You should always replace a cracked radiator cap as soon as you are aware it is damaged. Driving with a cracked cap can create extensive damage to the radiator and the engine. The caps are relatively inexpensive and easy to replace. Here's what you can do if you suspect you have a cracked radiator cap.
Step 1 - Ensure It's the Cap
A radiator cap is designed to allow steam and coolant to seep through the cap. This way, if the fluid boils, pressure won't build in the radiator. This is a problem that should be corrected. But if you notice fluid coming from the cap you should check to ensure it is a crack in the cap. To do this simply allow the car to cool off completely and take off the cap. Inspect it for damage. If the cap has no damage, it's not the cap that needs attention.
Step 2 - Find Radiator Cap Replacement
While all radiator caps serve the same purpose, they do have slight differences between the different makes and models of vehicles. You can either take the old radiator cap with you to an auto parts store, call your local dealership, or just call an auto parts store with the make, model, and year of your vehicle. It's important to use the correct cap. Just because it appears to fit doesn't necessarily mean it will have the necessary pressure and venting specifications.
Step 3 - Replace the Cap
With your cars engine off and cool you can just replace the cap. It's a good idea to wear gloves or cover the cap with an old towel. You will want to turn the cap counterclockwise. Do not completely remove it yet. After you've twisted it a bit, allow it to sit for a few minutes. This will allow any pressure that has built up to be released. Failure to follow this step can result in injuries, so just be patient and wait. After about 5 minutes you can go ahead and finish removing the cap.
With the old cap off, you can simply toss it and replace it with the new radiator cap. You will want to turn the new cap clockwise and be sure it's as tight as you can get it without using any tools.
Step 4 - Testing
Start the engine with the hood still open. Be sure you stand away from the radiator in case the cap is faulty. You are checking for bubbling or any discharge around the cap. As long as you don't see any, you're good to go.
It may be tempting to purchase a generic or cheaper version of the radiator cap. This isn't recommended. Getting the cap made specifically for your vehicle will ensure the cap is the right size, has the right pressure controls, and will have a lower chance of failure.
If you are driving and stranded because of a radiator issue, it's best to have the vehicle towed to a repair shop. Driving with fluid leaking from the radiator will cause damage to the engine since it can't cool properly.