M923A2 problem with the cooling system

[MarcM35A2]

Hey guys, I have a problem with my m923A2. When I got the truck the cooling system was leaking at the radiator, at the tubes to cool the compressor and on the hoses on the rear of the engine right above the transmission. So I dicided to change all hoses and the radiator. All brand new! But since that I have the problem that the truck presses out coolant through the lower tube on the radiator or the tubes on the rear of the engine. For me there seems to be to much pressure in the system. But it does noch come up through the ventilation..... now I changed the ventilation cap but the problem is still there.
I thought about compression comming into the cooling system, but there are no bubbles in the water and no pressure in the system. but when i drive a few minutes, the pressure is up and the system is leaking again. And... the pressure in the cooling system stays in the system several days. When I oben the cap, the pressure releases.
Have you any ideas? Thanks a lot!!!!

Oh... changed the compressor too, because I thougth that the compressor might be responsible for the failure......

 

[Suprman]

Sounds like a blockage somewhere is causing pressure to build or you are getting combustion pressure into the system. Did you check the thermostat?

 

[Carlo]

Hi Marc. Can you take photos of where the leak is happening please?

 

[MarcM35A2]

Well, i don´t think that the thermostat is bad. because when the engine is getting hot, the system starts to leak. when the truck cools down, one or two days, ther is still pressure in the system. sounds like you´re opening a coke when I open the cap...
May be the cylinderhead gasket is done....???

Will take photos tomorrow!!

 

[Suprman]

If the thermostat stays closed and the water pump is pumping away I could see
pressure building in the system and it bleeding out. Usually when you are blowing combustion out where it does not belong there will be other signs though. If it was me I would take out the thermostat and completely flush the cooling system and radiator. Who knows the radiator could be all sludged up.

 

[MarcM35A2]

Will put in a new thermostat! Radiator is brand new! The curious thing is, that the pressure stays in the system when the truck cools down and that the pressur is not enough to blow out of the ventilation hose.......
I am really helpless....
But don´t want to take off the cylinder head for nothing!!!

 

[Carlo]

Hey Marc. I have 2 M923A2's and 1 M923A1 and all 3 have pressure at the cap days after it's been shut off. These are pressure systems. If you had nothing it would mean it's bleeding off as a leak. Send me a photo to where are leak you said is.
Plus ALL 3 of my trucks run hot. The A2's in particular. I'm trying to find a actuator that will come on at a lower temp to try.

 

[goldneagle]

Hey Marc. I have 2 M923A2's and 1 M923A1 and all 3 have pressure at the cap days after it's been shut off. These are pressure systems. If you had nothing it would mean it's bleeding off as a leak. Send me a photo to where are leak you said is.
Plus ALL 3 of my trucks run hot. The A2's in particular. I'm trying to find a actuator that will come on at a lower temp to try.

Your temperature sensors may be bad. I had that on one of my M923 trucks. With the new sensor the temp has a lower reading. I checked my cooling system with a infrared thermometer. 20 to 30 degrees cooler reading than the gauge on the truck read. Found them cheap on Ebay.

 

[Carlo]

Your temperature sensors may be bad. I had that on one of my M923 trucks. With the new sensor the temp has a lower reading. I checked my cooling system with a infrared thermometer. 20 to 30 degrees cooler reading than the gauge on the truck read. Found them cheap on Ebay.

Where are the temp sensor located?

 

[dochowz]

Yeah...i'm in for a new sensor also...my m923 runs pretty hot for my taste.

 

[goldneagle]

Where are the temp sensor located?

On the NHC 250 it is on the passenger side of the engine next to the heater hose connection. There is at least one thread on SS that mentions it and may even have the part number. Not sure of location on the M923A2 engine. Check the TM for that.

I found the original listing for the temperature sensor:
Temperature Sending Unit M-Series MS24537-1 - Temp Sender - Military Surplus Box

 

[goldneagle]

Yeah...i'm in for a new sensor also...my m923 runs pretty hot for my taste.

Does your fan kick on at all? It make a loud racket when running. On my truck i thought the thermal switch was stuck and the fan did not run -even though the gauge went over 200 deg. F. When I took reading with the infrared thermometer the temp of the engine and radiator were under 200. The reason the fan did not come on was the engine temp did not go over 190. The new temp sensor gives more accurate temp. reading.

The best way to check is using the infrared thermometer. (The sensor I bought was $12.50) I think it also fits the transmission.

 

[Carlo]

Yeah...i'm in for a new sensor also...my m923 runs pretty hot for my taste.

Nice truck!

 

[bulldog_mack13]

On the A2's its behind the alternator and a hose behind that you will see it screwed in with a Military wire plug sticking out of it. You will have to move the alternator and hose to get to it. Pain in the butt ,but not too hard to do.

Seems like a lot of those go bad.

 

[jscottrichardson]

Coolant Problem M923A2, In need of advice.

I'm new member to Steel Solders and new owner of a 1990 BMY M923A2 with a coolant problem.
I have placed around 1500 miles on the truck and this is my first mechanical problem since owning the vehicle.
(Problem)
The truck is leaking coolant from a fan driven pump on the engine.
The leak appears to be a relief valve (see picture).

Can someone assist me in fixing this issue?

Thanks,
Scott

 

[Suprman]

Water pump

 

[jscottrichardson]

Thanks !!