Coolant expansion/overflow tank


With your help I hope to put an end to the mystery of the coolant expansion/overflow tank in my 1997 Honda CRV. According to the dealer, there are times that the tank will be empty and there are times that there will be coolant in there, especially when the weather is warm. They advised that I should not add coolant to the tank; it will overflow. My husband says that they must be wrong and adds coolant to the tank. When I find coolant all over parts of the engine, we think the radiator is leaking. I had it pressure tested and there was no leak.

Another mechanic looked at it (non-Honda) and he said there must be a leak because the tank is empty.

I have had this car since 2002. It runs very well. I am looking forward to an answer to the ‘real’ workings of this tank. Thanks, in advance.

The tank should never be completely empty, because if it is, air can get sucked into the radiator. Typically, with the car off and the engine at ambient temperature, the tank should be filled with a 50/50 mix of coolant so that the level is between the “Full” and “Low” marks on the expansion tank. This leaves enough room for the tank to accept more fluid when the engine heats up and the coolant expands, but also ensures that when fluid is sucked out of the tank into the radiator, the tank won’t be sucked dry to the point that air enters into the system.

If the tank is repeatedly going empty, than you do probably have a leak, even if the initial pressure test didn’t indicate this was the case. If you search the Car Talk letters, Tom & Ray have a good description of the different ways to find a leak, and describe an “over-night” method of testing that helps determine hard-to-find leaks.

The owner’s manual should tell you where the proper fluid level should be in the overflow tank.

I’ve never heard of one that goes completely empty as part of normal operation.

The tank should never be empty. It should be at the full mark when it’s cold and somewhat higher (but not overflowing) when it’s hot.

Sounds like a bad radiator cap.

Your dealer is incorrect. Did you get this information from the “service representative” at the counter, as I suspect? If they really knew cars they’d be in the bays rather than at the counters. They often provide bogus information.

Riggy explained it beautifully, except I’d add that the fluid is sucked out of the tank into the radiator during normal cooling and contraction. The engine’s cooling system breaths in and out into the tank.

I also agree with Riggy that if you have to keep putting fluid in to keep the tank from going dry you have a leak. A good shop should be able to find the cause of the lost fluid without much trouble, whether it’s leakage to the outside world (regardless of the conditions under which it happens) or leakage into the combustion chamber(s) through a bad headgasket.

This isn’t rocket science. You need a good independent shop.

It was a service representative! They make it impossible in some of these places to get back where the mechanics are so they act as a go-between. I have to rely and the information that is relayed from the mechanic to the rep and vice versa. I appreciate your help and will go armed with this information. Now, if I could just find a good independent shop.

Thank you! I will do some more searching for how-to. Overnight? I will look.

Ask everyone you work with, everyone you know, and your relatives who they use…and who they recommend avoiding. Generally the cream will float to the top of the data.

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