Humvee Fan Fighting with Thermostat? (almost always on but temps are good)

[Kevin Donahue]

Hi all,

Took my M1123 on a 4 hour round trip plus some fairly hard trails this weekend. Needless to say I was impressed! ATV Riders on the trail were bewildered that a truck was there with them making it through the tough obstacles so well.

However, the fan is running a lot on the highway/ main roads. Water temp will stay somewhere between about 201- 220 degrees the entire time (verified this is pretty accurate but maybe the gauge is reading 5-10 degrees hotter than it actually is compared to the temp gun) and once it comes on it never really turns back off.

Isn't the fan supposed to cool the engine all the way down to like 185 then shut off only to come back on again around 220?
Mine seems to kick on around about 210 and just run all the time. Sometimes the motor will cool to maybe 201 and sometimes it will hit almost 220 but never go a hair over that or a hair under 201. Even when I throw hard trails at it with little air flow I see the same operating temps.

The only time the fan seems to shut off on its own is when I am slow cruising gravel roads around 40MPH or less but operating temps read almost exactly the same.

My thought is the thermostat is running the motor on the hot side, maybe failing to open all the way so even though the fan is doing its job the fan cannot cool the engine down to those sub 200 temps like it should. I have a new Thermostat, fan switch, and time delay module on order, will report back the results but let me know what you guys think.

Note this is a pretty clean truck. Odo read only 300 miles when I got it and the undercarriage was show car clean so I believe that to actually be accurate. I highly doubt I have any debris between the tranny cooler and radiator and seems unlikely my radiator is shot. However it has been sitting for a long time I'm sure so maybe there is some scale buildup internally.

 

[911joeblow]

These are NOT like your gas trucks. Temps below 200 are NOT good for a diesel. Diesels need temperature to run properly and achieve proper burn. Your gauge is indicating correct operating temps. The fact that your fan is on a lot can be due to several things but here is a short list.

1. If you have not sprayed high pressure water from the backside of the radiator toward the front yet I would do that right away. These trucks get dunked in dirty rivers and driven in dusty sandy environments and the radiators can be nearly completely clogged with debris.
2. Completely and thoroughly flush the entire cooling system including the heater core. These often get field serviced with the same dirty river water and can be pretty clogged with debris.
3. As part of above consider a cooling balance kit from Paradox by Design. Doing a flush is a good time to install one. A lot of the fan cycling and weird temperature fluctuations happen when the system burps overheated coolant and trapped steam from the back of the block and heads. These engines run very hot at the back and over cool at the front which causes a lot of issues. Read this blog to fully understand what is going on in these engines. https://paradoxbydesign.com/blogs/news/the-humvee-diesel-engine-cooling-paradox

Good luck!

 

[Kevin Donahue]

These are NOT like your gas trucks. Temps below 200 are NOT good for a diesel. Diesels need temperature to run properly and achieve proper burn. Your gauge is indicating correct operating temps. The fact that your fan is on a lot can be due to several things but here is a short list.

1. If you have not sprayed high pressure water from the backside of the radiator toward the front yet I would do that right away. These trucks get dunked in dirty rivers and driven in dusty sandy environments and the radiators can be nearly completely clogged with debris.
2. Completely and thoroughly flush the entire cooling system including the heater core. These often get field serviced with the same dirty river water and can be pretty clogged with debris.
3. As part of above consider a cooling balance kit from Paradox by Design. Doing a flush is a good time to install one. A lot of the fan cycling and weird temperature fluctuations happen when the system burps overheated coolant and trapped steam from the back of the block and heads. These engines run very hot at the back and over cool at the front which causes a lot of issues. Read this blog to fully understand what is going on in these engines. https://paradoxbydesign.com/blogs/news/the-humvee-diesel-engine-cooling-paradox

Good luck!

Really appreciate the reply!

1. I have not done this but will today when I take it to the self service wash. Seems unlikely to me because it visually looks really clean but I could be wrong of course.
2. I seriously doubt this is necessary in my case. The reservoir is sparkling clean antifreeze and when I took off the heater core to paint the humvee it was nothing but clean and green antifreeze that came out. I don't think my humvee saw much action.
3. Will look into this. Wondering if I have air in the system since I took the heater core out. I did attempt to burp it by running with the radiator cap off but maybe didn't do it enough idk.

Thanks!

 

[JetbikeAnt]

Really appreciate the reply!

1. I have not done this but will today when I take it to the self service wash. Seems unlikely to me because it visually looks really clean but I could be wrong of course.
2. I seriously doubt this is necessary in my case. The reservoir is sparkling clean antifreeze and when I took off the heater core to paint the humvee it was nothing but clean and green antifreeze that came out. I don't think my humvee saw much action.
3. Will look into this. Wondering if I have air in the system since I took the heater core out. I did attempt to burp it by running with the radiator cap off but maybe didn't do it enough idk.

