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Originally Posted by Diseasel300
A crack in a cylinder head can most definitely allow combustion products into the cooling system, but not coolant into the cylinder.
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Only if the crack is opening under combustion pressure. It is possible but not common and I've never seen on in person or discussed in a trade magazine. I'm basing this on decades of real shop experience across many makes.
The world is made up of more than friction-less pulleys and ideal gasses. ( Think about the ideal gas law for a bit, what I state is more subtle than you think. )
As for checking for a combustion to coolant leak with sub 90 PSI air, read this
M104.980 too much radiator hose pressure?
This links to a rad pressure on an aluminum head M104. Post 18 says
Quote:
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A leak down test confirms it. I went in order of 1-5-3-6-2-4 and 3 was the one that was leaking into the cooling system. Pressurized the cylinder at TDC with around 30psi and I heard bubbles in the coolant reservoir.
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Have a look at the pic, the fire ring on 3 is intact and only slightly dark on the drivers side. Granted we can see the underside of the gasket but gee, our guy found the leak at 30 PSI. Number 1 is at bottom.