Im having some overheating issues. Ive replaced the thermostat, flushed the heater core, flushed the radiator and flushed the block. All seems clear.
Whats the best way to check if the viscous fan is doing it job?
Right now the motor is stone cold. The fan spins freely, somewhat. Halfway through a full revolution, there is some resistance. That normal?
Im assuming I should feel greater resistance when the fan clutch is hot, correct?

 

should feel like it is stirring through peanut butter......creamy not chunky.
should fully engage clutch as temp rises.
1/4 to 1/2 revolution with a spin by hand at cold is okay.
best to compare warm operation against the cold for comparison

 

Just got it up to operating temp. It didnt feel any different. I could get a 1/4 to 1/2 turn with medium to light pressure of turning by hand.
Here is a video.
I just turned the motor off at 203 degrees

 

it should spin more freely when warmed.
can you hear it drop in sound from idle to op temp? if no then then need new vc
does your truck only overheat at idle and not at freeway speeds? if yes then need new vis clutch

 

I did notice a drop in sound while getting up to operating temp.
It only overheats while driving. It will not overheat while idling.

 

If it is overheating while at highway driving speeds, you have another problem not vc. Highway speed produces more air than the fan
Air in system? Are you hearing the slosh waterfall sound at the heater core?
Do you get good heat in cabin?

 

Yes. I have good heat in the cabin.
No waterfall sound in the heater core.
I flushed/backflushed the heater core. Same with the radiator. Coolant hosed were firm during idling. I've bled this system about 5 times total so I feel semi-confident that all the air is out.
When idling, before driving, the temps stay 186-197 degrees. Then, while driving around 20mph the temps will rise to 200-210. Once it reaches 210 the temps will not drop, even if you drive/coast faster. The temps just start to shoot up really fast. It also doesnt matter if the cabin heat is on or off.
I just when for another drive, overheated again. I pulled it back into the garage and pulled all the spark plugs. All the plugs look "normal", meaning the tips had a small amount of white residue. Kind of like a burnt look. Non of them were super clean or dirty. These plugs have about 35k on them.

I did notice that while idling earlier today for about 45 mins, there was alot of what looked like condensation coming out of the tail pipe. Ambient temps today where around 55. I dont really have a sweet smell out of the tailpipe.

I replaced my HG's on my 6.0l diesel about 1.5 years ago. The D2 isnt really showing the same signs that the diesel did, albeit the D2 is a gasser.

Im stumped here...Any other suggestions before I start tearing off the heads?

 

I do need to mention that last monday, while bleeding the system, I drove the D2 around the block. It started to overheat and I had the cabin heat on full blast on high heat. When the temps got over 215 the cabin heat went cold.

 

that last post sounds like air pressurization in the system. ie. more exhaust from rpm driving , more exhaust gas into the cooling system from leak the only other elements would be the stat and it is a bypass system, so no change in cabin heat there. and the pump, but it is working at idle.
if it were me, I'd be putting the exhaust gas in coolant test on it

but really wierd that you have no other air in the system symptoms, otherwise.
I hope I am overlooking something and someone more knowledgeable will chime in with a solution.

 

Thanks for your help. Ill test for combustion gases in the coolant tomorrow.