1978 300 D Cracked head?

The 78 300 D that i rescued from the crusher about 30 thousand miles ago ran hot recently. The driver told me it just steamed up...started knocking and died. (she was not paying attention to gauges).

the water pump had gone south...no water left in system.

installed a new pump. it fires up easily. runs..no bad noises. bo big smoke. no water in oil...But it starts to miss when it heats up and the radiator cap is on. pop the cap and relieve the pressure it smoothes out.

Im thinking it has a cracked head or block and the pressure is forcing steam into a cylinder(s). anyone have any ideas on that. makes sense to me...
but i have never dealt with this type problem before.

anyone ever have any luck with block sealers they sell over the counter.

this is a spare car and not one i want to spend much on repairing.
will probably part out...but i just hate to see one of these old beasts
dissapear.

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