Truck Repair: Cummins N-14 overflow, head gaskets, international eagle

Question
Our N-14 is boiling water out of the low coolant overflow tank. We are loosing approx. 7 gallons in 200 miles. We tried a new overflow cap, but it did not work. We have 750,000 miles on truck and it is a 1999 International Eagle. Do you have any suggestions?

Answer
Boiling or blowing water out?You can check for compression in the radiator by simply getting a clear 20 oz coke bottle,fill it half full of water.Place overflow line(one next to reservoir cap)in the bottle until it is submerged.Run engine & see if it bubbles.If it bubbles,try disconnecting the air compressor discharge line & retest.If bubbles go away,air compressor head is the problem if it is a Holset QE compressor,but most IHCs have Bendix.Problem is probably the head gaskets.If one is bad & they have never been off,I would change them all & get the liner height checked.Spec is 0.004-0.007.If they are 0.004-0.005,Put a special liner shim on top of the liner.Cummins makes a 0.002 steel shim for the top of the liners.You have to put one on each liner under one head.