I think my fan clutch is not disengaging. Looking for a fuel efficient fan clutch if one exists. I did a web search and came up with this article. Just skimmed so far, but it has a lot of good info. Maybe Michael wrote it? http://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf Would love to discuss more here on maximizing fuel economy without getting the thread closed due to decisive conversation. It's good for all of us!

Maybe Michael wrote it?

I'm not that good... As for stock fan clutches they are fine. If they perform as design they have very little if any impact on MPG's. Because once the engine is running and the fan clutch is fully unlock you should be able to stop the fan with a old radiator hose at idle. Electric fans typically have been reported to flow less air than the mechanical fan does. You might be taking the load off the belt at the OEM fan but you'll be putting load back on the belt again at the alternator. So a fan is a fan basically. As long as a the cooling system and the fan clutch is working properly there should be no load on the engine as long as the coolant temps are below 205*F typically by 205-208*F I hear mine going into full lock up and now you drawing HP from the engine.

This morning prior to start up I felt the fan rotational resistance. It was significant. Would not rotate at all without pressure. I here that you should be able to spin in a bit when cold and have it free spin a half turn or so. same resistance after driving an hour

Start the truck and let idle for a minute or so and then shutdown and check. Check with it cold and hasn't moved its not a good way to check it. Even mine is solid like a brick if the engine has not ran. But after the engine has been ran the oil in the fan clutch is moved around a bit and should feel free then. In the dead of winter I get a locked fan for the first 30-60 seconds of run time and then the oil flows in the clutch and the fan unlocks.

This morning prior to start up I felt the fan rotational resistance. It was significant. Would not rotate at all without pressure. I here that you should be able to spin in a bit when cold and have it free spin a half turn or so.

I have noticed with mine, when I park overnight and start it the next morning I have 100% fan engagement for 1-2 minutes before it slows down. I assume this is the amount of time it takes for the fluid to disperse and return to normal. Been like this for as long as I have owned it(7 yrs). The fan still kicks in when needed and there is no mistaking it, when it does. Checking it on a cold engine may not give an accurate diagonise. Run it for a little while, then check.

According to the article referenced in the opening post, 10-12% penalty for a faulty fan clutch is possible. That's a huge number!I still believe that my fan clutch always feels the same. I have not checked it in hot weather. But last summer I could hear it kick in during high temps. My belief is that I'm not kicking out when temps are cool or moderate. I'll try the hose or rolled up newspaper test tonight and report. Be it will shred the newspaper, but I could be wrong.