Probably an air bubble. You lost a lot of coolant and didn't bleed when you refilled. Refill while cold- max out the heat, crack the cap to the 1st notch and let it warm up a bit. When it belches, that's most of the air- add coolant to top it off then tighten the cap. Check coolant and add as needed until it settles.

PS must've been one heck of a hit to do THAT to it. Splices or not, you moved something you shouldn't have...

rjp

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)