Q: The other day, I took off the radiator cap in my 1995 GMC Yukon with the 5.7-liter to check the antifreeze, and it was filled with this thick gooey stuff. There's no smoke or anything like a bad head gasket.
A: That's never a good sign. Typically, if it's a pinkish goo, it's transmission fluid, and a brownish goo means engine oil. Most transmission coolers are built into the radiator. As the air passes through the radiator removing heat from the engine's coolant, it also removes heat from the transmission fluid flowing through the radiator core. Sometimes, a crack will form in the cooler, which makes a direct connection between transmission fluid and coolant. The transmission fluid being under a higher pressure travels into the coolant system. The other possibility is a cracked engine block, cylinder head, or bad head gasket. In this situation, the high engine oil pressure pushes the oil into the coolant system. You can install a coolant system pressure tester, remove the transmission cooler lines, and inspect for coolant coming out of the transmission cooler connections on the radiator. That would confirm the need for radiator replacement and a complete flushing of the coolant system.
While pressurizing the coolant system, you can also drain the engine oil and inspect for coolant mixed with the oil. That would confirm the engine is the source of contamination, which would require disassembly to determine the exact point of origin. These tests are not always 100 percent reliable due to the thickness of the goo as opposed to free-flowing antifreeze. It may be a good idea to have it inspected by an experienced tech who can determine if it's oil or transmission fluid contamination just by the look and feel.
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