced belt, and both pulley and tensioner pulley. Did oil change used a quart of mystery oil and a synthetic blend as I use synthetic oil but wasn't aware of history so didn't want to just switch. Car still was over heating now I just thought maybe the new thermostat got damaged cause it ran very hot so replaced it yet again. No luck still over heating it gets extremely hot like 240 to 265 and heat stops working about 235 and gets cold fans run no problem at all. I noticed the coolant boil and was in the reserve tank also boiling. I did notice today after it being filled that when it over heated I am 100% losing coolant just don't know where and it's not leaking on the ground I have a bad feeling it's a head that's bad but just want to ask someone's opinion. The oil is clean no antifreeze in it I will say when I flushed the system there was a massive amount of black sludge looked like oil but it came out of the drain where the transmission line connects, during oil change I noticed a bolt missing on trans pan and all bolts were hand tight so those are all set now. Hope that's enough info to get some ideas. I did buy a coolant temp sensor but didn't put it in cause I only bought it thinking it wasn't really overheating but now no it is for sure so I doubt that sensor will do anything. Also replaced all hoses and clamps just to be safe. The top hose gets very warm and the hose that goes to water pump not so much but pretty sure that's normal when it gets hot if I lift the cap on the res it bubbles through the entire system and you can hear it perfectly and the engine cools much quicker but the fact that I had to add 2 gal today after filling it last night tells me it's not good at all thanks for any help!!