Although you don't give much detailed information about this problem, I did
have something very similar in my '95 Chevy C1500 with the 6.5L turbo
diesel. I had rpm fluctuations between 45 and 50 mph if I gradually
accelerated up to that speed and then kept it there. Turns out it was the
EGR valve on top of the motor. There are two EGR valves available - one for
the normally aspirated (non-turbocharged) 6.5L diesel, and one for the 6.5
with the turbo. You must get a new EGR valve specifically for the
turbocharged motor. The one used on the 1500 series trucks was for the NA
motor - even if it had a turbo on it. Look for the part number for the one
used on the 2500 series trucks and that will be the proper EGR valve. It
takes two bolts and one vacuum hose, and took about 15 minutes to replace.
Hope that helps - Jonathan
Jonathan A. Race
Lieutenant, EMS Supervisor
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