Bubbling in the reservoir

Question

So I have a 2004 chrysler sebring 2.7l. It is bubbling or boiling in the reservoir. I have replaced the thermostat, the rad, the reservoir itself... all brand new and it still does this. There is no sign of it being the head gasket... no milky substance on oil cap, no oily sheen on the coolant and no smoke. It barely gets above half way on the temp gauge. And when I press on the cap the bubbling stops in the reservoir... did it with both old and new. Water pump is not making a sound and there is no coolant on the ground. But it still continues to bubble and boil in the reservoir. I'm at a loss and need some advice please.

Answer 1

You can have a tiny leak in head gasket that is only leaking from the cylinder to a water way. That would cause exhaust gasses to leak into the coolant as you may be experiencing, seen as bubbles. Pressing down on the cap is likely just increasing the pressure in the cooling system sufficient enough to stop the tiny leak occurring for now.

To get oil in the water or water in the oil, the gasket would need to be leaking from an oil way to a water way. In that case you wouldn't necessarily be getting bubbling in the water but you would get the usual emulsified oil or oil in the reservoir.

It is also possible that you have a very small crack in the cylinder head or even the block. If the crack just goes between the cylinder and the water way, then you would just get bubbling.

Answer 2

Yes I’ve had similar with my Landrover Discovery.New hoses fitted etc,nothing unusual other than losing fluid.Appently at running temp and the cap off if it bubbles ,look for head gasket which is not the first time,other engine.