Q: Following a recent water pump replacement on my Dodge Durango, the truck has been losing considerable coolant (overheat temperature and warning light comes on). I see no coolant leak at all under hood or on the ground. This condition keeps happening after about 150 miles of driving. The local shop says all seems normal including the water pump, hoses etc. What can be the cause/cure?

A: If coolant isn't steaming out from under the hood or dripping on the floor of your garage, it's either coming out the tailpipe as vapor or going into the engine's crankcase—but it almost always comes out the tailpipe. The most likely cause is a bad head gasket or cracked cylinder head allowing coolant into the combustion chamber, where it's burned up and emitted through the exhaust. On rare occasions, a crack in the engine block can pass coolant into the crankcase. But that's a tough one to miss. The engine oil would look like Pepto-Bismol. Testing involves pressurizing each cylinder one by one and checking to see if the compressed air comes gurgling up in the radiator. Once the leaking cylinder is confirmed, the affected cylinder head comes off for a visual inspection. The head should then be checked for warpage or cracks. A warped head can be corrected by machining, but cracks require replacement. Another rare one is a crack between the engine block's cylinder wall and a coolant galley, which would also burn coolant. But I'd put money on the head gasket.

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If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at trucktrend@sourceinterlink.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.

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