Switch off engine, check coolant level (but coolant is fine!)

[eyescream]

Hello guys,

I know there are tons of coolant threads here, but i got the message as above, but my coolant levels were fine... faulty expansion tank sensor maybe ?

Any thoughts ?

Thanks in advance !

 

[TYb]

Hello guys,

I know there are tons of coolant threads here, but i got the message as above, but my coolant levels were fine... faulty expansion tank sensor maybe ?

Any thoughts ?

Thanks in advance !

Check again once the car has cooled down and see if it drops.

 

[eyescream]

correct me if im right, but shouldnt coolant level increase when car is cooled ?

 

[L_G]

No, given water expands when it is heated why would the level go down?

 

[Lehn]

correct me if im right, but shouldnt coolant level increase when car is cooled ?

You’re right, if the car is hot the coolant level is low until you open the lid of the coolant tank then it rises pretty fast. I can confirm this since I got the warning myself and checked the coolant level (be careful if you check it while the engine is hot since there may be released some steam when you open the lid and the hot air is getting out).

It’s because the air in the top of the tank is also getting hot and thus expanding as well which will push the coolant down since the hot air has nowhere to go. When you open the lid the air can get away and the coolant rises.

The coefficient of expansion of air is 0.0034/k while it’s 0.000214/k for water. In other words the same volume of air expands almost 16 times more than the same volume of water.

 

[AlS3BE]

Check the contacts on the coolant sensor. Sometimes they get corroded so remove quick clean and back on and see if that helps.

 

[TYb]

You’re right, if the car is hot the coolant level is low until you open the lid of the coolant tank then it rises pretty fast. I can confirm this since I got the warning myself and checked the coolant level (be careful if you check it while the engine is hot since there may be released some steam when you open the lid and the hot air is getting out).

It’s because the air in the top of the tank is also getting hot and thus expanding as well which will push the coolant down since the hot air has nowhere to go. When you open the lid the air can get away and the coolant rises.

The coefficient of expansion of air is 0.0034/k while it’s 0.000214/k for water. In other words the same volume of air expands almost 16 times more than the same volume of water.

Every day is a school day.

 

[eyescream]

Thanks guys ! I swapped out the expansion tank for a new one and all is well again ! Probably faulty sensor

 

[Mikeso51]

It’s because the air in the top of the tank is also getting hot and thus expanding as well which will push the coolant down since the hot air has nowhere to go. When you open the lid the air can get away and the coolant rises.

I think there is a misunderstanding of the physics here. Water is incompressible, and when it gets hot it expands. Nothing can stop this. Air on the other hand is compressible, and obeys the gas laws. PV=KT. So if the volume is fixed and the temperature rises, the pressure rises. That’s why the water in your radiator can exceed 100 C without boiling - the increased air pressure in the space above it raises the boiling point, just like a pressure cooker. If you are unwise enough to remove the pressure cap while the engine is hot all kinds of things may happen, including instant boiling if the engine is very hot. However this does not change the volume of the water.

 

[Lehn]

I think there is a misunderstanding of the physics here. Water is incompressible, and when it gets hot it expands. Nothing can stop this. Air on the other hand is compressible, and obeys the gas laws. PV=KT. So if the volume is fixed and the temperature rises, the pressure rises. That’s why the water in your radiator can exceed 100 C without boiling - the increased air pressure in the space above it raises the boiling point, just like a pressure cooker. If you are unwise enough to remove the pressure cap while the engine is hot all kinds of things may happen, including instant boiling if the engine is very hot. However this does not change the volume of the water.

I stand corrected. But then I can't explain why the coolant level is dropping with a hot engine.

 

[Mikeso51]

I haven’t checked on my own car, but assuming the level does drop when hot it is probably due to the internal volume of the coolant system and its plumbing increasing when hot and under pressure by more than the expansion of the coolant itself. The coolant volume increase is around 20 cc max, so quite small effects matter.

 

[Neil C]

I hope everyone is taking note of the lesson. . . . there may be a test !

 

[Lehn]

I hope everyone is taking note of the lesson. . . . there may be a test !

I sure don’t know whom or what to believe anymore

 

[Flying Scotsman]

Just check your coolant level on a cold engine on a level surface, it must be between the high and low levels, if dead on the low level add some and then keep an eye on it on a cold engine again and the same level surface, if it drops, you have an issue.

 

[Mikeso51]

Just checked the handbook and it says the level should be mid-way between max and min marks when cold, and may rise just above max when hot. If your level is dropping when the engine is hot, you have, as Flying Scotsman says, an issue.

 

[Lehn]

Just checked the handbook and it says the level should be mid-way between max and min marks when cold, and may rise just above max when hot. If your level is dropping when the engine is hot, you have, as Flying Scotsman says, an issue.

Yah that’s what I have concluded by now. I’m sure I got the leaking thermostat. Always driving with 1 litre of demineralized water in the boot. I just thought it was like that before I got the warning low coolant level but I might just remember wrong.

 

[rory182]

Possible airlock. I had a TTS until recently and after a coolant system flush I got similar warnings. Checked the header tank and level was down, checked it again and it was empty, but no leaks... As I was on hold on the phone to the garage I opened the expansion cap and the airlock suddenly vented and the reservoir bubbled up with coolant. Need to ensure no air trapped in the system when filling.