Hidden Coolant Leak Mystery: No Drips but Level Drops

As someone with moderate DIY repair experience, I dealt with similar symptoms on my 2010 GL6. Initially thought it was just low coolant, but it turned out the issue was more complex. The recurring check engine light combined with mysteriously disappearing coolant often points to electrical problems with the cooling system sensors or wiring. In my case, the culprit was a damaged wire harness connecting to the coolant level sensor. The faulty connection was causing incorrect readings and triggering the warning light. The coolant would appear low because the sensor wasnt getting accurate readings. After checking for air pockets in the cooling system and inspecting all radiator hoses, my mechanic found the wiring issue. The repair involved replacing the damaged wiring and connector, which solved both the sensor readings and check engine warnings. Total cost was around 95 for parts and labor, much better than replacing the entire sensor unit. Before suggesting more specific advice, could you share: Any diagnostic codes from the check engine light? How quickly does the coolant level drop? Does the engine overheating warning ever come on? Have you noticed any white exhaust smoke? This will help rule out more serious issues like head gasket problems versus electrical connections.

Had my last service at 161953 KM and experiencing the same issue. My mechanic checked for air pockets in the cooling system and tested the radiator cap pressure, both came back normal. The coolant level kept dropping though. We finally traced it to a hairline crack in the coolant reservoir that was really hard to spot. The leak was so small it would evaporate before hitting the ground. The water pump and all hoses tested fine, but that tiny crack was enough to cause a slow leak. After replacing the reservoir and doing a complete system flush, the engine overheating warnings stopped and coolant level has stayed stable. Check around the edges and seams of your reservoir carefully, sometimes these cracks only show up when the engine is warm and under pressure.

Thanks for sharing those details about finding the hairline crack. That actually matches my experience too, these GL models seem prone to developing tiny stress cracks in the coolant reservoir that are incredibly hard to spot. After my wiring fix, I also discovered a similar hairline crack when the check engine light returned a few months later. The coolant loss was so gradual that the leak would evaporate before leaving any traces. Once the engine was warm and the cooling system was under pressure, you could see tiny beads of coolant seeping through near the upper seam of the reservoir. A complete system flush and new reservoir fixed it completely. Just make sure to check for air pockets after replacing components in the cooling system. These engines can be sensitive to air bubbles, especially around the coolant level sensor. Your solution proves why thorough pressure testing is so important, sometimes the obvious signs of a coolant leak just arent there. The cooling fan might run normally and engine temps can read fine while that slow leak continues unnoticed.

Thanks for those helpful responses about the coolant reservoir. You both confirmed my suspicions about these slow leaks. The diagnostic codes show P0217 and P0116. The coolant level drops about an inch every two weeks, and no white exhaust smoke or engine overheating warnings have appeared. Getting it checked tomorrow and will have them pressure test the entire cooling system, especially around the reservoir seams. The coolant loss timing matches what others described with hairline cracks. Strange how these tiny leaks can trigger the check engine light while the cooling fan and temperature readings stay normal. Glad I asked here first, will update once the mechanic confirms if its a cracked reservoir or something else causing the gradual radiator fluid loss. At least now I can point them in the right direction rather than paying for unnecessary diagnostic time.