The block of the engine is of open deck design: the cylinder walls are not braced at the top. Under boost and heat the unbraced cylinder wall moves sideways, and this micro-movement puts a shearing load on the fire ring of the head gasket. The ring cuts further into itself with each thermal cycle, and because there is no solid backing underneath it between the cylinders, it extrudes into the coolant slots. In the end it lets pressurised gas and coolant through.
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It is not the mileage that decides but the year of casting and the use. On the blocks cast between 2015 and 2019 there are grooves forming slots between the cylinders, and the risk is highest on those. In 2020 the maker revised the casting: in place of the open slots came small coolant passages drilled diagonally from the outer wall, and with that the metal backing returned. The number of failures dropped noticeably as a result.
The load is what counts, not the distance: the critical case is a heat-soaked restart after hard use.
The symptom is coolant going down with no visible dripping, white smoke, an uneven, stumbling idle, misfires (codes P0300 and P0316), mayonnaise-like oil and running hot.
The repair starts with a pressure test, then a revised gasket, a new cylinder head or a complete block replacement follows, depending on the extent of the damage. An engine that had already overheated before the repair can remain distorted, and then the coolant loss comes back.
What to check
- Check the casting year of the block: a casting from between 2015 and 2019 is the risky one, post-2020 is the good one
- If it is from between 2015 and 2019, ask for the repair history of the head gasket or proof of a block replacement
- Look at the coolant level and ask whether it has had to be topped up