That's because old vehicles had metal radiator caps.

 

....and plastic coolant reservoir caps, just like is the problem on his S560. I've never seen a metal reservoir cap that wasn't aftermarket. And even then, they'd still have to have a seal of some sort, which is presumably what failed.

At no time has Steve intimated that there is a problem with his radiator cap.

 

Of course not....not on newer vehicles. But they were standard on older vehicles...as were many metal parts that are now plastic. In fact, on the cars I grew up with and learned to drive on, there were very few if any plastic parts, period, except for distributor caps and maybe windshield-wash-bottles.

 

We’re talking about the reservoir cap, not the radiator cap. The MB reservoir cap is pressurized though, which other cars I have had it’s not…

Cant even see the radiator cap in this car it’s all under the plastic shrouding

 

All the cars I remember from the 70s, including three we owned, had plastic coolant reservoirs with plastic caps. I don't have much experience with cars more than 50 years old, but for example, here's the coolant reservoir from a 1955 Buick Super:



Pretty sure the whole thing, including the cap, is plastic.

 

I love the smell of reservoir cap in the morning.

 

Yes, that just repeats what I said earlier......that cars of that vintage had a few plastic parts, but very few.....mainly distributor caps, underhood fluid-reservoirs, and (sometimes) red taillight covers. In fact, the cars I learned to drive on (60s-vintage Plymouth Valiants) still had metal master-cylinder-reservoirs for the braking system.

Now, that's not to say that everything about plastic parts is bad....plastic doesn't rust or corrode like metal. But, in general, they lack the strength and heat-resistance that metal parts do...and they heave a tendency to crack and break.

 

Clearly there’s something up with the MB cap…

 

Plastic can also be made into shapes that are nearly impossible to do with metal, even with complex casting techniques.

Plus, it's cheap to make plastic stuff aka just pennies per unit piece.

 

Bingo. There's the main reason you see so much of it today.

 

That is not a radiator or tank cap, that is a pressure bottle and that cap is the moderator for system pressure and vapors.

There is no rad cap on cars that use that style of coolant system at all, it is a non-overflow tank type setup and that cap is what functions similarly to a traditional overflow type system rad cap except that it is NOT intended to allow coolant to freely flow past it to a storage bottle when the system reaches operating PSI. It is also the emergency vent point if the coolant level reaches a very high state under heat or for other reasons, it is not supposed to allow/have a need to pass coolant. The reason I PMed Steve is to further explain the system and to tell him what to check since there may be an actual issue with the cap being the first victim.

 

That is my main concern, that there is some issue causing too much pressure which is causing the cap to fail…

 

Excessive pressure in the cooling system is usually caused by overheating. Is the temperature gauge (or warning light) normal? The S-class has a good-sized engine and power, which can produce a lot of heat even without a turbo.

 

No overheating ever, but it can also be caused by failing headgaskets etc...

The cap should be designed to withstand the heat the engine makes under normal conditions.

I did have this thought, remember how I added those covers that mysteriously Mercedes stopped putting on the car in 2019? I wonder if maybe this is why they removed the covers? This reservoir is underneath one of those covers



How mine came from the factory:



We assume that this was a cost savings measure, but maybe it wasn't...

 

Failing head gaskets cause the oil and coolant to mix, deleting the effectiveness of both and causing overheating.

Totally agree....no argument there. I've never been fan of plastic engine parts.

Yes, covers can and do add to heat-retention...I have never been a fan of them, either, as you know from my full-reviews.



However, I can't state with credibility that the cover you stuck on directly caused the cap-problem...I'm not enough of an engineer to do so.