My expansion tank keeps, requiring a top up, at least once a week. The vehicle went for a pressure test, which indicated a small leak on the cap, which I replaced. After this I took it for a chemical test. But the cylinder head and gasket is in order. Another pressure test indicated that there are no leaks on the system.

1. Is it normal to top up the tank? If so how frequently?
2. On other vehicles, the coolant tanks have a low and high mark. Apparently, the anti freeze, expands when hot, and goes back to the original state when cold. Which would give to levels, depending on the time you check the level. Is there not a low/high mark on the bottle on my mercedes

 

Hello and welcome to the forum! The expansion tanks are known to go bad after a period of time and even start to turn yellow. You'll likely see these micro-cracks which can cause the tiny leaks that you're talking about. It's not a bad idea to replace them after a prolonged period of time, the procedure isn't too difficult - I'm attaching a link to our site's DIY tech article on the install below. Please let us know if you have any questions or if we can assist with any future part needs.

https://www.pelicanparts.com/techart...eplacement.htm