Cooling issue on NA diesel

[Pat Dolan]

I wonder if the water pump isn't pumping hard enough?? And is letting the coolant sit in the rad too long without pushing it into the engine fast enough?

If you can warm the engine up and have the top hose hot and bottom hose cold then it should NOT be overheating since the cold water in the lower hose is what's going into the engine.

How fast does the antifreeze shoot into the top of the coolant tank?

You are kind of arguing with yourself on this one.  Yes, IF there is sufficient flow, the different rad hose temps will let you know what is going to the engine, but - as you correctly surmise - if there is something not circulating that cold water in the bottom hose isn't going anywhere.

While I too don't trust the accuracy of the stock instruments, seeing it move significantly under load is all that is important.  If the cooling system (including thermostat) are doing its job, once up to operating temp, the gauge should rise to some point on the face (varies from car to car) and just stop under ANY further load.

I have had a lot of trouble with re-built water pumps from really bad domestic and import (as in NOT imported from Europe) suppliers.  Some have such poor quality impellers (stamped steel like a typical Yank Tank) that they don't pump at idle and cavitate like mad at high rpm.  Just something else to consider.

[rabbitman]

Yesterday I put in a new 87C t-stat, my old one was napa and they called it 180F. My needle used to ride at halfway all the time, since the new 87C stat is actually 188.6F the needle sits a little higher, back when I had a 195F stat the needle sat even higher still.

All that to say, your needle shouldn't flunctuate unless your not getting airflow through the rad or it's too cold out to stay hot.

Oh yeah, I have a stamped impeller.........

[Black Smokin' Diesel]

I know you said you replaced the waterpump but I've heard of pumps with the impeller slipping on the shaft when hot. Also, are your belts tight enough? Did you test the t-stat in boiling water?

Could be an air lock. How are you filling and bleeding the system?