M923 Cummins NTC 250 Overheating

[fuzzytoaster]

I've got a cummins 250 that is acting up on me and over heating. I've flushed the system 4 times to remove the build up and checked the water pump so all feels to be in good working order. The engine will creep up to 220 if I let her idle for about 10 minutes and the radiator doesn't get above 120. I've cleaned the thermostat and confirmed it works (first assuming bad sending unit), the fan too engages correctly. I've been using a temp gun to hunt down hot/cold spots and the hottest spot is the air compressor (due to location on the hot side of the water pump?)

My thoughts are a clog, bad/weak pump, clogged radiator, or..what else to cover my bases. I'm getting good flow through the system as with a water hose running she'll stay at 180 at idle. This leads me to think bad pump. Thoughts?

 

[dmetalmiki]

Take the thermostat out and then try it.

 

[Scar59]

Sounds like your headed in the right direction. Coolant is not circulating, if clogs are not discovered, the pump is the next culprit.

 

[goldneagle]

Are you sure it's not the temperature sending unit? Had that happen on my M931. Gauge would show it hot but fan did not kick on and IR gun read no signs of overheating. Yes the fan does engage properly. I road tested it. Usually comes on after climbing a long slope, then shuts off after a few minutes.

 

[Ferroequinologist]

I had the same problem once. Took the pump out, all looked good. Still overheated. Did everything else still had problems. Took pump out again and discovered the impeller had sheared from the shaft... but would stay on it and look fine and turn with the pulley. Only when you held one then tried to turn the other did it show itself.

Either way I agree coolant is not circulating properly. If the thermostate is good and you tested it, I would focus on the pump. A blockage would have to be huge to block enough of the ports in that block to cause what you are experiencing.

I also agree with GE, are you verifying the temp at the sending unit location and not just relying on the gauge?

 

[juanprado]

If you choose to pull the pump, I sell the elusive to find water pump mounting plate gasket which does not come with the pump. I would recommend changing it, ask me how I know......

 

[fuzzytoaster]

Are you sure it's not the temperature sending unit? Had that happen on my M931. Gauge would show it hot but fan did not kick on and IR gun read no signs of overheating. Yes the fan does engage properly. I road tested it. Usually comes on after climbing a long slope, then shuts off after a few minutes.

Sadly yes, it was my default troubleshoot. I can feel the heat radiating off the engine with the hood open, it's unnaturally hot. I've used the temp gun to trace coolants flow and it does seem to stay hot behind the pump and cool after it so the pump is what I'm looking at next.

I had the same problem once. Took the pump out, all looked good. Still overheated. Did everything else still had problems. Took pump out again and discovered the impeller had sheared from the shaft... but would stay on it and look fine and turn with the pulley. Only when you held one then tried to turn the other did it show itself.

Either way I agree coolant is not circulating properly. If the thermostate is good and you tested it, I would focus on the pump. A blockage would have to be huge to block enough of the ports in that block to cause what you are experiencing.

I also agree with GE, are you verifying the temp at the sending unit location and not just relying on the gauge?

Yes, in fact the temp gauge is only marginally higher than the readings are on the gun (5-10 degrees when over 200), still it shouldn't be reaching 200 from 5 minutes at idle or else I have one warm blooded beast. I did pull a thermostat from another truck and install it with no change in reported temps or behavior. I'll be diving into the pump next, I was able to source a good take out from Castle Bravo. I'll be noting juanprado about the plate gasket if it doesn't come with it already.

 

[74M35A2]

With your heater on max, does the discharge air stay hot or cool off? This can also tell you coolant flow, or lack thereof.

 

[WillWagner]

Take the thermostat out and then try it.

NO, not this, two loops in the cooling system. This will recirculate the coolant to the path of least resistance, the block loop. These aren't a sb Chevy.

The heater operation is a good troubleshooting step. If it is hot, w/p is doing it's job. If the engine hits 220 sitting still at idle, the pump is junk

 

[fuzzytoaster]

Just to wrap this thread up it was the water pump going bad.