Dear Ole Mechanic;

I had my driver's window down the last nice day we had, and I noticed that the engine was roaring more and louder than usual.  The faster the engine went, the louder it got; however, the engine seemed to be running fine--just a little low on power.  It did not matter if the truck was in gear going down the road or sitting still in neutral; the roar got louder as I revved up the engine.  It is not an exhaust leak.  Some one said that a fan clutch going bad could cause a roar like that.  My questions are: What does the fan clutch do?  How does it work?  If it is bad, is it going to tear something up?Fan Clutch


Oh, the truck is an F-250 super duty with the Power-Stroke diesel, a 97 model.    Thanks   

Noisy Thru Town

Dear Noisy;

Yes, a stuck fan-clutch can drive the radiator cooling-fan at full speed all of the time, and the fan noise will then sound like the roar that you are hearing.  When the fan clutch is operating correctly, it is designed to engage the fan and pull air through the radiator when the engine is working hard or is getting hot.  When the engine is cold, not working very hard, or going over 50 MPH with a light load, the clutch releases and allows the fan to freewheel.  Now it does not engage or disengage suddenly; it has such a smooth, gradual engagement and disengagement that it is not usually noticeable.  The ability to freewheel reduces the fan load on the engine to help improve fuel mileage and to make more power available to pull a load if the extra cooling is not needed.  Since the power needed to drive the fan can exceed 10 HP, this can result in a significant amount of fuel savings--as much as one to three miles per gallon.  Also, when the engine is cold and you are trying to warm up the engine, you do not need the fan trying to cool it down so, the clutch helps get the engine up to operating temperature.

OK, that is what the fan clutch should do if it is operating correctly.  Now for how it works.  There is an aluminum housing that has an internal part driven by the water-pump shaft.  The fan is attached to the outer part of the housing, and there is an internal fluid that keeps the inner and outer part lubricated, so that the fan can freewheel.  Usually the aluminum has fins or ribs cast into it, so that it will be at about the same temperature as the air coming through the radiator.  Since the air has picked up heat or the lack of it from the radiator, the aluminum will be at about the same temperature as the engine and coolant.  The aluminum transmits that heat to a temperature-sensitive spring mounted to the front of the housing.  The spring can be either a coil or a flat one.  As the spring heats up, it pushes on the clutch, starts engaging it, and causes the fan to start spinning faster.  That pulls more cooling air through the radiator to keep the engine from overheating.  As the engine and the spring cools down, it disengages the clutch, and the fan freewheels.  The lubricating-liquid in the fan clutch does try to spin both parts of the clutch, so the fan never does stop completely. When it is really cold outside, the thick liquid can cause the fan to roar right after you first start the car.

If the spring gets loose or goes bad, or the liquid leaks out, the clutch can fail and keep the fan fully engaged.  Fully engaged, the fan will be noisy. That could be the roar you hear, and it will take more fuel, decrease power, and make it harder to warm up the engine. However,  it should protect the engine by keeping it from overheating.  After all, a new fan-clutch is a lot cheaper than a new engine--especially a new Power-Stroke diesel.

Is it going to tear something up?  Not likely in the short term, as it should fail engaged instead of freewheeling.  However, it will put an additional load on the fan, fan belt, and water pump, and it will use more fuel.  If you try to "get by" for very long, then all bets are off, as there is a lot of stress on all of the components when the fan is running fully-engaged all of the time.  Also, it will not take too long to pay for a new fan clutch when you figure the extra fuel you will use.  It is possible to have the fan clutch fail completely and fly apart, especially if you try to get by without replacing it once it has gone bad.  The flying parts usually destroy a number of things like the radiator and the batteries and put dents or holes in the hood.  In the unlikely event that it fails and freewheels, you will probably overheat the engine sooner or later, and that can get very expensive.

The replacement of the fan clutch is not a complicated job until you realize that the working room is very limited and that a special tool is needed to remove and replace it.  It takes a very thin, specially bent 1 7/16 inch wrench, as the clutch is threaded onto the front of the water pump shaft.  A Crescent wrench or pair of water pump pliers just ain’t going to cut it, and a regular wrench is way too thick.  It will not fit, and I know this because I have been there, and I have the bloody knuckle to prove it.  Of course I am cheap, so I made my own tool.

Herr Professor Nuzanbolts