Expansion Tank Cap / Coolant Reservoir Cap for auto

Heater/Cooling System auto parts: Expansion Tank Cap / Coolant Reservoir Cap

How to detect the coolant reservoir cap failure

The reservoir cap examination is performed in several steps. First, the component should be visually inspected and checked for tightness of its fitting to the coolant reservoir, the absence of cracks, scratches, rust and soot deposits on the surface. Then you have to test its ability to maintain pressure within certain limits.

For assessment of the intake valve operation you need to remove the cap, tightly squeeze one of the cooling system hoses (so as to let the air out of it), and then close the reservoir. If the hose immediately returns to its original shape, the valve must be in good working order.

To test the outlet valve, start the engine, warm it up to the working temperature, and after that unscrew the reservoir cap. If you hear a hiss, it indicates that the part is tightly sealed. But at what pressure the valve activates cannot be determined in this way.

For more comprehensive diagnosis, you will need a pump with a manometer, and an expansion tank identical to the one installed on the vehicle. The pump should be connected to one of the tank fittings, and the remaining ones should be plugged up with improvised means. The reservoir neck should be closed with the cap which needs to be tested. After that, you need to switch on the pump and observe at what pressure the outlet valve opens: a characteristic click followed by a hiss is usually heard. If in the course of further pump operation the hiss increases, but the pressure stays at the nominal level, the part is properly working.

You may identify that the coolant reservoir cap needs diagnosis by the following symptoms:

  • antifreeze boils during normal engine operation;
  • insufficient performance of the heater in the passenger compartment;
  • coolant leaks out from the hoses and pipes connections, and also from under the expansion tank cap;
  • after a long parking, the hoses are warped;
  • engine overheating becomes more frequent;
  • steam is coming from under the hood;
  • the coolant level has sharply decreased.

Causes of the coolant reservoir cap failure

The component failure is mostly caused by contamination. The accumulated dirt leads to jamming of the valves, prevents them from tight fitting to their seats, causing the seals to fail prematurely. The cap leakage may also occur due to slackening of the return springs or impaired threaded connection.

Deposits in the expansion tank are formed for various reasons: because of delamination of the cooling system hoses, delayed coolant replacement or its low quality, usage of water instead of antifreeze in the cooling system, oil ingress into the tank, etc. To prevent jamming of the cap valves, you have to periodically flush the cooling system with special agents and regularly clean the expansion tank from dirt.

Replacement of the coolant reservoir cap

Even a beginner driver can manage the replacement of the coolant reservoir cap. For installing a new component just open the hood; carefully, so as not to burn yourself with coolant, unscrew the cap; clean its seat from dirt and install a new part.

When selecting a cap, consider its actuation pressure: the values ​​indicated on its surface must comply with the specifications of your vehicle cooling system.

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