Coolant overflows

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check air tanks for coolant . I have seen compressor with cracked head that was pumping air into cooling system

 

no. look for something else

 

I don't have suggestions yet. I was thinking could be bad air compressor...I had similar situation on series 60 and it was liner problem. your is different situation if no gases are there in cooling system

 

my experience ? the coolant level in reservoir was getting high and the coolant was coming out . after stopping and waiting about 20 min level was going back and I was getting low coolant level check engine light.
it was not cracked. it was sunk just a little. I diagnosed myself. what I did. I made coolant level in reservoir about half. I took a plastic coke bottle and filled it half of water. there is a small hose coming out from reservoir. I put that hose inside of coke bottle. I took out the reservoir cap and hold the neck by hand. when engine was hot I see few bubbles and after 10 min coolant level raised in reservoir . I drove my truck like that about a mount and then big bubbles started coming out...

 

Does Volvo have air controlled heater valves like western star and some freightliners? If they leak air pressure into the heater hose it might push water out. It should only do it when there is air pressure in the system though.