So, I noticed one day that that the truck ('95 RAM 1500) is running above 200F, and its never done that before.

I replaced the thermostat and flushed and cleaned the radiator. Put in 180 thermostat. It made it better, but on a hot day, in stop-and-go traffic, it would climb to 200, which I am certain it shouldn't.

I looked at the fan clutch. It spins free when cold, and it's got somewhat of a drag when hot. I am wondering however if it is not enough. When the motor is cold, the fan is turning very slow - you can see it moving...

Do I have a weak fan clutch?

 

How about that water pump?

 

Rad cap might not be holding pressure. Could have a small leak in the heater core and not holding pressure. Could be the clutch.

 

Well, I am thinking, I have 0 issues at highway speed and the temp stays on 180, so, I thought it was the air flow issue and not the water pump...

Anyone ever try to rig the clutch to be "more rigidly connected" to the pulley (e.g. with some bailing wire just to check if it improves the cooling?

 

Is there a way to measure the pressure?

 

Most auto parts stores will have a tester that you can buy. Some will loan them out. It looks like a radiator cap with a hose and a manual pump attached to them.
IIRC the fan clutch is not supposed to spin freely like that.

 

+1 on the cap. Its a cheap replacement and if its the original on your 95 or looks pretty old, it probably needs replaced. If the rubber seal is cracked or stiff, again, its a cheap deal.

The easiest way to check your fan clutch is to run the motor to temp, open the hood while the motor and have a helper shut off the motor while you observe. If you don't have a helper close your door and shut motor off through the open window.
At temp. the fan should only spin a revolution or two before stopping. If it's spinning freely, its bad. Also grasp the fan and try to wiggle it (with the motor off! ) If there's any play your clutch is going out.
Also, you might want to try flushing the outside of the radiator. If you can't see through the vanes it may need a cleaning.

EDIT...thought of one other thing. If when you flushed the radiator and changed and there was a lot of corrosion, you might want to clean or change the temp sensor. It's next to the T stat in the water jacket. If its covered with corrosion that could affect your temp reading.

 

Update: got new radiator cap and new fan clutch.

New fan clutch seems to behave a lot like the old one. When I run the truck to temp (needle almost at 200) then shut it off, the fan spins (and spins and spins). When it stops, I tried it by hand. It has a drag, but I can push it with one finger (even when hot). But, the old clutch did pretty much the same thing.

One thing I noticed though, revving up the engine at idle (not much, 2000rpm or so) brought the temp level right down to about 180 or so...

EDIT: When opening the radiator after bringing to operating temp, -- nothing. I wrapped my arm in a towel, expecting an overflow of coolant, but it didn't happen.

OH, and while i had the belt off and tried to spin the fan (with new clutch), it spun the water pump pulley freely... I'd expected to hear a "glop-glop" sound, like Sheriff420 mentioned in another thread, from the pump trying to push water around.

So... Water pump?

 

Did the pressure test of the cooling system over lunch. Went to oreilly and borrowed their pump with adapters. Pressurised it to 15lbs, and it held up there...