10/06/2026 11:03 am
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Automotive Q&A
2017 GLC 300 M274 loses coolant with no puddle after parking
Review the evidence, add a focused answer, and distinguish confirmed measurements from assumptions.
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This GLC 300 needs about 200 ml of coolant every few weeks, but the garage floor and belly pan are dry after an overnight park. There is a faint sweet smell after a highway drive and a small pale residue below the intake side of the engine. The oil and transmission fluid show no sign of coolant contamination, and there is no persistent white exhaust smoke.
A cold pressure test at the cap rating held for 20 minutes without an obvious drop. Is there a practical way to narrow a heat-dependent seep on the M274 before removing covers and replacing the usual cooling-system parts by pattern?
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11/06/2026 5:52 am
Start by cleaning the residue so you can distinguish an active track from an old spill. With the undertray removed, inspect the thermostat housing, pump area, small vent and bypass connections, and the turbo coolant-line joints using a mirror or borescope. A leak that evaporates on a hot casting may never reach the floor.
If the cold test is inconclusive, repeat the inspection immediately after a fully warm drive, without opening the hot system. UV dye approved for the coolant can help, as can leaving the pressure tester attached while the engine cools. The goal is fresh residue at a joint, not a parts list based only on engine family.
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14/06/2026 9:26 am
Cleaning first made the difference. After a warm drive, a borescope showed a new wet line starting at the thermostat-housing seam and evaporating before it reached the undertray. The repaired housing now holds pressure hot and cold, and the level has remained fixed through a week of commuting.