BMW 540i Expansion Tank Cap
Telltale Signs of a Failed BMW 540i Expansion Tank Cap
Your BMW 540i expansion tank cap is a tough component that was built to withstand and maintain the pressure inside your expansion tank, but it can still give in to old age and eventual wear and tear. Knowing how to check for a defective expansion tank cap can save your Bimmer from more serious cooling system problems. Here are some of the commonly encountered BMW 540i expansion tank cap issues:
Rapidly overheating engine
Among the many possible things that can cause an overheated engine is an expansion tank cap that can't hold the pressure inside the expansion tank. When this happens, your coolant won't boil until your engine has already become way too hot. Try using a pressure tester on your expansion tank cap to see if its relief valve opens under the proper pressure range specified in your vehicle's service manual. If the results fall even just slightly above or below this range, then you have to get a new expansion tank cap for your Bimmer.
An errant drop or two of coolant around your expansion tank is pretty normal, but if you notice that a good amount of coolant has been pooling around it, then you should check your expansion tank cap. Try tightening your expansion tank cap and see if doing so will take care of the leak. If not, your expansion tank cap might have some cracks or dents that are allowing coolant to escape. A rusted expansion tank cap can also result in a coolant leak, so make sure that you replace your expansion tank cap if it shows any of the signs of damage mentioned.
Other troubleshooting tips
Sometimes, your expansion tank cap might produce a hissing sound accompanied by some steam. This can be caused by a damaged O-ring seal on the cap, so be sure to check this component as well. If the rubber O-ring seal has become brittle, cracked, or warped, you will probably have to replace the entire expansion tank cap. A stuck pressure relief valve on your expansion tank cap can also cause problems for your cooling system, so make sure to look out for this problem as well.