Usually if you shear off the bolts on the thermostat housing you can take the cover off anyways, and then remove the stubs with some vice grips, works fine....clean out the hole, use some new bolts, and you're set. I had it happen on both of our SD's, had it fixed in about 10 minutes.

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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
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