Yj over heat

Nope. Leaky valve did cause a miss, but not the overheat. Throwing a rad at it tomorrow. I know the cylinders aren't bleeding into the water jacket, so that's the only thing it could be, although it seems clear. It takes longer than the block to drain totally.
 
You probably flushed enough crap into the core to handicap it a bit, especially as gummy as that stuff looks. Mystery solved though.
 
Oh and for what it's worth, my old used and not clean brass/copper radiator cools better than the brand new single core parts house unit that is spec'd for a 91 yj 4.0. I couldn't get it over 210 at idle with the old rad & if I revved it up, it dropped below 210. Parts house radiator, idle up to 215-220, revving wouldn't drop the temp but going down the road did. So much for oe quality.
 
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