How about filling up the system with water, and use a pressure pump, put about 12-15 pounds of pressure on it, and wait. You will be rewarded with a drip.

My water pump leaked when I installed the serpentine belt correctly, for the first time, then stopped. 5 years later, it started dripping again, and I replaced it.

It took an hour and a half for the leak to start under pressure, but it did start. Ended up replacing the radiator due to a cracked tank, too. Glad I did, as the coolant temp went down nearly 15 degrees. It's a happy Benz these days.

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My first Mercedes Benz. 4/6/07.

1984 190E (201 024) 2.3L four.