Thanks!

I just installed the PARADOX HMMWV 6.2/6.5/6.5T COOLING UPGRADE KIT (patent pending) and I'm very happy with the results. Running the hoses was challenging, but I feel I came up with a best way to do so and if you have any interest I will share it with you. Good luck!

 

[DREDnot]

Hi all,

... the fan is running a lot on the highway/ main roads. Water temp will stay somewhere between about 201- 220 degrees the entire time (verified this is pretty accurate but maybe the gauge is reading 5-10 degrees hotter than it actually is compared to the temp gun) and once it comes on it never really turns back off.

Isn't the fan supposed to cool the engine all the way down to like 185 then shut off only to come back on again around 220?
Mine seems to kick on around about 210 and just run all the time. Sometimes the motor will cool to maybe 201 and sometimes it will hit almost 220 but never go a hair over that or a hair under 201. Even when I throw hard trails at it with little air flow I see the same operating temps.

The only time the fan seems to shut off on its own is when I am slow cruising gravel roads around 40MPH or less but operating temps read almost exactly the same.

Sounds like the way mine ran. Never stayed on long enough to get the temp down that far.
Running steady above 40 would get the temp up to the 220 mark and the fan would kick on for a minute or two then cut off before the temp gauge had a chance to move. After a lag of a minute or two, it would get down to maaaybe 205.

It seems hot-ish, but jives with the numbers in the TM.

I wired in a manual override switch just to have a max cool option.

You are replacing the right parts to see if there is room for improvement

 

[Kevin Donahue]

Sounds like the way mine ran. Never stayed on long enough to get the temp down that far.
Running steady above 40 would get the temp up to the 220 mark and the fan would kick on for a minute or two then cut off before the temp gauge had a chance to move. After a lag of a minute or two, it would get down to maaaybe 205.

It seems hot-ish, but jives with the numbers in the TM.

I wired in a manual override switch just to have a max cool option.

You are replacing the right parts to see if there is room for improvement

Interesting... I cannot tell for sure but I don't think mine ever even shuts off when I am rolling at higher speeds. It might be though because there is so much other noise and unless I am not in overdrive its a bit hard to hear the fan over everything else. Also seems like once it has had to come on for those long periods of times it really wont even start shutting off at slow speeds anymore until I shut it off for a bit and then restart the motor.

My guess is the engine actually is building heat to quickly but that massive fan actually is doing a great job in keeping it from overheating. It probably has the original thermostat from 2003 so I am thinking that is the culprit but we will see I guess.

 

[frank8003]

So how many running engine minutes You think the entire engine block gets up to temp. Is the temp sensor in front, rear, middle. The engineers designed Her to run.

 

[Milcommoguy]

Go easy with the HI PRESSURE water back wash.

Unless one wants to buy a new radiator. CAMO

 

[Retiredwarhorses]

220 is operating temperature, everything sounds like it’s running normal.....leave it alone.
understand that doing a flush can create more problems than it solves...

 

[Ajax MD]

The guys saying 220F is normal are correct but I feel that the fan should not run non-stop while driving. Clean the radiator from the back, and consider replacing the thermostat.

My truck operates between 205F and 220F. During balls-hot summer temperatures in Maryland (90F-95F), in stop-n-go traffic, my fan comes on every 7-10 minutes and runs for 30 seconds, max. If I'm driving at a steady pace, providing consistent airflow the intervals between fan cycling are definitely longer than 10 minutes and the fan will run for 15 seconds.

The only thing I've done to my truck is replace the coolant with fresh and put a new thermostat in it. I used the NSN/OEM thermostat, I didn't get clever and put a cooler thermostat in it. Oh- I did buy a "fin comb" and spent 2-3 solid hours straightening all the fins on the oil cooler/radiator assembly.

 

[Mogman]

""Interesting... I cannot tell for sure but I don't think mine ever even shuts off when I am rolling at higher speeds. It might be though because there is so much other noise and unless I am not in overdrive its a bit hard to hear the fan over everything else. Also seems like once it has had to come on for those long periods of times it really wont even start shutting off at slow speeds anymore until I shut it off for a bit and then restart the motor. ""

Are you kidding??? you will know if the fan is "running" or not, scares the snot out of many when it kicks in and roars like hell and the truck will slow down.
One item I did not see mentioned was the belts, maybe they are slipping enough that the fan "runs" all the time and you cannot distinguish if it is running or not because it is slipping and not pulling the massive amount of horse power and roaring as it should, the belts must be in very good condition and pretty darn tight, even then the belts will likely squeal when it first kicks in..

