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Q: Coolant seems to disappear

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My car runs good. No weird noises or smells. I recently changed the water pump pulley, drive belt and tensioner. It would oveeheat real fast I couldn't even drive it. After I changes those parts I filled it with coolant and it ran good for a couple days then it was overheating again. But now it will get hot and sometimes it cools itself off before I can pull over. I checked and it has no coolant. I left the cap off and started it and put coolant in it and it was sucked down. I could feel the coolant in the upper radiator hose and the upper hose did get hot. The radiator fans come on. The radiator cap is clean and has the rubber and spring still work fine. The radiator isn't blocked. It doesn't show its leaking coolant on the ground. I've heard that these cars do go through coolant. But a gallon in 2 days seems a bit much. So where could the coolant be going?

My car has 153735 miles.
My car has an automatic transmission.

If this "just started", it may be that there was trapped air in the system, since you did the water pump replacement, and so all you are doing is providing make up coolant at the moment. But, if the coolant level continues to fall in the coming days, and you have to keep adding coolant, clearly that would be an indication of a leak. If the leak is not visible externally, it could be leaking at the head gasket or at a crossover passageway through the manifold. A static pressure test (cold engine) would help as if the leak is external that test would help identify the location and mark the area needing repair. If pressure won’t hold on a static pressure test and there is no evidence of any external leak(s) that means there is an internal leak which will necessitate partial engine disassembly to diagnose and repair. I would recommend you request a cooling system leak diagnostic to be sure that the leak is not external (external, other than from head gasket area) and thus potentially repairable without major disassembly.

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