Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Coolant

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LEVEL CHECK



See Figures 1 through 5

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Fig. Fig. 1: To check the coolant level, remove the radiator cap and observe the fluid level ...



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Fig. Fig. 2: ... then add the correct amount of coolant and water to the system



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Fig. Fig. 3: Be sure the rubber gasket on the radiator cap has a tight seal



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Fig. Fig. 4: Periodically remove all debris from the radiator fins



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Fig. Fig. 5: Cooling systems should be pressure tested for leaks periodically

Dealing with the cooling system can be a dangerous matter unless the proper precautions are observed. It is best to check the coolant level in the radiator when the engine is cold. This is done by removing the radiator cap and seeing that the coolant reaches the mark on the bottom of the filler neck. On later models, the cooling system has, as one of its components, an expansion tank. As long as the coolant is visible above the "Low" mark on the tank, the level is satisfactory. Always be certain that the filler caps on both the radiator and the reservoir are tightly closed.

In the event that the coolant level must be checked when the engine is warm on engines without the expansion tank, place a thick rag over the radiator cap and slowly turn the cap counterclockwise until it reaches the first detent. Allow all the hot steam to escape. This will allow the pressure in the system to drop gradually, preventing an explosion of hot coolant. When the hissing noise stops, remove the cap the rest of the way.

If the coolant level is low, add equal amounts of ethylene glycol-based antifreeze and clean water. On models without an expansion tank, add coolant through the radiator filler neck. Fill the expansion tank to the "Full" level on cars with that system.


WARNING
Never add cold coolant to a hot engine unless the engine is running, to avoid cracking the engine block.

If the coolant level is chronically low or rusty, refer to Body for diagnosis of the problem.

The radiator hoses and clamps and the radiator cap should be checked at the same time as the coolant level. Hoses which are brittle, cracked, or swollen should be replaced. Clamps should be checked for tightness (screwdriver tight onlydo not allow the clamp to cut into the hose or crush the fitting). The radiator cap gasket should be checked for any obvious tears, cracks, or swelling, or any signs of incorrect seating in the radiator neck.

Check the freezing protection rating at least once a year, preferably just before the winter sets in. This can be done with an antifreeze tester (most service stations will have one on hand and will probably check it for you, if not, they are available at an auto parts store). Maintain a protection rating of at least -20-F (-29-C) to prevent engine damage as a result of freezing and to assure the proper engine operating temperature.

DRAIN SYSTEM, FLUSH & REFILL



See Figures 6, 7 and 8

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Fig. Fig. 6: Cutaway view of a typical cooling system flow



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Fig. Fig. 7: To drain the cooling system, unscrew the radiator drain plug ...



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Fig. Fig. 8: ... then drain the coolant into a suitable container


WARNING
When draining engine coolant, keep in mind that cats and dogs are attracted to ethylene glycol antifreeze and could drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain coolant into a sealable container. Coolant should be reused unless it is contaminated or is several years old.

The cooling system should be drained, thoroughly flushed and then refilled at least every 25,000-30,000 miles. This should be done with the engine cold.

  1. Remove the radiator cap and the expansion tank cap (if so equipped).
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  3. With the caps removed, run the engine until the upper radiator hose is hot. This means that the thermostat is open and the coolant is flowing through the system.
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  5. With the engine stopped, position a suitable drain pan (or pans), then open the radiator draincock located at the bottom of the radiator. To speed draining, also open the engine block drain on the right-hand side. Before opening the radiator drain, it is a good idea to soak it for a few minutes with penetrating oil to loosen it. The radiator can also be emptied by siphoning, using the type of siphon used for gasoline, or coolant can be drained by removing the lower radiator hose.
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Do not attempt to siphon coolant by sucking on the end of a hose. The coolant is poisonous and can cause death or serious illness if swallowed.

  1. Completely drain the coolant, and close the draincocks. Add clean water until the system is filled.
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  3. Repeat Steps 3 and 4 several times until the drained liquid is nearly colorless.
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  5. Tighten the drain valve and then fill the radiator with a 50/50 mixture of ethylene glycol or other suitable antifreeze and water.
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  7. With the radiator cap still removed, run the engine until the upper radiator hose is hot. Add coolant if necessary, replace the caps and check for any leaks.
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