HOWLING DIESEL
My GMC dealer is at a loss for an answer to a problem I seem to be having with a 2004 GMC Sierra 2500HD with a 6600 Turbo Diesel that I bought used earlier this year. The problem is a recurring howling noise (I think it’s the turbo). This problem only occurs when I use the truck to tow a travel trailer and after a two- to three-hour period while cruising at 100km on level highways. Outside air temperature is usually pretty warm at more than 30 degrees C (86 F) when this happens. Once the howling noise begins, my oil pressure drops to about ¼ on the gauge and the engine loses power. Oil and coolant temperature gauges remain normal. This howling noise lasts for about six to eight minutes; after that everything reverts back to normal. The noise disappears, oil pressure goes back to normal (1/2 on the gauge), and engine performance returns to normal.
This phenomenon also occurs when I pull off the highway after some hours of driving and when the transmission downshifts to a lower gear when in the Tow/haul mode. Since I frequently drive down to Florida from Canada towing a travel trailer, I do not want to run into mechanical problems with the diesel. Help is appreciated.
Wolfram Schlegel Woodbridge, Ontario, Canada
Could this be the engine-driven fan clutch engagement that you’re hearing? The factory fanclutch operates with a bimetal coil that responds to changes in air temperature between the rear surface of the radiator and the fan. This bi-metal coil will rotate a valve inside the fan-clutch as air temperature changes, which in turn engages/ disengages the fan. Diesel owners describe the sound produced by an engaged fan differently. Some owners are alarmed when they hear the fan engage because of the roar it can produce. Generally, sound volume produced by the fan is related to engine speed. In lower gears and higher engine speeds, the roar will be more pronounced. While cruising at lower engine speeds, you may not notice it nearly as much. Tow/haul mode keeps the transmission’s torque converter locked in second through fifth gears as you slow down while taking an off-ramp. As the transmission downshifts, engine speed increases and fan noise will also increase if the fan-clutch is engaged. I can’t explain why you’re seeing a change in engine oil pressure unless it’s also linked to engine speed. I suspect it is. If you can’t demonstrate the problem for your dealer, it may be difficult to diagnose. In the meantime, with the engine off, I’d remove the serpentine fan belt, then rotate the various engine-driven accessories, checking bearing condition for the alternator, A/C compressor, power steering pump, belt tensioner pulley bearing and fan hub. The springloaded belt tensioner arm is also a possibility, if it’s intermittently binding. Make sure the belt tensioner can be moved through its range of motion without binding or having rough spots. A tensioner generally has a life span of something approaching 100,000 miles before its pivot point wears out and possibly contributes to binding. Good luck.