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HOW-TO: Replacing 126 coolant overflow tank


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Some weeks ago, I was thinking about the coolant expansion tank on my 560SEC. It's the original tank, now 22 years old, and made of plastic that gets heated up and cooled down every time the car is run. Although thick plastic, it gets discolored and weakened with age over the years. I was thinking that this is a catastrophic failure point; if this tank ever cracked or broke while I was driving the car, I would at minimum be stuck somewhere, and at the worst could overheat and ruin my engine due to rapid and catastrophic coolant loss.

Of course, this is the same exact situation with our E500Es too. Our tanks on those cars are 15+ years old now, and also get discolored and brittle over time. Many of you have replaced them, but others (such as myself) have not but will plan to shortly.

I was placing an order to parts.com recently, and took a quick gander at the cost of the 126 coupe expansion tank and coolant level sensor. Both were inexpensive (I think the tank was around $25-30) so decided to tack them onto my order and replace them when I got the chance. Well, tonight was that chance. I took my time with the job and it took about 90 minutes. Generally it could be an easy 60-minute job for the average DIY'er.

I have detailed the job here in photographs for everyone. In general, this same process would apply to the W124 too.

An overview of the job:

First, you should drain the coolant. I did this using the pet-cock at the bottom of the radiator, draining about a gallon of coolant from the radiator into a large, flattish oil drain pan (well cleaned beforehand, of course). This was necessitated because the car is lowered and thus you can't get much under the front spoiler of the car. I used a large blue pint funnel under the pet-cock and above the pan to direct the spraying stream of coolant into the pain neatly. Once the coolant was drained, I closed and tightened the pet-cock with a "stubby" screwdriver (which I'd also used to loosen it in the first place).

Second, check the overflow tank. My drain job was enough to totally clear the expansion tank of coolant, so I was "good to go" in loosening the hose connections and the electrical connection to the coolant level sensor.

Third, after all connections are removed from the tank, remove the three screws that hold the ends of the tank in place to their underhood brackets.

Fourth, remove the retaining circlip that holds the coolant level sensor into the side of the tank. Insert the new coolant level sensor into the new tank (be sure there's a black rubber gasket to seal it from leaks) and press the retaining circlip into place with your snap-ring pliers and some even pressure to compress the black rubber gasket. I used a socket with appropriate diameter over the top of the end of the coolant sensor to press it down to compress the rubber gasket enough, to expand the circlip into the groove. Took a bit of work but turned out not to be nearly the job I thought it was going to be. Once the circlip is seated, you should be able to rotate it back and forth in the groove with your fingers. This way you will know it is seated properly.

Fifth, install the tank back into the car. Reconnect and tighten the clamps for all hoses, carefully checking the hoses for abnormal bulges or other oddities that would necessitate replacement. I found that the fabric-covered rubber overflow hoses on my car is brittle (though it's not immediately critical for the operation of the car) so I will order a new one or check for it at an auto parts store soon. I cut about a half-inch off of the end of this hose before re-installing it.

Sixth, CAREFULLY add coolant back into the tank, using a funnel at the top. Start the engine and put the climate control system in "defrost" mode (left-side button on the ACC). Warm the motor/coolant up and continue to both watch the temperature gauge, and add coolant back into the tank as the level begins to go down. It is probable that you will have an air bubble in the engine's coolant passages, and the only real way to work this out is to drive the car and CAREFULLY and REGULARLY monitor the car's temperature and add coolant until the temp gauge acts normally, and you've put all of the coolant back into the system that you originally removed. I was lucky in that my cooling system took all that i removed after the engine warmed up and BEFORE I went for my test drive, so I had a pretty good assurance of no or a very small air bubble in the system that I knew would easily work itself out.

Lastly, CHECK FOR LEAKS at all hose connections to the expansion tank, and on the coolant level sensor itself. Once you've got all of the coolant into the system that it will handle (hopefully all you removed) replace the cap and carefully take the fully warmed-up car for a test drive. Bring any remaining coolant with you in a gallon jug (or don't venture too far from home) and add any remaining coolant if there is room in the tank to accept it.

Then have a nice, cold beer.

Below are my photos from this operation.