1988 GMC Sierra Overheating - coolant loss please help.

[Jj1985]

Truck is a 5.7 with 155,000 miles on it. About a year and a half ago we had an overheating problem in the winter, put a heater core in it and it was okay for about 6 months, we then put a thermostat in. Again. It helped for about 6 months, in the winter time the truck pretty much sat because I was sick. And when it was driven there was NO heat. When spring hit, the truck ran warm, the heat would be on and would get ice cold and temperature climbed sky high, then it seemed like a dam broke and the heat got hot and temp dropped back down. So I bypassed the heater core, and the truck still did the same thing at the gauge. Thats pretty much when I started suspecting a head gasket problem.

Fast forward to about 3 weeks ago, truck was fine. It had gotten warm but not boiling once. I was on the highway, the truck ran great, temp gauge stayed where it should have, then i had a series of stop lights when I got off the exit, the temp gauge climbed and climbed until it pegged and the check gauges light would come on. Then I would take off and the temperature would drop back down, to around 220 to 230, until eventually it just hovered around 250. Which is not okay with me, So I parked it because I have another mode of transport.

Well two days ago my 96 caprice lt1 radiator split, and the radiator couldnt be found in town for it, so I threw a new radiator in the truck so i could have something to drive hoping it was maybe clogged. Filled it with new coolant and ran into the SAME issues. Each time it would over heat(high temp at the gauge, bubbling in reservoir) id have to add about a half gallon of coolant/water mix. Never more than 50/50. The last test, I took the thermostat completely out, the temp gauge never went over 140-150 approx, but the radiator was again about a half gallon low. The reservoir however at this time was full and boiling. Full of chocolate milk water.

EVERYTHING regarding the cooling at this point has been replaced. Except the clutch fan, water pump, and heater hose at the back of the intake. Which shows absolutely zero signs of leaks. And im still having this problem. Which is why I think there is a bad head gasket, heres the kicker though, the oil is pristine and there is no white smoke, sweet smell or condensation coming out of the tail pipe. I took some photos and video of the coolant conditions at different times.

This truck is sentimental to me, it was my grandpas and we spent many of time in it together when i was younger. So its not going anywhere, I just want to get down to what problem I am having so I can take care of it. Any and all help is greatly appreciated! If i have left out any details that you need to know please enlighten. Lol. Thanks is advance.