I have a 2007 Sierra Classic 2500HD 6.0 with 10000 miles and just this weekend it started acting strange. As I started out the engine sounded like it was revving, but the RPM's were at 2000 and it didn't seems to be moving hardly at all unless I gave it more gas and got it up to 3000 RPM or so. I took it to the dealer today and they said it was hot out and the revving noise I was hearing was the engine fan working. It does this towing or not.
To me I think the trany is like slipping or something. It was even doing it on the highway at certain times. You could hear it revving, but not accelerating the way it should.
Watch your temp gauge. If it gets up to 200 plus, then I would expect the engine fan to run and make a lot of noise like a jet engine. Usually at 50 MPH highway speeds there is enough air flow that the engine fan will shut off.
Watch the tach. As RPMs increase there should be increase in MPH speed. (Disregard the noise) If no speed change with RPM increase, it sounds like transmission problem.
Pete W4WWQ DX70 IC-2800 Smartire X-10 camera
2006 Chevy 3500 CC LB 4WD DRW Duramax auto-6
2003 Cardinal 29WB LX EU3000is Roto-Choks 300W from www.amsolar.com
Could be the torque converter locking and unlocking. When the torque converter unlocks, rpms will go up a bit with very little corresponding increase in vehicle road speed, as the torque converter is slipping. When it locks, rpms will go down with very little change in vehicle road speed.
My '02 Suburban behaved like that once when I let it run out of gas. The engine revved, seemingly free of the transmission, but I did make it the last quarter mile or so to the filling station.
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