Step by step instructions to determine if your radiator cap is bad. This is very easy!
First … You will see white residue under the cap and on the reservoir and or radiator. The white residue is from the bad cap allowing air into the system and water out during evaporation. The more water that evaporates, the more the vehicle overheats thus allowing even more evaporating.
White Stain Coming From Cap
Second … When flipping the cap over you will see a seal around the inside of the cap. The seal is normally made up of rubber or cork. Inspect the seal for cracks or an area that is smashed more on one side than another. The seal could be torn completely or in some cases missing completely.
Third … You should pressure test the coolant system. In most cases seeing these tale signs, you will not need to, but if you are unsure if it is the cap, then pressure test would be best. When you pressure test you will either find a leak, or you will not. If you find the leak, then you will make the repair, but if there is no leak, and you know your thermostat and the water pump is good, then it must be the cap. The cap is removed during the pressure testing, therefore, you have eliminated everything but the cap. So if you’re overheating, it must be the cap.
Coolant Presure Test Kit
Fourth … Replacing your cap. Be sure when you replace the old cap with a new cap, that you push down and turn in a clockwise rotation. Just turning the cap and not pushing down will not allow it to seat or seal properly. On newer models with the reservoir only, you may not need to. The rule of thumb is if your cap is partially or all metal, push down then turn. If it is all plastic, just turn clockwise until snug.
Fifth … There is an OBD code for a bad cap. If you check your OBD codes and you receive a P2181 code, this is telling you that the cap is bad. Click the video below for a detailed understanding.