Can I Add Water To Mercedes Coolant? -Mechanics advice


Warning light on, low coolant level, but what if you’re in a pinch and you don’t have coolant, but you do have water. 

It is common practice to top up Mercedes coolant with water, however, constant top-ups using water will weaken the cooling, antifreeze, lubricating, and anti-corrosion protection.

In this post, you’ll learn when it’s Okay to top up a Mercedes with plain water. You’ll also learn how to test the strength of your coolant.

Merc GL

Avoid Constant Water Top Ups

It’s Okay to top up Mercedes coolant using just water, however, using coolant to top up is always best. The problem with using water is, it dilutes the coolant strength. I should point out when we say coolant we are also talking about the antifreeze, they are one and the same.

You’ll find a link to Mercedes compatible coolants here on the consumables page.

Coolant

Coolant is a concoction of chemicals designed to perform four main tasks: 

  1. To cool
  2. Offer freeze protection
  3. Lubricate
  4. Prevent engine corrosion

To cool – Using plain water as coolant will challenge your Mercedes coolant system. Coolant is specially formulated to have a higher boiling point, and that’s a good thing. In the dead heat of summer or a five-mile traffic jam. 

Allowing a car to overheat risks blowing the head gasket or worse warping the cylinder head.

Having the correct quantity and strength of coolant will make the difference between steam by the side of the highway or sitting pretty behind the wheel with the a/c on full.

Antifreeze – The antifreeze additive offers frost protection down to -35F, constantly topping up with plain water reduces the effectiveness of the antifreeze, effectively increasing the chances of frost damage. 

Frost, as you know, causes water to freeze and expand, when the water inside an engine freezes it cracks the cylinder or block causing thousands of dollars in damage. Antifreeze is super important.

Lubrication – Your Mercedes coolant system employs a pump to move coolant around the system, the pump shaft needs lubrication, rubber seals also need to be lubricated. Without the correct quantity of lube in the coolant, seals and pumps are likely to fail prematurely. 

Blocked car rad

Anti-corrosive protection – Corrosion isn’t just for the bodywork, without the correct quantity of anticorrosive additives in the coolant, the inside of the engine would begin to corrode. Head gaskets are usually the first to corrode. 

Corrosion is difficult to remove and causes issues like pump failure, and total engine failure in really severe cases.

How To Test Mercedes Coolant

Visually check your coolant, it comes in many different colors, pink, blue, green, yellow. Color is less important than how it looks. It should be strong in color. A weak washy-looking color indicates the coolant may be weak, old and exposing your Mercedes to frost damage.

fresh coolant

Coolant should be changed every three years, old coolant loses its ability to protect, and worse than that it becomes acidic. The acidic coolant eats head gaskets, seals. 

In severe cases, the engine metal is eroded and this type of damage can’t be repaired, the engine is totaled.

If your coolant is older than three years, go ahead and change it out. 

You can check the acidity of the coolant using a voltmeter. 

Set the multimeter to volts, place the black probe on chassis ground and place the tip of the probe in the coolant. A reading close to or above one volt means it’s eating your motor.

However, it’s more common to simply check the strength of your coolant. Three different tools are commonly used. 

  1. Refractometer
  2. Disc type tester
  3. Dip strips

I use a Refractometer, it’s fast and accurate. Any reading below minus 20 F is good.

The disc-type tester is super user-friendly, it’s a turkey baster-type tool with floating discs inside a clear tube. To test your coolant, suck a sample of coolant from your reservoir using the tool and the disk that floats inside the coolant-filled clear tube identifies the antifreeze strength. Super easy to use.

Dip strips are user-friendly too but a little wasteful as they’re one-time use. To use them, dip the strip into the coolant and read the strip color.

You can check out all these tools on the Coolant system tools page.

Car coolant

Coolant is sold as pre-mixed or as a concentrate. The concentrate is generally purchased by owners that live in extremely cold conditions. Using the concentrate allows the user to mix the coolant to a max of 70% coolant concentrate to 30% distilled water. This offers protection below -35 F.

Regular coolant is mixed at a ratio of 50:50. Meaning for example. This typically offers antifreeze protection down to -35 F.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty-five years, and I've worked for GM, Volvo, Volkswagen, Landrover, and Jaguar dealerships. My passion is cars. I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of car ownership, including buying advice, maintenance, and troubleshooting.

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