Low coolant level

[babyduke]

2005 X5 3.0i sport

A couple months ago, I had the thermostat and coolant sensor replaced w/ coolant flush/fill. I just randomly checked the coolant level and it was low so I re-filled it. It went back down again. I don't see any wet spot on the floor of the garage. Before I take it to the shop, where should I look for any clues?

Note: Low coolant level warning never came on.

[Clockwork]

Do you see any smoke coming from your exhaust?
Do you smell a sweet smell inside the X5's cabin?
did the person/people repairing the thermostat refill according to proper procedure, OR did they perhaps capture an air bubble in the system?
Do you need to run the heater in your X5 much? If not perhaps the coolant was NOT in the heater core until recently you turned on the heater therefore bringing fluid into the heater core and therefore lowering the fluid level in tank reservoir as it was now filled with air. And sometimes this can happen by accident even to good mechanics. Coolant bleeding can take a few tries to get all air out of system
keep filling up with coolant in mean time. Perhaps it was just an air bubble in the system.

[Joshdub]

I'd top it off and check again in a little while. It is difficult to get all of the air out of the system after opening it up. It is likely that a bubble was trapped somewhere and broke free after driving around for an amount of time. If it is losing coolant frequently then I'd start to look for a leak.

[babyduke]

Thanks for the suggestions!

[jdstrickland]

Depending on how low the level is, filling the expansion tank will lead first to filling the shortfall in the radiator. Keep filling until the expansion tank stays at a constant level, then check regularly to see how much is leaving and how fast.

[babyduke]

I keep filling to eliminate the possible trapped bubble. Coolant level continues to go back to low level and stays there. What can be the causes? It's obvious it's not emptying out but stays at a certain level.

[bcredliner]

If you have not bled the system do so. Fill it up put on the cap start it up and let it run, turn on the heater and let it run until up to operating temp. Open the bleed screw. When only coolant comes out the system should be free of air. FYI-the expansion tank is not completely full when the system is at appropriate level.

[jdstrickland]

The cooling system is a vessel. The radiator cap is the portal to that vessel. The cap seals a portion of the vessel -- the expansion tank. As you fill the tank, the fluid either goes into other portions of the vessel -- the radiator and block, for example -- OR it comes out of the vessel. If it is not coming out, then the vessel is more empty than you thought, so keep filling.

You are not having any issues with over-temp, or not telling us of any, so you should check the level each morning before you start the car and top as needed. You should find that after 2 or 3 more cycles, the coolant should stay at the correct level. There is a bleed screw that allows trapped air to escape, but generally this screw only needs to be opened as one of the steps of completing a service call that requires draining of the system to accomplish. The trapped air can be displaced by the coolant over time, and in this case the expansion tank -- the tank under the radiator cap -- will empty, which is the symptom you are reporting.

[bcredliner]

I have never found it necessary to do anything other than bled the system. That's what the bled screw is for. It is a closed pressurized system and coolant moves throughout the system. Trapped air is moved when the thermostat is opened and heater is on. It doesn't stay trapped. Air exits via the open bled screw. I usually bled it twice when I first fill it up but I can't remember the last time air came out the second time. If you are losing coolant, start looking for a leak.

[jdstrickland]

Originally Posted by bcredliner

I have never found it necessary to do anything other than bled the system. That's what the bled screw is for. It is a closed pressurized system and coolant moves throughout the system. Trapped air is moved when the thermostat is opened and heater is on. It doesn't stay trapped. Air exits via the open bled screw. I usually bled it twice when I first fill it up but I can't remember the last time air came out the second time. If you are losing coolant, start looking for a leak.

You're right. Bleed and be done is the preferred way to do it. It is faster than warming up and cooling down then checking and filling, then repeat. The warm-cool-fill-repeat is easy for people that don't know about the bleed screw, and the OP seems to me to be in this category.

The OP is not inclined, I think, to know what you are telling him. He's not going to bleed the system...

Coolant does not move quickly from the expansion tank to the radiator. That is, just because the expansion tank is full does not mean the radiator is full until the expansion tank is checked and found to be full after the car has been parked overnight (a period long enough for the engine to cool completely). If the tank is low and is filled, the radiator may not be full. The engine has to come to operating temp and then cool again to be checked. The system is not full until it is full when cold after it has been to proper operating temp.

If the system comes to full when cold, then it has to be observed for a few more operating cycles, always at the cold side of the cycle, to see if it remains full.

If no, then one of the possible leaks that does not present as an over heat condition is a cracked head where coolant enters the combustion chamber and is vaporized in the hot exhaust. This leak can be so slow as to be unnoticeable to the naked eye, but on a continuous basis can allow the expansion tank to empty in about 6 weeks, with no other symptoms.

External leaks present as puddles on the floor, or even visible steam from the source.

A blown head gasket will mix coolant and oil, and present as an overheat condition.