Hi. I’m swapping a gen 4 JDM motor into a Gen 3 Prius. On the new motor, below the EGR, there are some new cooling bits. On the Gen 3 head there is a large radiator hose connection, then a cooling temp sensor, then a smaller radiator hose connection. On the Gen 4 head there is a large radiator hose connection, a temp sensor, BUT then 2 more electrical plugs? And I don’t see the smaller coolant port coming out of the head? Can anyone explain what I’m looking at on the new engine? IMG_9510 by Tbic posted Apr 1, 2026 at 1:10 PM IMG_9507 by Tbic posted Apr 1, 2026 at 1:09 PM
You might find some explanation in this video series, about 2/3 through: Swapping in a Gen 4 Prius' 2ZR Engine into the Prius v | PriusChat
Old head had two hose nipples coming right out of it, gen 4 has one big outlet for the "water-outlet fitting" to bolt on, and the hoses attach to that. US market got a simpler water-outlet fitting, while Euro and Japanese markets got a more-elaborate one. The gray thing is an electrically-operated "flow shutting valve" that the ECM can use to influence the temperature of the water going to the EGR, cabin heater, and EHRS. There is more info in this thread. This drawing's not from Toyota, but from an auto-research firm. Funny that they've labeled the same valve "flow shutting valve" in the physical layout, but "selector valve" in the schematic. When the valve is closed, those paths only get cool water straight from the water pump. That cools the EGR better, and picks up more heat from the EHRS, but doesn't offer much heat to the cabin. When the valve is open, hot coolant exiting the cylinder head is supplied, so you get better cabin heat, but less heat picked up from EHRS, and less cooling of the EGR. One approach you can take in an engine swap is to buy the US version of the water-outlet fitting (which doesn't have the flow shutting valve), and cap off the extra coolant line coming around from the water pump.
