I have an ’04 Chevy Silverado 2WD pickup with the 4.3 6cyl and auto tranny. Between 1100 @1300 rpms I get a strong vibration that you can feel in the floor of the truck. It happens mostly while coasting or decelerating. It feels like the vibration you get driving a manual transmission in a high gear at too low RPM, therefore “lugging” the engine.
The trans fluid looks fine and is at the proper level. The truck has 45000 miles and a fluid change is probably in order as I bought the truck used with 36000. Any ideas? The truck runs and shifts well otherwise.
I had the same problem, though mine felt like a misfire at 1100rpm. After pulling the plugs, checking the (new-ish) wires, replacing the coil packs (2) and driving myself nuts, the condition worsened to your current status. A set of engine mounts fixed it. D’oh! Once again, I kicked myself for throwing parts at a problem.
So a good way to check for a bad mount is by torque-ing the motor when parked. Open the hood, look at the engine while running and have someone gun the throttle. HARD. If the motor jumps up pretty violently, odds are your mounts are bad.
Just for kicks, I checked the Technical Services Bulletins (TSBs) for this vintage Silverado and found this: “Coolant leak at engine mount bolt, NHTSA Item Number – 10010663. “ Who knew GM engine mounts were filled with coolant?
Just kidding. Also remember that TSBs are frequently red herrings. Who knows if this problem has any relevance to 4.3L motivated trucks, or if the (three?) customers who complained about this problem aren’t related. Just kidding, again. I think.
[Send your queries to firstname.lastname@example.org]