My 2020 S560 has 27,000 miles on it, twice now I have gotten a low coolant warning on startup and needed to add coolant. Has anybody else experienced this? I have never had a car I had to continually add coolant to like that...

 

You likely do have something to look into with that consumption. It could be something simple like a pinhole in a hose or something more complicated.
Would advise to inspect all hoses and water pump for signs of dried up coolant as a next step. Also check reservoir for signs of contamination.
If nothing found an oil analysis at next oil change.

 

Its under warranty, so its gonna be their problem lol

 

The pressure cap is a place where coolant can leak (in vapor form) without leaving traces--the water leaks out but the glycol does not.

I had an incessant leak in my Ferrari until they fingered the cap.

If the rubber seal does not look abso-friggen-lutely perfect; suspect the cap.

 

You need to add UV tracer dye to the system, and find the leak.
Also get an oil analysis in case it is internal.

Dealers will just start swapping parts and are just as likely to cause more problems later.

 

closed system which means should not loose coolant:
So have the system pressure tested to see if there is a leak and where it is located.
And have cap tested which is all Standard diagnosis of coolant leak.

In mean time check oil for chocolate mousse looking stuff in engine oil fill cap which is sign of possible head gasket issue or at worse a cracked block.

 

i had to top off my 2020 560 last winter. dealer said just put some water in. lol. i did! it worked fine. this summer, i took the car in for its annual maintenance, they found a cracked coolant reservoir and replaced it. when i asked them why i didn't see any evidence of a leak on the garage floor, they said there is a pan under that section of the reservoir to catch drips. musta been a slow drip! anyway - yes it happens. it is an easy fix. mine was under warranty.

 

Interesting ... I had my coolant light come on after first year of ownership and topped it off with about 8 oz of distilled water. My Indy did a pressure check and found no leaks, so I'm just going to keep an eye on it.

 

Definitely nothing in the oil filler cap...one of those times I'd like to have a dipstick to see the oil. I'm going to add distilled water, I'm about 2,000 miles from the 30K A service and I will bring it to their attention then. The dealer filled it around 10k miles and they didnt seem concerned that it was low at all.

I only get the low warning in the morning at the first startup, then I don't get it again.

 

A single fill after being placed in service is normal as the fill from factory could have had an air gap. But OP continued need to add is a problem.

 

As Tom said above, once 2 years ago I had to add a couple of ounces of coolant. I had same concern, where did it go? I have seen some say they had leakage at a joint in some hoses . Easy fix. But not my issue. I have a lift in my garage, and give a good inspection with led lights everywhere, and have never found any seepage from anything in 46,000 miles. So me adding was 17,000 miles / 2 yrs ago, nothing since. I'd have it checked, but wouldnt be too concerned.

 

The UV dye make finding even seepage much easier.

 

Yeah I'll bring it up at the next service stop and report back

 

The pressure cap was the source of leak on my 2019 S560. Took them forever to find it.

 

So I finally pulled the cover and checked the reservoir, and I'm pretty confident its the pressure cap:

 

Oh yes, the telltale white residue, that's definitely coolant. Oh, and btw, one of reasons I took off those useless covers to be able to keep an eye on things like this.

BTW, didn't mention in my reply earlier but when I first got my car in 2021 it had a bad cap with white residue underneath and needed coolant. I bought a $6 replacement on Amazon and no more white residue, but could still be losing a little bit of vapor here and there. Does seem to be a bit of a weak link in the cooling system

 

I'd say you saved yourself a dealer visit! Clean that up and put a new MB cap on, and I bet your good. I'd inspect weekly for a while. Looking at the dealer parking lot and service drop off gives me anxiety about getting a ding or scratch. I avoid the dealer at all cost! Good find, thanks for posting!

 

Yep, thats my plan. I'm going to order a new cap.

 

Glad you figured out the problem. My problem is I get a low coolant message every time I start the car but the coolant level is fine when I check it.

 

Add a little anyways. Mine took very little, so the sensor is really sensitive to any low coolant level.

 

Nice to see you found the glitch as I was not aware of caps could cause such weird things to happen

 

remember caps are the RELIEF VALVE of the coolant system.
They are what limit the pressure build up in the system so yes they can vent sometimes.

Pressure in system is important to increase coolant boiling point keep boiling point lower but too much can cause other leaks.

 

The mayonnaise in the oil filler cap being caused by a failure in the cooling system is largely an urban myth.It can happen but normally It's caused by using the car on short trips where the engine doesn't get get to full running temperature. Head gasket failures and cracked heads show far more obvious symptoms before you see milky oil.

 

A little moisture in the oil can be normal with a car used for short drives where it never has time at temp to be evaporated.

Lots of moisture is a sign of a problem.

 

Got that bass ackwards friend. Point of pressurizing the cooling system is to raise the point at which the coolant boils as once it turns to a gas there is not much ability for it to get heat out. Point of having ‘Anti-freeze’ being several.... Lowers temperature at which coolant freezes to prevent cracked heads, damaged radiators... raises temperature at which coolant boils and prevents corrosion in the cooling system while being friendly to water pump seals and preventing electrolysis between dissimilar metals.

The pressure cap has multiple tasks.... Prevent leaks.... Keep the cooling system pressurized as the coolant expands up to the point where it will blow off pressure to keep radiators, hoses from blowing up.... Allow air to enter the reservoir once coolant has cooled so the cooling system is not ever in a partial vacuum.