12.7 Detroit 1994 build date

First pressure test rad cap or replace. Next check for any type of air supply getting back to coolant system, Third get a block tester kit from parts store and test for combustion gases in coolant. Simple and effective.

 

more than likely nothing wrong with it if i fill mine up does the same thing leave it wear it likes it

 

You can borrow a pressure test kit from Autozone or O Reilly’s. Pump it up to 20 lbs. or so, see if it holds pressure. If not, it obviously has a leak. Might have a pin hole in a cylinder liner? Head gasket leak? No coolant in oil? Might be high enough most of it’s going out the exhaust, but still, some should show up in oil, from leaking past the rings after shutting it off. I doubt that’s the problem. Usually caused by not maintaining coolant with proper additives, using coolant/ tap water mix, or any hard water. If it holds pressure, no leaks. Start over with about 5 or 10 lbs of pressure on the system. Start it up, see if it builds pressure. It will probably build up fast, if there’s a problem. Another way to easily check for coolant building pressure is to put the end of the overflow tube submerged in a pop bottle half filled with water. Start the engine, while it’s running, look for steady stream of bubbles. If so, you may have a leaking head gasket. Usually from a dropped cylinder sleeve. Possibly a cracked head. If so, it will keep getting worse, using more and more coolant. Might suddenly blow out, and hydro lock the engine. Before going to the worst case scenario. If you have a Sleeper, check the bunk heater valve. It’s inside the Truck, under the bunk, attached to one of the heater hoses going to the heater box/core. See if it’s an air over electric type. You’ll see, it has some small wires running to it, and if it has a 1/4” air line going to it.then it’s an air/electric type. If so, the valves are known to fail. Symptoms are usually intermittent. The Truck won’t push coolant, until the bunk gets up to temperature. Then the air control on the valve overrides electric, and closes the valve. If it’s bad, air blows through and past the valve, and puts 100+ lbs of air into the system. You can do the bubble test, if bubbles are present, turn off the ( bunk heater hose ) water valves on the engine going to the bunk. See if bubbles quit. If so, replace the valve under bunk. Other possibility is a bad compressor head gasket. Take the SS line off at compressor, run the Engine. See if it quits building pressure. If so,compressor could have a cracked head or bad head gasket.