Regarding whether or not replacing the water pump was irrelevant because the problem predated the replacement; I have found that on medium duty Hino trucks a failing (but not yet apparently failed) water pump can introduce air into the cooling system. In our case this led to air pooling in the radiator, forcing coolant out of the overflow and setting off the low coolant alarm. Since everyone these days seems to rely on the overflow bottle as a measure of coolant level, this took a little while to figure out. Hence, it is conceivable that the water pump led to an air pocket before it was replaced.
Did you notice any overheating or unusual noises?
The reason I ask is because one time I replaced a heater hose and I think I must have introduced air into the coolant in the process. Immediately after I replaced the hose, I started the engine & I heard this rather loud knocking/popping sound (sync’d with the engine rpm) coming from near the top of the engine, seemed to be from the valve/injector/water-pump area. It went away after about 15 minutes of driving, but I have been wondering if air in the cooling system caused the knocking noise? A very clearly evident noise I’d never heard before. Or since. There were no signs of overheating or any other symptom, just the noise.
1990’s econobox. Any ideas?