Cooant resivoir tank overflowing

[Bon Voyage]

I went out to do the monthly start on my bus today, I ran it for maybe 15 minutes and after I shut it off I noticed the coolant reservoir tank was full and started to leak out the cap hole. The engine didn't get up to full temperature so it wasn't too hot, the reservoir tank was always a little too full like maybe half to 2/3 at most but definitely not overflowing. Any ideas what it could be other than a head gasket issue?

[cadillackid]

most of these busses dont have overflow recovery bottles that can push out and suck back in coolant.. if the system gets filled a little full.. natural expansion will push out coolant when the engine warms up.. the condition is exacerbated when the heater valves are open as there is more coolant expanding into the same expansion space..



I added a recovery bottle to my DEV bus for this reason.. it was always seemingly a little low on coolant as I have lots of heaters.. so now it pushes ut and sucks back in coolant like a car... runs a little cooler in summer and I have zero air issues in my heater loops now