Posted: Aug 11 2007 at 11:18pm | IP Logged

what you have is called a viscous fan hub.  It contains a gel that reacts to the engine bay heat and uses the gel reaction combined with internal springs to apply or disengage the fan.  The fan should never spin freely, there is gel inside resisting that.  However, if you start your truck and watch the fan at idle, it will alwaqys sping and it will see mquite fast until you see it when it is hot, then you will know the difference.  It usually will run quite quietly when cold in comparison to when it is hot.  If you get a mirror and use it to look at the front side of the fan hub you'll see an aluminum round rad-type setup.  THe fins and such are the hub.  If the centre is dirty, your gel has most likely leaked out and your hub is shot.  There mayh not be neough leaked gel to get the fan blades dirty, but it can be really obvious upon closer inspection.  If you are not losing coolant ther is no point in pressure testing the system.  Fan hubs used to be designed to fail to the engaged position to prevent engine damage.  Nowadays they are engineered opposite, for some reason.  I beleive what happens inside is that as the engine heats up it heats up the gel in the fan hub assy as well.  As you know, hot liquidsw expand.  This expansion overcomes the force of the internal springs and applies the fan clutch.  Once the fan is turning to match engine rpm, it'll lower the engine temp.  This causes the gel to cool and thus contract.  Now the springs have overcome the gel andthe fan clutch is disengaged.  It is actually an extremely relaible system.  Go with an electric replacement and free up some hp!  Most vehicles with this setup will run 12 years with no problems.  They are maintenance free, infinitly automatic adjusting, cheap, sealed and reliable.  All in all, not bad.

I hope this helps.  You might also have a plugged rad.  This will take a little more tricky investigation.  It's not hard, it just takes some dedicated time and knowhow.  The rad could be plugged internaly or externaly.  If you need to flush out the fins, always flush from the engine bay out.  You wantg to blow out the debris the opposite way it came in.