I did a few searches on this topic & didn't come up with anything that I had not already tried.

My coolant temperatures climb pretty quickly and can reach 210 while climbing a hill with normal RPMs and unloaded. Cooling system has been flushed & mixed properly, thermostat replaced, water pump replaced, radiator cap replaced.

I do notice that if I engage the air conditioning compressor (and therefore engage the cooling fan) the temperatures will fall back to normal and stay there for the most part. Maybe my cooling fan clutch is weak?? How does one test this? Also, how long should a cooling system maintain pressure? I seem to loose pressure after 1/2 hour of sitting..not a scientific test, just squeezing the upper radiator hose.

I don't notice any coolant leaking, however the level has been slowly falling after changing the thermostat and water pump...I attributed this to air in the water jackets working itself out.

NO evidence of coolant in the oil.

I guess I am looking at either an undetectable leak or a bad fan clutch? Any other ideas?

Thanks for your time!

 

After your truck cools down pull the radiator cap and see if the radiator is full to the top. If it is not then you still have air in the system somewhere.

 

Do you have a tranny temp gauge? I have seen the tranny temp cause the coolant temp to go up. You have a heat exchanger on the driver side of the motor that both go through and can cause it to read high.

 

It's supposed to be a closed system. Air or no air, if the level is slowly dropping, it's going somewhere. Take a coolant sample to a local repair shop and have them do a test for by products of combustion; +=head gasket leak.

 

Thanks for the replies...I understand that it is a closed system, but if
there are air pockets taking up volume in the water jackets
and they are slowly purging as the engine runs, wouldn't that cause a drop
in coolant level? I don't like to jump
to the worst possible case when troubleshooting, but I do undersand your point here.

I don't have a tranny temp guage, but it is a recent rebuild with a stage 4 BD kit, mag hytec pan, etc... So I can't imagine it's getting hot, but I guess I shouldn't assume that, either.

Thanks for the ideas and taking the time to post!!

 

you said you were going up a hill. If the TC was not locked (50mph or slower)the tranny temp would go up quick.

 

have you checked the bottom radiator hose. sometimes the spring in the hose will rust away and the hose will suck shut.just a thought

 

Nope...hadn't checked the lower hose. I'll do that tomorrow...I like the simple stuff first. Wish I had thought of that when the coolant was drained from the radiator...I'd have just replaced it

I guess I should have a trans temp gauge anyway...maybe I'll install one. I really just cant see that being the problem, though. I had a worn out stock 48re and some pretty ridiculous loads for 150K miles with no overheating problems. I understand that without TC lock, there is slip, but the old tranny slipped like crazy....this one is a whole different animal...

 

Here's a little info on the fan clutch.

5.9L DIESEL (ELECTRONICALLY CONTROLLED FAN DRIVE)
The Engine Control Module (ECM) controls the level
of engagement of the electronically controlled viscous
fan clutch by monitoring coolant temperature, intake
manifold temperature, air conditioning pressure and
transmission oil temperature. Based on cooling
requirements, the ECM sends a signal to the viscous
fan clutch to increase or decrease the fan speed.
Fan speed is monitored by the ECM. Fan speeds
above or below a calibrated threshold will set a DTC.
Circuit concerns will also set fan clutch DTC’s.


DIAGNOSIS AND TESTING
VISCOUS FAN DRIVE
ELECTRONICALLY CONTROLLED VISCOUS FAN DRIVE - 5.9L DIESEL
NOISE
NOTE: It is normal for fan noise to be louder (roaring) when:
² Fan duty cycle high. This may occur when ambient (outside air temperature) is very high.
² Engine loads and temperatures are high such as when towing a trailer.
² Operating conditions where transmission temperatures may be high
² Cool silicone fluid within the fan drive unit is being redistributed back to its normal disengaged (warm) position.
This can occur during the first 15 seconds to one minute after engine start-up on a cold engine.
LEAKS
Viscous fan drive operation is not affected by small oil stains near the drive bearing. If leakage appears excessive,
replace the fan drive unit.
ELECTRONICALLY CONTROLLED VISCOUS DRIVE – 5.9L DIESEL
If the fan assembly does not free-wheel and a metallic
grinding sound exists, replace the electronically controlled
fan drive (Refer to 7 - COOLING/ENGINE/RADIATOR
FAN - REMOVAL).
NOTE: The following test may take up to 15 minutes
to perform.
temperature.
1. Set the parking brake and verify the transmission
is in park or neutral.
2. Set air conditioner (if equipped) and blower fan to
OFF.
3. Start and allow engine to reach normal operating
temperatures.
4. Stop engine, connect the scan tool and select
appropriate model year and engine option.
5. Check for and correct existing DTC’s
6. Using Tool 6801, backprobe pin 1 of the electronically controlled viscous fan drive connector, with the harness
connected located at the lower fan shroud to battery ground.
NOTE: The fan drive control coil is energized to 12 volts at this time.
WARNING: A spark may occur when connecting pin to ground. Be sure that no combustible material is in
the area.
7. Using the scan tool, verify that DTC 0480 set. If it is not set, verify that a good ground is available.
8. Start the engine.
9. Actuate engine speed to 2000 RPM..
10. Go to the SENSOR screen and observe the fan speed.
11. Run the engine at 2000 RPM until the fan speed increases to 1850 RPM or more for 30 seconds.

 

Thanks for the information! The only time I have ever heard my cooling fan "roaring" is when the ac compressor is engaged. Even with the engine temperature at 210 on a hot day, I don't hear it at all. Also, if my ac is on, I don't seem to have a problem with engine temps. I'll check all the elecrical connections for then fan clutch...I don't have any dtc's, though. Seems that since I hear the fan when the ac is on, it probably works fine...maybe a sensor controlling it's operation or an elecrical connection is bad.

Thanks again

 

might be the water pump maybe??? mine did this about 4 days in a row before the water pump failed and seized up. it threw the belt off and i lost power steering, brake booster and fan.... and my water pump never leaked out the weep hole to let me know it was going bad. just happened.

not sayin it is that, but it could be

 

Water pump is less than 5k old, thanks for your input, though!

 

Are you using HOAT approved coolant. That also makes a difference. Either get some from the dealer or buy Zerex Hoat coolant at Napa.

 

Ok...well, i used peak's global "final charge" heavy duty diesel engine coolant. The specs claim it is approved for just about every diesel engine...including cummins. It's "OAT" or organic acid technology...is there much difference? I can't imagine this stuff is not ok to use??

This is starting to get expensive

 

Oat antifreeze may not be compatible with all the seals, gaskets, etc... but I dont think it will cause overheating problems unless you mixed the two types together, which you did as the engine holds nearly 3 gallons that doesnt come out by draining the radiator. I cant say for sure but rumor has it that some types of antifreeze when mixed cause problems. FWIW the cooling fan speed sensor will only set DTC when below 50rpm or above 6000 rpm according to the service manual, sounds like anything in between does not set a code. A side note to the testing section says it could take 15 minutes for the engine fan to speed up, does not say if thats because of the fan clutch operation or if it's waiting for the engine to warm up. If it really takes 15 minutes for the fan clutch to lockup it's no wonder so many of us are overheating.