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You can add a bit that way, but if the expansion tank is completely empty there's a chance the radiator is so low with coolant that it might let the engine over heat. If you do fill it that way run the engine until the temp gauge comes up about half way, then fill the coolant expansion tank to the top. That will allow the system to suck in the maximum amount into the radiator. Once the engine has fully cooled check the expansion tank and refill as necessary. You'd need to do the heat and cool cycle once more to make sure the expansin tank stays full.
One thing to consider is where did the old coolant go? If you're losing coolant it's going somewhere. It's normal to lose a very small amount over time but if the expansion tank is going dry there's a leak somewhere, either in the plumbing between the radiator to the expansion tank, somewhere in the cooling system or there's a head gasket leak.
You can add a bit that way, but if the expansion tank is completely empty there's a chance the radiator is so low with coolant that it might let the engine over heat. If you do fill it that way run the engine until the temp gauge comes up about half way, then fill the coolant expansion tank to the top. That will allow the system to suck in the maximum amount into the radiator. Once the engine has fully cooled check the expansion tank and refill as necessary. You'd need to do the heat and cool cycle once more to make sure the expansin tank stays full.
One thing to consider is where did the old coolant go? If you're losing coolant it's going somewhere. It's normal to lose a very small amount over time but if the expansion tank is going dry there's a leak somewhere, either in the plumbing between the radiator to the expansion tank, somewhere in the cooling system or there's a head gasket leak.
