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996 Coolant tank cap leaks - what to look for

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A common complaint with the 996 coolant tank is dried coolant residue found around the coolant cap. This indicates that at some period during the operation of the car, coolant is leaking out of the tank mouth.

This condition is different from leaks under the tank. These types of leaks are indicative of the plastic tank cracking. The solution for this is to replace the tank.

The seepage from the tank mouth can be caused by two things.

1. The Coolant tank cap being defective.

2. The Coolant tank cap being not fully screwed on and seated on the tank.

Early 996 coolant tank caps have a design defect in them that will cause them to start 'weeping' coolant over time. These caps have a part number that ends in "00". If your car has one of these caps, it is best to replace it.

Later caps with an 01 or higher part number suffix do not have this problem, but can be difficult to fully seat because of the improved seal. Later caps that exhibit 'weeping' may not actually be defective, they are only not fully screwed on and seated.

The issue is that the cap gasket is slightly thicker, and will bind against the tank top and threads for the last 1/8th turn. Because the cap is hand tightened, it is easy to think that the cap is fully seated. In order to overcome this, clean off all of the dried coolant residue from the mouth of the tank, and the inside seal and threads of the cap. Then spray some silicone lubricant on the cap seal and threads prior to screwing on the cap. This will allow the cap to be fully hand-tightened.

The cap has a cruciform pattern on it which makes it easy to verify if it is fully seated. Further, the when the cap is fully seated, it will make a "clicking" feel as it is fully seated.

Here is an example of a coolant tank cap in which the owner reported that there was leakage. Notice the cap cruciform alignment (and that it is an 00 cap).


Here is an example of a coolant cap that is fully seated and has no leaks. Notice the cap cruciform alignment.


Before you replace your coolant cap or tank, make sure that your problem is just that the cap is not screwed on all the way!

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