I have a 2005 Sentra that loses coolant. I’ve been to a dealer and 3 different mechanics, had the head gasket replaced, the car has been pressure tested for 2 hours without any leaks showing up, and there’s never any telltale coolant on the ground where the car is parked. And it’s still losing about a quart of coolant every 10 days. Help!
This is very unusual, however, sometimes leaks are heat related. That is, if heat combined with pressure causes the leak, it may only leak when you are on the road, not when you are parked or during a pressure test. A pin hole leak onto a hot component such as a manifold, would cause the leaking fluid to evaportate, thus no puddle on the ground. Run the engine till hot, then open the hood, have someone rev the engine while you look for a tell tale sign of vapor. Good luck with this. PS, if the leak is from an expansion plug that is covered by the transmission housing it could be hard to find.
If the radiator cap has not be replaced, do so. It would not show on a pressure test since it’s not part of the test. It can hold pressure when hot …but bleed coolant as you’re driving down the road until it heats up. It only pushes a little since it seals quickly after the thermostat opens. It can have no “show” anywhere and will draw coolant from the recovery tank as normal.
I had this happen on both of my jeeps. I too was baffled. No leaks …held pressure …no puddles.
I once had a water pump that leaked a little only when running before it warmed up.
This car is almost due for a timing belt if it uses one.