Does it have the original radiator? It might be plugged up and not getting good enough air flow.....I had that problem on my diesel. My temps dropped 10-15C under hot-day-load driving by simply installing a new radiator....it wasn't internally plugged, but the fins were filthy and blocking air flow.

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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
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