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How much pressure to test coolant tank and cap?

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I've searched but didn't find anything. I plan to pressure test my coolant system to track down a leak. How much pressure is safe? 20PSI? Also, at what pressure is the coolant cap suppose to open if I pressure test it?


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For the coolant system the manual recommends 18.85 PSI.

For the cap:

1. The pressure relief valve opens at an excess pressure of approx. 20.3 PSI; the pressure then slowly decreases again, and the valve closes at 14.5 PSI. If the pressure decreases further, replace the cap.
2. If the test pressure of step 1 is maintained, test step 2. Apply excess pressure to the pressure relief valve of step 1 by pumping rapidly. The pressure relief valve of step 2 must open at an excess pressure of approx 26 PSI. If the valve does not open, replace the cap.
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  • 4 years later...
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14 hours ago, KK911C4 said:

Loren, I need to do this. I rented pressure testor from Auto Parts store but none of the adapters fit expansion tank.  What kit or adapter fits on coolant reservoir opening to pump to 19 PSI  for 1999  996 C4? Thanks 


The adaptors are often specific to the pressure tester brand.  The one used to attach to the surge tank is a VW type.

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