Volvo MD11C Overheating
Whats the volumetric cooling water flow rate at X rpm? Dont have a manual or the data - call your distributor for the info.
Go to your manual and obtain the data, then run the engine at the spec. rpm until the thermostat fully opens, take a bucket and stop watch and catch the water coming from the exhaust. If the output is not up to spec. then you have a blockage somewhere: broken impeller vane jammed downstream in a hose, fouled thermostat, exhaust manifold starting to form ''platelette'' rust in the cooiing passages. Cast iron exhaust headers will form huge rust plateletts which break off from the interal cooling passage walls and block or partly block the flow - especially happens if you drain the water from engine during layup and let the cooling system "dry-out". If so, just poke a large stiff wire into the header to break up the flakes.... then check for pin holes into the ''gas side''.
You dont state if this is a fresh water (exhcanger) system or a raw water cooled engine. If the volumetric flow is within spec. then consder to ''pickle'' the engine to remove carbonate ''boiler'' scale by using an organic descaling agent such as Marsolve or RydLyme. If you use acid, you can dissolve the base metal of the engine as well as the scale.
When was the last time you changed the pump impeller(s)? Should be yearly!