When I'm sitting at a stop light sometimes the transmission does something weird. It will give one good little surge. It would've moved the truck forward if I wasn't on the brakes hard. What do you think would cause this. Its a 5.7L engine and 4l60e trans. Thanks

I would be looking at engine running problem.
Can you feel the engine rpm come up when it does it?
MT

the rpm doesn't raise when it happens. Thats why I was thinking something in the trans

the rpm doesn't raise when it happens. Thats why I was thinking something in the trans

Could be lockup clutch trying to lock in or locking in.
See if you can disable lockup solenoid for a test and try it.
MT

What if it isn't down shifting to 1st as you come to the stop. Then as you sit there awhile, the transmission clunks down to first gear and away you go.
That's my guess, but I don't know how to fix it.

This sounds like a problem with the driveshaft yoke where it is inserted into the back of the transmission. I believe there was a service alert on this problem on various GM, 2wd trucks around this vintage. As the rear suspension moves up and down, the driveshaft yoke slides in and out of the transmission. When you stop at a light, the yoke is back slightly further because the rear suspension unloads as you stop and the yoke doesn't slide smoothly forward because the grease is dirty, sticky, whatever..... As you are sitting at the light, the vibration from the engine idling causes the shaft to slide forward to where it wants to be. It moves with a lurch. GM's fix is to remove the driveshaft, clean the splines and use a special grease on the splines. I believe the part number is 12345879 (I know, I know, look it up)!

Hope this helps.

This sounds like a problem with the driveshaft yoke where it is inserted into the back of the transmission. I believe there was a service alert on this problem on various GM, 2wd trucks around this vintage. As the rear suspension moves up and down, the driveshaft yoke slides in and out of the transmission. When you stop at a light, the yoke is back slightly further because the rear suspension unloads as you stop and the yoke doesn't slide smoothly forward because the grease is dirty, sticky, whatever..... As you are sitting at the light, the vibration from the engine idling causes the shaft to slide forward to where it wants to be. It moves with a lurch. GM's fix is to remove the driveshaft, clean the splines and use a special grease on the splines. I believe the part number is 12345879 (I know, I know, look it up)!

Hope this helps.

many years ago this was a problem that a co worker had and i believe that this was how it was fixed...