Now the hood won't stay up. (or the tailgate on your wagon)What's up with that? Later model Mercedes use gas pressure hood struts instead of the older style hood springs to keep the hood in the up position. After time, these gas struts will lose pressure and become ineffective. Wagons use the same set-up for the tailgate. You might notice the struts becoming weaker the cooler the ambient temperature is. This is a sure sign they are losing pressure and will need to be replaced soon.