Under the Hood: How a radiator expansion tank works

McClatchy-Tribune Information Services

May 20, 2019

Q: My question is about radiator overflow tanks. Iíve always had cars and trucks that had an overflow tank that you would keep full to the cold or hot mark. Checking over my daughterís car, she has a tank with a screw-on cap, and I canít find any other way to check the radiator fluid at the radiator. Can you tell me why the difference and how this should be maintained?

ó Jim S.

A: It sounds like your daughterís car is equipped with a radiator expansion tank rather than the overflow container youíre familiar with.

Older domestic vehicles typically came equipped with a top or side tank radiator utilizing a pressure relief cap. As engine coolant heats up it expands, causing a buildup of pressure. Operating at up to 15 psi is a good thing as this raises the boiling point of water by about 3 degrees F per pound of pressure (257 vs 212 degrees F). A 50/50 mix of coolant and water raises this even further to about 265 degrees F. With the typical operating temperature range of most vehicles being about 195 to 230 degrees F, this insures the coolant remains a liquid and does the best job of transferring engine heat to the radiator. Older vehicles would discharge coolant via a tube to the pavement when excessive fluid expansion occurred ó not the best thing!

Adding an overflow container to capture spillage and a combination pressure/vacuum cap was a good idea, as any lost coolant is contained and can be drawn back into the radiator as the system cools down (the vacuum valve in the cap allows this). These systems typically have a simple container cap and the overflow tube extends near the bottom of the container for draw-back.

During normal operation, the coolant level in the container varies between the cold and hot lines. If the coolant level in the container is maintained and the tube and cap do their job, one might assume the radiator is full ó although I still like to check it! Should even a small leak occur, cool-down vacuum may not be sufficient to draw fluid back to keep the radiator full.

Your daughterís car uses a much sturdier pressurized expansion tank with a pressure relief cap instead of the simple overflow container. The expansion tank is located higher than the radiator and functionally becomes the top of the radiator, adding fluid capacity, with plenty of room for coolant expansion. This method allows a low hood line, as the radiator can be a bit shorter in stature and doesnít have itís own cap.

Another advantage is that by keeping the radiator completely full of fluid, fewer air bubbles occur, which improves heat transfer. An expansion tank also has cold and hot fluid lines that should be periodically checked, and one can observe true radiator fluid level without removing the pressure cap. Never remove a pressurized cap unless the system is stone cold!

My neighbor was recently told by a repair shop his BMWís expansion tank needed replacement due to losing fluid, at a cost of $400, among other service work performed. He declined the job and we looked at it together over a period of several days (a luxury most shops donít enjoy). It turned out the tank was fine! It seems the technician had overfilled the tank and it would discharge coolant from the cap when driven, which ran down and collected around the tankís top seam, causing it to appear faulty.

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