that happened to me a bit too until i unscrewed the expansion tank cap to the first tab (not all the way off but still loose) and drove it around a bit. got the last of the air out and it stopped doing that.

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1985 300D - 1984 Euro 280E AMG Clone (SOLD) - 1978 280CE (SOLD) - 1983 300D (SOLD) - 1981 300D (SOLD)