I am the owner of a 1986 359 Peterbilt equipped with a 400 low flow Cummins engine. Recently, I have replaced the water pump, thermostats, hoses, and thoroughly cleaned the entire system, including rebuilding the radiator. Interestingly, the engine idles perfectly without overheating, but after just a short drive, it starts to overheat. I noticed that there was very little flow when I removed the plug from the top of the water manifold while the engine was running. However, when I removed the radiator cap, water began to flow and was pushed out of the hole. Upon replacing the cap, the flow stopped. I am curious to understand why the radiator cap is causing the system to pull a vacuum so quickly when removed. Your insights on this matter would be greatly appreciated. Thank you.
It appears that there may be an air lock in the cooling system, possibly caused by a blockage in the small line connecting the highest point of the system to the radiator tank. The high flow component may have been defective from the start. Any high flow part compatible with an 855 engine should fit, unless an STC is required.
Answer: - The issue of overheating despite replacing components like the water pump and thermostats could be due to a lack of coolant flow in the system.
Answer: - The low flow observed when removing the plug from the water manifold could indicate a restriction in the cooling system, possibly caused by a blockage or air pocket.
Answer: - The phenomenon of water flow starting only when the radiator cap is removed and stopping when it is replaced may indicate a problem with the radiator cap not maintaining proper pressure in the system.
Answer: - The rapid vacuum formation when removing the radiator cap could be a sign of a faulty radiator cap or an issue with the pressure regulation in the cooling system.
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