Over the winter I picked up a reasonably clean Grand that wouldn't run. Seller said he was just driving down the street and it died. Sure, whatever, got it for a great deal.

After getting it home and checking the oil and finding 5 quarts of antifreeze sitting under the 5 quarts of oil (no it wasn't all whipped into milkshake) figured they fragged the engine. Pulled the plugs to do a compression check and found one plug sheared off and piston jammed into the head. Bingo!

Picked up a running used motor from a local salvage yard a few weeks ago. Got it in, fired it up started checking things. Everything looked good. Until the temp started creeping up. The overflow started bubbling, temp was still climbing, so I shut it down.

Next day drained the coolant out. The new coolant came out a muddy greyish-brown. Emptied it all out, disconnected the hoses, pulled out the garden hose, put the pressure trigger on it and started flushing the radiator. Started seeing a lot of metallic flakes coming out. Eventually a big blob of mud looking stuff plopped out. Kept the water flowing until it was clean. Looked out the blob of mud and it was shimmering metallic. I'm guessing the previous owner was trying to seal up a perceived head gasket leak or something.

So pushed a lot of water through the radiator. Hooked everything back up, reloaded the radiator with coolant, restarted and began watching. Everything proceeded normal, temp was climbing to normal range, around 210 the electric fan kicked on, temp slowly came down, fan turned off. This cycle repeated several times. Temp seemed stabilized and wasn't climbing over around 220, but after some time, bubbles in the overflow began again, so I shut t down. Figured I'd pick up some radiator cleaner and try another flush.

All that work was on Sunday. Tonight, Wednesday, I decided to drain the radiator. When I began to turn the cap, it was under pressure. A lot of pressure. Like 18# worth of pressure and it started blowing antifreeze out from under the cap..and it was cold antifreeze. So I turned the petcock to drain things. It only took a few turns to get flow started. Had to modulate how far I turned the petcock to keep a steady flow instead of a shooting stream.

So now I'm wondering of a cycle of cleaner and water will work at all. I have never encountered a cooling system that held that much pressure over several days . But it also has me wondering if I might have a cap issue or a bigger problem inside the radiator.

Thoughts?