I found out where my coolant has been going. It slowly has been dripping out of my overflow tank vent. Why does it keep going to the overflow?? I thought my thermostat was bad so I took it out to test if it was bad but it still does the same thing. What else could be wrong??? Thanks

could be one or 2 things causing it..

Check the radiator cap, its spring loaded to keep the right amount of pressure in the system and vent the rest. If the spring is weak or the rubber sealing gasket is ripped it will flow into the resevoir

The more expensive solution is you have a bad head gasket and the compression of the motor is over pressurizing the coolant system.

sounds like a head gasket to me.

If I'm not mistaken, a small amount of hot coolant is supposed bleed into the holding tank and then return back into the system once the cooling process creates a vacuum. Maybe the cap is not allowing this to happen. If so, you would think the level in the holding tank would continue to rise.

It still sounds like a head gasket. But keep tourchering your self.

Another good test other than compression, is a leak down test.

Put the piston at top dead center of the power stroke. Timing mark on the T and both valves closed. Put a gauge and a air nozzle (on a tee) in the spark plug hole. Put 7 PSI's in the cylinder and it should hold it for 10 minutes.

What ever you come up with good luck.

I'll probaly order a head gasket and a radiator cap. It has to be one or the other. Thanks for the help guys.

It is probably a head gasket and could possibly be a bad cap, But before you tear it down check to make sure it is simply not too full of coolant. The coolant expands quite a bit and if it is filled to the brim in the radiator and the overflow is at the normal or over mark, it will blow out expanded coolant untill it reaches its proper level. My Mo's do not have overflow tanks and if I fill the radiator while it is cold it will spit coolant out when hot until the level is about 2 inches from the top off the radiator when checked cold again. There is no use to add above this as it will just spit it out when hot again.

What alerted you to this problem? If you had not added water for any reason in a while and everything was OK and all of a sudden it started spitting out coolant then you may have a gasket blown. However if you did some work that required refilling the coolant or just checked it and noticed that it was low and added some pryor to this problem starting it may be just spitting out the expanded coolant.