Expansion Tank 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.