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1994 GMC C2500 RPM's at cruising speed

Tiny
JEROWE
  • MEMBER
  • 1994 GMC C2500
Engine Performance problem
1994 GMC C2500 V8 Two Wheel Drive Automatic

I have a '94 GMC 2500, 6.5l turbo diesel. With stock equipment, when traveling at 70 mph, it was turning 2100 rpm's. The average fuel economy was 15 mpg. I installed a k&n filter and 3" custom straight pipe exhaust. Now, when traveling at 70 mph, it is turning 2500 rpm's. What would cause this increase? If anything, I would have expected lower rpm's at cruising speeds. Also, the fuel economy has not changed, or possibly decreased slightly. Of everyone I have spoken to, nobody has any idea as to what is happening. Thank you for any help you may offer.
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Thursday, April 22nd, 2010 AT 8:47 AM

2 Replies

Tiny
DR LOOT
  • MECHANIC
  • 2,311 POSTS
It sounds to me like you lost your overdrive, that is the lock up converter, which brings your RPM down 400 to 500rpm, are you getting an engine light on? When you are in 4th gear shift into overdrive and see if there's any change.
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Friday, April 23rd, 2010 AT 6:15 PM
Tiny
JEROWE
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No, its definately in overdrive. I can tell when it shifts into each gear, including overdrive. When I shift into drive, to disable overdrive, it disables 4th gear as well. Also, there is no engine light illuminating.
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Monday, April 26th, 2010 AT 12:31 PM

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