1997 C-2500 misfire but not a misfire...

[Guntar D]

1997 C2500 2wd 5.7 4l80e 

truck runs like a sewing machine, all normal maintenance is kept up.  Only issue I have is cruising speed (above 60).  The truck shifts perfectly through all the gears and once it gets up to around 60 it feels like it intermittently misfires.  Now staring at the rpm gauge on the dash as well as a live data scanner it is not the engine misfiring (rpms don’t change).  So this leads me to the transmission.  Upon a ton of research I have replaced the TPS (delco), new fluid and filter, new rebuilt valve body with all new valves and solenoids, and larger updated external cooler.  After all this it does the exact same thing with no change.  After the the valve body and fluid it shifts even better and smoother.  I can see that according to the computer the TCC is staying locked the whole time while cruising.  I can manually unlock it by tapping the brakes and it goes right back into lockup.  I’m lost for a direction to turn now and any help is greatly appreciated!

[Bruhman]

 

 It might be an ecu issue. If you have access to another ecu I would try and swap them out to see what happens. It's the 4L80E I'm assuming. I've also seen instrument panels go out and do this sort of thing. That too can be swapped with a used one. Both are easy to swap. Good luck I hope this will help. 

[ChevMech]

Not to hijack this post, but I'm having a similar issue on my '97 C1500 2WD 4.3l. When you get above about 65, the truck starts to buck like it has a misfire. The other odd thing is that the speedometer is reading about 10 mph too fast. For what it's worth, the cruise control doesn't work either. Do these symptoms point to the cluster being bad?

 

Before we go any further, the tires are the correct size, the gearing has not changed, and the shifting is like it should be. This seems to eliminate the need for a VSS....at least from what I've read. 

On 5/2/2020 at 9:39 PM, Bruhman said:

 

 It might be an ecu issue. If you have access to another ecu I would try and swap them out to see what happens. It's the 4L80E I'm assuming. I've also seen instrument panels go out and do this sort of thing. That too can be swapped with a used one. Both are easy to swap. Good luck I hope this will help. 

 

[Bruhman]

My cluster went out as well. It wouldn't charge at first when I got that fixed it would shut off going down the road. It's a good possibility the cluster is bad yes.  Because once i replaced mine it corrected a number of issues I was having. A used cluster shouldn't be very expensive.