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A posting regarding high speed cooling is on the mark. That is, the Hummer does not cool as efficiently at high speeds due to the design of the front end et all. Due to the aerodynamics, the fan will cycle at highway speeds, especially if the A.C. is used. That is why the hood scoop seems to work for some installations.
Most hummers do not overheat much in traffic because of the diesel's intrinsic nature. These engines are always running wide open throttle as far as air intake is concerned. If there was a throttle plate, not enough air would be available to fire the charge under compression. Because of the diesel fuel's increased choleric values, this "lean" mixture does not result in any strained idling similar to the so called lean burn engines of the early seventies. The cold air coming in with minimal fuel intake results in dramatically lower exhaust gas temperatures and combustion pressures. Only the injector squirt and advance is varied to increase power. Usually a bit later squirt under high power settings which is similar to the vacuum advance mechanisms of the gasoline engines.
If you begin to pull a hill while in torque converter lockup, you may notice the diesel rattle diminishes with increasing throttle. On the computer driven turbos, the Stanadyne pump retards the squirt just a bit for more efficient burning just like it's gas counterpart.
A trip to your local Stanadyne service shop will reveal lots of info about these pumps and all of their modifications throughout the years.
On our trips around the country, Kathe and I usually run between 55 and 65 so we have not experienced heating problems. We need a bumper sticker that says "Smile as you pass under".
John W. Watkins Jr.
I am going to try and remember my shop class here so feel free to correct me if I am wrong (and I am sure you will!!:-) The fan has a viscous coupler that is affected by the heat of the air. This is a fluid (if I remember right a type of silicone) that gets thicker at higher temps and locks the fan into direct drive and at lower temps allows it to free wheel. Jeez, seems like I am forgetting something but I think that is the gist of it.
The Hummer fan is, as you say, a belt driven unit. The actual blades are connected to a clutch that is driven by the belt. It is sort of a binary device. When "off", the blades are not driven that much from the engine and the blades rotate slower than the pulley. When "on", the blades get coupled to the pulley and they spin faster.
The clutch is temperature sensitive - when it gets hot (sensed via the heat conduction through its mounting plate), it turns on so to speak. I believe it is a bi-metal type unit. It differs from the older viscous units that you couldn't place horizontal.
What happens on the Hummer is that at highway speeds, the coolant temperature gets hotter and the clutch turns on. Hence, the fluid cools and then it turns off again. The air flow through the radiators is not that great due to some aerodynamic concerns. However, the primary reason (IMHO) is that the Hummer has way too much drag. Power consumption due to drag rises _cubic_ to speed. More load on the motor means higher temperatures. The trucks were never designed to run at 75+ MPH.
Note that when the fan kicks in, the truck isn't overheating (yet). If it pulls the temp down then everything is working as it should. That's the way the system works. If it cycles too much or stays on, slow down. If there is still a problem, go to 3rd gear. 3rd gear increases the flow rate through the radiators and moves the engine into the HP band. The turbo diesels present interesting problems b/c they have too much torque. They are able to pull themselves up hills while still in 4th gear. The problem is that their temperatures soar through the roof (220). Note once again that if it can hold the temp outside of the redline, it is technically normal operation.
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