Keeping Your Cool Radiator Stack Cleaning, Fan Clutch, Motor Mounts

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Hummer in Mud

This is one of those things that happen due to a chain of unrelated events. I was investigating a ‘new’ noise my truck was making. While checking under the hood I saw that the fan shroud was cracked. Not a big deal but something to add to the list. When I finally got the truck to the shop and up on a lift we found a broken left motor mount. The engine torque lifted the engine up off the broken mount and caused the fan to hit and break the shroud.

Here's how you check your motor mounts. With the hood open, engine running, transmission in Drive, firmly hold the truck with the brake and gently apply throttle.  If the engine rises up more than about 1/2 inch, the motor mount is most likely broken. Be careful with this test; if the fan begins to hit the fan shroud stop right away. You can damage the fan, the shroud, the fan clutch, and even the water pump. 99% of engine mount failures are on the driver's side mount because the engine torque in drive wants to lift the left side up.  Checking the passenger side is the same. Just use Reverse instead of Drive. You can also use a floor jack. Try to raise the engine on either side and observe the motor mount. See if it is separating.

You will have to lift the engine up a bit to replace a motor mount. Be careful that you don't lift it by the oil pan; you will crush it. One Hummer tech that I spoke to said the job is easier if you remove the engine bracket that the 4 bolts of the motor mount attach to. Some have jack against 2x4's resting on the edge of the pan. Once again, when you raise the engine don't break the fan. In some instances you will have to remove the fan.

Left MotormountLets go back in time about a year. My truck is a 98 turbo diesel wagon and over the years it’s seen off-road use. Each time it’s been off-road I spent hours meticulously cleaning the truck and rinsing out the cooling stack. The cooling stack is a series of radiators bolted together on top of the fan shroud. On the top of the stack is the air-conditioning condenser. Next is a large split radiator that cools transmission fluid on one side and engine oil on the other. Under these is the radiator. My stack always looked new. Common sense says that a radiator stack full of mud will cause the truck to overheat, but my truck never overheated. In fact last winter I had a problem with the truck not warming up and running cooler then normal. I wasn’t getting the nice hot temperatures from the heater like I used to. Replacing the thermostats solved some of the problem but the truck still never came right up to 195 and stuck there like it used to. I also checked to see if my fan clutch was stuck on all the time. If your fan is always blowing cool air over the radiator the heater won't give you as much heat. That's why truckers cover up their radiators in the winter. I replaced the thermostats a second time with no change.


Cooling Stack Dirty Radiator

Coolers Back to the present. Replacing the shroud is not an easy task. You have to remove all the coolers from the truck which entails disconnecting and pumping out the refrigerant in the air-conditioning system, draining the radiator and disconnecting the oil cooler lines. As the coolers came off one by one I couldn't believe my eyes. The split oil cooler was full of dirt and the radiator was half plugged up with mud. The fan clutch was also coated with dirt. Everything looked clean on the outside. The lesson learned is; the only right way to clean the coolers is to take the stack apart.

Over the next few days I was closely watching the temperature. The truck was running like it used to. It would warm up normally and stick right at 195. The heater is even hot like it used to be.

Here's what I believe is happening, especially in cooler weather. The thermostats open in a non-linear way. As the temperature of the engine coolant approaches 195 a heat sensing element in the thermostat begins to move a valve that lifts off its seat allowing a low volume flow of coolant into the radiator. As the engine warms up under ‘normal’ operating conditions the valve rises and falls within a small range tightly controlling the flow of coolant keeping the engine temperature right at 195. If the engine is placed under a load that requires more cooling the thermostats will further open flowing an increasingly large volume of coolant into the radiator. Once the thermostat is open past the fine control point small changes in temperature will not have much effect on the flow of coolant into the radiator. Because my radiator was plugged up it required the thermostats to operate in their ‘high flow’ range to keep the engine at normal operating temperature. Because the stats can’t tightly regulate coolant flow in this range the temperature would fluctuate some and never rise up and stay at 195 like it used to.

The thermostats don't regulate fluid flow in a linear fashion. This is due to its physical construction; i.e. needle valve design at the beginning of it's travel. I believe that when the thermostat is operating in a 'close to wide open' range it's ability to tightly regulate flow is much less then when it's operating 'close to closed'.

Clutch Fan

How to check your Fan for Proper Operation:

Your fan has a clutch in the center that is filled with fluid (oil). The amount of fluid that is let into the clutch determines if the fan spins or free-wheels. The fluid is controlled by a valve connected to a bi- metallic coil strip at the center of the fan. If the coil is caked with mud it probably won't work to well.

With the engine off attempt to rotate the fan. You should feel a noticeable drag. A properly operating fan should have enough drag so that as soon as you stop the fan will stop. If there is no drag or if you can't rotate the fan at all the clutch has failed.

The moral of this story is if you’ve ever been wheeling in mud, splashing through puddles or you bought your truck used and don’t know where it’s been my advice is to get the cooling stack cleaned out.