Typically not much flow through the expansion tank. Most cooling systems, the return feed to the expansion tank is for overflow under extreme heat, or a little expansion/contraction in the system during cool down, but really not enough to move a lot of volume under normal conditions. If sampling, IMHO, you would be better off capturing from the radiator drain.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)