 

[Coug]

""Interesting... I cannot tell for sure but I don't think mine ever even shuts off when I am rolling at higher speeds. It might be though because there is so much other noise and unless I am not in overdrive its a bit hard to hear the fan over everything else. Also seems like once it has had to come on for those long periods of times it really wont even start shutting off at slow speeds anymore until I shut it off for a bit and then restart the motor. ""

Are you kidding??? you will know if the fan is "running" or not, scares the snot out of many when it kicks in and roars like hell and the truck will slow down.
One item I did not see mentioned was the belts, maybe they are slipping enough that the fan "runs" all the time and you cannot distinguish if it is running or not because it is slipping and not pulling the massive amount of horse power and roaring as it should, the belts must be in very good condition and pretty darn tight, even then the belts will likely squeal when it first kicks in..

I had the same thoughts, if you can't tell then likely something is wrong.
Even running AC on high, having the BEOD (large box basically) on back of the truck, and towing a loaded DRASH type trailer at 65-70mph the fan comes on, runs a minute or so, then goes back off for a few minutes. If yours is running non-stop then either your cooling system isn't cooling it enough, or your fan clutch is slipping or something else is going on.

 

[Kevin Donahue]

So how many running engine minutes You think the entire engine block gets up to temp. Is the temp sensor in front, rear, middle. The engineers designed Her to run.

I have no idea what you mean by this statement.

 

[Kevin Donahue]

220 is operating temperature, everything sounds like it’s running normal.....leave it alone.
understand that doing a flush can create more problems than it solves...

I am definitely not going to flush it, but fan running 24/7 doesn't seem normal to me. Just the fact its killing my fuel mileage alone makes me want to look into it let alone the power suck.

 

[Kevin Donahue]

The guys saying 220F is normal are correct but I feel that the fan should not run non-stop while driving. Clean the radiator from the back, and consider replacing the thermostat.

My truck operates between 205F and 220F. During balls-hot summer temperatures in Maryland (90F-95F), in stop-n-go traffic, my fan comes on every 7-10 minutes and runs for 30 seconds, max. If I'm driving at a steady pace, providing consistent airflow the intervals between fan cycling are definitely longer than 10 minutes and the fan will run for 15 seconds.

The only thing I've done to my truck is replace the coolant with fresh and put a new thermostat in it. I used the NSN/OEM thermostat, I didn't get clever and put a cooler thermostat in it. Oh- I did buy a "fin comb" and spent 2-3 solid hours straightening all the fins on the oil cooler/radiator assembly.

I am gonna go ahead with the new thermostat as well, considering the constant running happens even in mild temps is concerning let alone 95 degrees,

 

[Kevin Donahue]

""Interesting... I cannot tell for sure but I don't think mine ever even shuts off when I am rolling at higher speeds. It might be though because there is so much other noise and unless I am not in overdrive its a bit hard to hear the fan over everything else. Also seems like once it has had to come on for those long periods of times it really wont even start shutting off at slow speeds anymore until I shut it off for a bit and then restart the motor. ""

Are you kidding??? you will know if the fan is "running" or not, scares the snot out of many when it kicks in and roars like hell and the truck will slow down.
One item I did not see mentioned was the belts, maybe they are slipping enough that the fan "runs" all the time and you cannot distinguish if it is running or not because it is slipping and not pulling the massive amount of horse power and roaring as it should, the belts must be in very good condition and pretty darn tight, even then the belts will likely squeal when it first kicks in..

That is a good point on the belts, never even thought to check that. It is definitely loud in every gear besides overdrive. Overdrive it is audible but barely. Is that the case for you as well or does your Humvee not have overdrive?

 

[Kevin Donahue]

Thanks everyone for the replies- also update. New Radiator cap and thermostat came in today. Going to start with the cap and see if that does anything, then I will probably do the time delay module once it comes in, then the fan switch, then the thermostat, and lastly probably the belts. I am working on limited free time these days so I will go easiest to hardest until its resolved. Worst case the radiator cap fixes it and I end up with a bunch of spare parts... well actually worst case none of this fixes it and things get worse lol but lets stay positive for now.

 

[frank8003]

I have no idea what you mean by this statement.

I mean what is the real temperature of everything like hoses in and out radiator and what do you get out of the heater and what is oil temp and block temp?

 

[Kevin Donahue]

I mean what is the real temperature of everything like hoses in and out radiator and what do you get out of the heater and what is oil temp and block temp?

Every time I have hit anything with the temp gun it is 200 degrees or less. Now that's not to say things don't heat up when I am driving to about 210-220 and then cool down quickly when I stop before I can pop the hood open and check but I think my temp gauge probably does ready 5-10 degrees high.

Really paid attention last time I drove and my fan was kicking on around 210 and running constantly. Gauge stayed around 200-215 the whole time and fan seemed to have little effect.

 

[frank8003]

Do you have this one?