M931a2 cooling issue

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Hi guys first post here. I recently purchased a 1990 m931a2 with a custom lift bed, and roughly 10,000 miles. 339E4E98-1F58-4D00-A29E-811E770748CD.jpeg I’m pretty new to these trucks so bare with me. Anyways after fixing a few miner problems air leak, fuel leak, etc. I can’t seem to figure out how to fix or diagnose this temperature issue. Here is the story.... Purchased from a friend (who sells military trucks for a living), topped off all fluids and drove it home roughly 5 miles got up to 200 degrees and didn’t go past for the whole ride. After having it for about a week I drove up and down my street which is about 2 miles with no problems. Took it for a 5ish mile drive to town and it got up to 220 so I parked it and let it cool down for a while. Drove it back and it got up to 235 degrees as I pulled in the driveway. Here is what I’ve done so far.... added antifreeze (wasn’t that low) checked thermostat (hot water) it worked 058A3792-4DA3-4C5A-94F4-2845BE86A03C.jpegStarted it and let it ran for over a half hour which it got up to 195 then felt all the pipes. (Blue is what felt cold and red is what felt hot)C424F487-B010-477A-A10B-9E33E1302AD3.jpeg I drove it 3 miles the other day and got it up to 55 mph and after about 2.5 miles it reached 200 degrees and hovered right above it until I got home. I felt the same pipes then and everything seems hot except for the top of the radiator as seen in the picture above. Other thoughts.... from my understanding the person who owned this truck previously bypassed the sensor that told the fan to kick on at 200 and made it so it’s always on (although it doesn’t seem to be running at the high speed a fan should be running at) the guy I purchased it from (again, who sells these trucks for a living) thinks that has something to do with it. However a machine friend of mine seems to think it’s something with the coolent not circulating since it’s heating up so fast. Any thoughts, similar stories, and fixes would be greatly appreciated. See pictures below for any help.BE7FA646-789C-4421-A510-1C463BD3555B.jpeg A366300C-E844-4BA6-82F7-7B9F37B398B3.jpeg9F000B2E-6E5D-40A4-B37E-CA192AF4FFA1.jpeg
 

Csm Davis

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Okay so I would drain all the coolant and remove the lower radiator hose and look athe the inside of the radiator there to check how clogged the radiator is. I have seen many of these trucks that looked good from the top that had several inches of mud like buildup in the bottom of the radiator.
 

silverstate55

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I second what the good CSM just posted...flush it with Cascade dishwasher detergent thoroughly to clean out as much crud as you can....followed with a good flush with distilled water to clear out the detergent, then drain once clear and refill with the green low-silicate coolant.
 

Jbulach

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Flushing the cooling system and adding a filter is well worth while if it hasn’t already been done, but have you verified your temp gauge is not just off 40 degrees? They are common to be inaccurate. Also, if I remember right some of these motors had plastic impellers on the water pumps that liked to fail.

It looks like your shutterstat is bypassed, but with the truck empty, and mild air temperatures, the fan typically never comes on, so I’d say the previous owner was battling this as well, and was trying to put lipstick on a pig. With that... welcome aboard, beautiful truck, and finally we have one of us that nows how to take a picture!
 

frank8003

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Use your heat gun everywhere. It will tell you temperature wherever pointed. It is a very useful tool.
These trucks run cool. Forget the coolant temp and study the block temp for a while. Block/ engine oil must get hot to burn out the nasties. What is oil filters temp?
Maybe not an accurate gauge anywhere in any military truck, they just for relationships.
Can use the heat gun to find goesintah/goesoutah temperatures.
at 20 points across the radiator what is She doin?
These guns used to be real expensive but not anymore.
 

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7
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Maryland
Okay so I would drain all the coolant and remove the lower radiator hose and look athe the inside of the radiator there to check how clogged the radiator is. I have seen many of these trucks that looked good from the top that had several inches of mud like buildup in the bottom of the radiator.
Flushing the cooling system and adding a filter is well worth while if it hasn’t already been done, but have you verified your temp gauge is not just off 40 degrees? They are common to be inaccurate. Also, if I remember right some of these motors had plastic impellers on the water pumps that liked to fail.

It looks like your shutterstat is bypassed, but with the truck empty, and mild air temperatures, the fan typically never comes on, so I’d say the previous owner was battling this as well, and was trying to put lipstick on a pig. With that... welcome aboard, beautiful truck, and finally we have one of us that nows how to take a picture!
thank you, and yes I think I’m going to start with the temp gauge then try the flush. After that I will look into the water pump.
 

Andyrv6av8r

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Don't be afraid to pull both radiator side tanks . It can be done fairly easily with the radiator still in place. You might be surprised at all of the mud and goop in there.
 

74M35A2

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Your truck heats up awfully fast, like the water pump is not even working. Your belt, belt tension, and fan seem OK? You should hear the fan engage about 200F. The water pump is just an O ring and 3 bolts. I would pull that to check and see that the impeller is still complete and attached. Sounds like it is not.
 
7
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Location
Maryland
Your truck heats up awfully fast, like the water pump is not even working. Your belt, belt tension, and fan seem OK? You should hear the fan engage about 200F. The water pump is just an O ring and 3 bolts. I would pull that to check and see that the impeller is still complete and attached. Sounds like it is not.
Ok I will check that out thank you. I did just replace the belt. Also, the shutterstat is bypassed so the fan stays on.
 

charlesmann

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Your truck heats up awfully fast, like the water pump is not even working. Your belt, belt tension, and fan seem OK? You should hear the fan engage about 200F. The water pump is just an O ring and 3 bolts. I would pull that to check and see that the impeller is still complete and attached. Sounds like it is not.
what is the secret to removing the water pump? iv tried the heat, beat and repeat for bout 15 min. not an area that i feel comfy prying against to try and pry from top and bottom to wiggle it out. iv also got a penetrating oil soaking and even heated it up till the oil started bubbling then reapplied more oil, in hopes it'll seep in between the block and pump housing.
 

charlesmann

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After more heat and beat, i broke an ear off, but noticed it starting move outward, so i proceeded to beat on the pulley till it fell out. The new went in no problem, but i lubed it with an anti-seize compound, along with the mount bolts.

Next thing to clean will be the filling tank. Ill take it off and let it soak with clr and do a lil tapping to break up the heavy scaling and then purge it.
Iv ran the garden hose through the eng with it running 6x so far and getting a barely noticeable discolored water. Id say its clean enough for the time being.
Will be ordering a coolant filter and housing and install it when i get back from work in early jun.
 
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Maryland
Update.... first and foremost thanks for all the great reply’s, I greatly appreciate the help! With that said, I purchased a laser temp gun and got the truck gauge up to 200 degrees and then put the laser on certain parts of radiator hoses. Nothing was over 135 degrees. So I guess this means it’s the gauge and or the temp sender (not sure if it has a name). If any body has any insight on this please share. Or the name of the part and where to buy one. Thanks!
 

Jbulach

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Sunman Indiana
Update.... first and foremost thanks for all the great reply’s, I greatly appreciate the help! With that said, I purchased a laser temp gun and got the truck gauge up to 200 degrees and then put the laser on certain parts of radiator hoses. Nothing was over 135 degrees. So I guess this means it’s the gauge and or the temp sender (not sure if it has a name). If any body has any insight on this please share. Or the name of the part and where to buy one. Thanks!

Did you check the temperature of the engine block itself? The radiator will show very little heat until the thermostat opens and the water pump is moving water.
 
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