1998 Legacy Outback. 2.5

Car started missing and overheating last week. Did it a couple of times, then stopped. Drove it all week with no issues. Needed to go someplace on Saturday that us 30 miles away. Made it about 20 miles, and it started overheating again. Let it sit until it cooled down, then drove it home with no issues.

Thinking that the thermostat was bad, I took it to my mechanic about 5 miles away. Overheated twice and I had to stop each time. Sometimes it misses badly, other times it runs fine.

Mechanic said head gasket is blown because its pushing water and bubbles into the overflow. Drove it home with no issues.

Can anyone explain to me why it is having intermittent issues? If the head gasket is bad, why wouldnt it act the same all of the time? 

Edited July 1, 20205 yr by jeryst

They overheat when the combustion gas bubbles collect on the thermostat and cause the thermostat to close. Varying throttle conditions and driving angles means this happens differently. And the amount of coolant that's in the system has an effect also. It is generally expelled into the overflow bottle with combustion gasses when the system pressure rises above the cap's pressure rating - venting gasses and coolant - eventually leading to cavitation in the water pump. 

It's all academic though - you definitely have a bad head gasket. The mechanic is correct. If you want to fix it permanently you need to install 251 pistons and STI 770 gaskets. 

GD

Several reasons for odd behavior of head gasket leaks.

Expansion and contraction from heat and cooling changes the volume of the leak.
Carbon deposits can break loose inside the cylinder and plug the leak into the cooling system until the carbon is burned and then pushed through the leak.
Trapped air bubbles around the water pump vanes causing it to not move the coolant, and air around the thermostat causing it to not open.

The coolant flow powered by the water pump, with a closed thermostat, is engine and heater core going past the thermostats coil end, with part of the coolant tapped off to go through the Throttle Body and Idle Air Control Valve. Air tends to collect there and in the heater core and when that system is full of air the pump does not have water to pump.
If and When the thermostat opens the radiator is added to the inlet of the water pump.

On a side note: most people (even videos on YouTube) think the car thermostat spring is bimetal and moves when heated or cooled. Wrong. At the center of the thermostat coil is a sealed copper colored cylinder with a piston inside that is fastened to a rod that goes through the valve and connects to the peak of the thermostat. The cylinder is connected to the valve. The inside of the cylinder is filled with wax. When heated the wax liquefies and expands rapidly and pushes the cylinder away from the piston and rod and pulls the valve open. As it cools the wax shrinks and the spring pushes the valve closed. Originally, Bees Wax was used.

Correct - the thermostat is a form of wax motor. It is surprising how many people do not understand the basic operation of components like this. Then they go and make themselves look really dumb on YouTube. 

GD