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The tach's on these cars are not all that accurate. The degree of accuracy is about +/- 200 rpm. Its 99.9% likely the engine is still idling at the same speed as before....the tach is probably just "slow to react" sometimes....and they do seem to "reset" themselves after you've begun driving. Ours does it too. I'd not worry about it....just old electronics acting up.
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-diesel is not just a fuel, its a way of life- 
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
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