Personally as there seems to be issues with bleeding the coolant properly, I would be inclined to think it would be a coolant issue.
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If the block was cracked I would have though the coolant temps would sky rocket as soon as you boost the engine, not after repeated attempts.
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You would also probably blow coolant out of the engine as the compression stroke would pressurise the coolant channels and force the coolant out.
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From what I recall Dan hasnt had any coolant loss?
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If the block was cracked I would seriously doubt the engine would be making good power, and I expect Dan would experience rough running.
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Equally on that matter, I doubt its the HG as the engine wouldnt sound too go if this had failed?
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Could something like the water pump have failed?
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My old CRX siR had a similar issue, normal driving wasnt and issue unless it was sat in traffic
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The water pump came out and the impellor on the water pump had badly corroded and the blades reduced to stumps.
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Maybe inDans case he is getting restricted coolant flow?
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So he isnt getting sufficient cooling which is proving and issue after repeat runs?