T'stat has a small hole at rim, with a jiggle pin. This vents air from the system.

T'stat installs on a sloped plane, vent hole belongs at the high point. Bubbles rise through hose to radiator, and out the rad's smallest hose to the reservoir where they escape via the pressure-vent in the cap.

If installed wrong (many how-to manuals won't tell you about the vent), air bubbles will remain.

Also if you goop up the gasket area with sealant you could block the vent hole. If the gasket seating area is smooth and clean you shouldn't need sealant - the two metal parts mate metal-to-metal; the gasket within the space seals due to its thickness etc.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.