Fan clutch
 

ok i have what i thought was a 1997 chevy express 3500 with a 6.5 TD but after running the vin i find its a 1996 new body style.. well im trying to find out if the clutch fan is supposed to lock up when its hot or cold or when it is hot can i or am supposed to spin the fan with ease. .problem is when i drive around town it runs fine all day long system doesnt lose fluid no overheating no coolant in the oil no bubbles in the expansion tank or any of those... now when i hit the highway for 15 20 miles it runs just fine temp needle is one line above the first big line and good heat. but when i exit the highway the temp starts to climb rather quickly and i lose heat and it loses coolant out of the expansion tank. now as soon as i see this as i watch my gauges i shut the truck off and get off the road and let it sit for an hr or so and i open the cap and i add 1/2 to 3/4 gallon of coolant and i can then again drive 40 50 miles in the city and back roads where im not going over 40 and when i get home i put a piece of cardboard under the truck and i leak nothing... im only loosing coolant when i drive on the highway at 60 65, truck has new high flow water pump ac delco thermostates ac delco expansion cap , im at a loss:argh:

The fan clutch is supposed to engage when the engine is hot,it can only do this if there is clear unobstructed airflow through the cooling stack(rad,oil cooler,trans cooler ac condenser ect)-the air flow through those items is what heats the bimetal spring in the center of the fan clutch and causes it to engage.

If there are any fur patches on that spring it may not heat up properly and they are also an indication that the fluid inside the clutch has been leaking and may require replacement.

My Kennedy special calibration clutch has worked perfectly since installed and IMHO is the way to go.
Paired with the 20" steel fan or the plastic Dmax fan makes them even better.

Your symptoms do reflect a fan not engaging when hot and coolant flow is lower but a cracked block is not something I would rule out yet.

Have you had the cooling system pressure tested yet?

i replaced the fan clutch and radiator today and let it sit running for over a hr and all was good. i then tool it for a 15 mile backroad drive and about 30 mile drive on the highway and the temp stayed one line above the fist big line...when i got off the highway it went up to between the first and second line after the big line and didnt go any higher . for a slpit second the low coolant light came on but quickly went out.. i can home and parked the truck and cracked the cap off and let the air pressure out and checked the level and it was right where i left it before i took it for a road test.i have pressure tested the system before and it sits at 16 pounds over night . i did have a problem getting all the coolant out of the old radiator and it still sounds like there is some in there . and like i said before i am not getting any bubbles in my tank no white smoke out of the tailpipe or no coolant in my oil. so the theory on the cracked block or head is out the window .

I'm selling a Kennedy Special calibration clutch for 150$. It's like brand new.

So are you saying that the fan clutch and rad solved the problem?

so far it seems to be but im letting it sit overnight and go from there with it and update my findings

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well let it sit overnight and went out there and fired it up no white smoke from the exhaust on startup.. removed cap and tank was a little low so i topped it off and started checking all the hoses and found the line going from the tank to the top of the rad leaked when i touched it so im replacing the spring clamp with a screw clamp and go from there with it..