I have a 2004 f350 6.0. She has 250K miles on her. Runs great. I have ungraded things as they have wore out. Wicked wheel, 4 inch exhaust, Al radiator, Oil Cooler, etc. The delta for the ECT and EOT is usually between 5 and 8 with no load. We traded our travel trailer in for a larger 5th wheel toy haluler. She pulls great, but sometimes it seems the FSS doesn't respond fast enough or long enough. We went to Pigeon Forge and crossed Black Mountain. Plenty of power but the EOT was climbing right before the top got to about 245. The ECT max at about 225, so I figure it started de-fueling. I noticed the FSS was only about 1500 during this time with just under 3000 RPMs. The temp was in the 30s during this time, so I was surprised it got that warm. I have read about fan clutches working fine except at the higher RPMs. I got the idea to do the Blue Wire mod to lock the fan in 100%. I know the way the pulleys are, the FSS can be faster than the motor RPM. When I locked the fan in the FSS climed to about 880 at idle. I waited about 5 mins and run the engine up to about 2500 RPMs. The FSS shot up to 2530 but started to fall just a couple of seconds later and fail to about 1450 to 1500 and stayed there. The question is, can this be the early signs of a failing fan clutch? I would think with the Blue Wire Mod the FSS should be slightly higher than the engine RPM. I remember several years ago, before I changed the Oil Cooler and Radiator, the FSS would hit 2500 and even 3000 climbing a steep hill towing. The fan clutch is the original from the factory.

 

I had the fan clutch replaced on mine about 6 months ago and have not seen it get above 1200 to 1300. I may have gotten a bad one or possibly have another problem.