"If it was a blown head gasket there would be coolant in the oil."
That is not correct. When a head gasket leaks, coolant in the engine oil is by far the least likely outcome. More than 99 percent of the time coolant will be dawn into a cylinder, burned, and will show up as white smoke from the tail pipe. You'll be adding coolant on a regular basis, but it won't change color.
No transmission fluid, and to my knowledge, no engine oil flows through the intake manifold. If oil does, it is only related to drain back passages where that oil is not under pressure. Oil under no pressure won't sneak into coolant under 15 pounds of pressure.
My vote is for Factoryjack's comment about the transmission cooler in the radiator, especially since this is a GM vehicle. GM uses the red Dex-Cool antifreeze and they advertised it as "lifetime" coolant to make their cost of maintenance appear to be lower than that of their competitors, then they put a sticker under the hood saying to replace it every three years. Even the Dex-Cool company doesn't recommend waiting three years. Water pump lubricant, corrosion inhibitors, and other additives wear out in about two years, then the coolant becomes acidic and attacks metal parts. That's why we need to replace it. GM owners are all too familiar with leaking heater cores and radiators due to this Dex-Cool. A lot of mechanics won't put it back in their customers cars. There are alternatives that meet the same requirements for these engines. You can't mix in other brands of antifreeze either as it turns to what we call, "Dex-Mud". That is what happens too when transmission fluid mixes in. You will likely find the transmission fluid has turned pink or light brown.
To test to see if the coolant has turned acidic, use a digital voltmeter set to the 20-volt scale. Put the negative probe on the battery's negative post or a paint-free point on the engine. If you have a radiator cap, remove it, then stick the positive probe into the coolant, but not far enough to touch anything metal. The engineers forgot to put radiator caps on most of their radiators, so you'll have to go to the reservoir. Any reading over 2.0 volts is excessive. It's not uncommon to find 4.0 to 6.0 volts. By the time it gets that high, it won't be long before the heater core is leaking. Often that leak is blocked by residual casting sand from the engine settling in the heater core. When the cooling system is flushed, that sand is washed out, then the leak becomes apparent and the mechanic gets the undeserved blame for causing the leak.
Sunday, December 17th, 2017 AT 6:34 PM