Not only is it a heat sink to cool the fuel on the return side but I imagine it also acts as a baffle to smooth out surges in the fuel supply.

The CIS system is a fairly primitive set-up. Effective yes but not exactly technologically advanced. 'Course, "if it ain't broke, don't fix it" is a pretty safe way to ensure reliability.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
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