I was driving one day in my caprice classic, retired police car, and i noticed that when im driving somtimes at steady speed at like 40Mph it shakes. I thought it needed an alightment for the tires but thats not it. it turns out to be the engine. Its a 4.3 265 V8, i asked around why the car did that and some guy said it might be a Sticky Valve. It might be skiping a cyinder... not sure,, Please help me!!

Make sure it's the engine first. Get it to the point where you have the vibration, put it in neutral and kill the engine. Just make sure you have enough empty highway so you can pull off and restart the engine afterwards.

If it is the engine, you can disconnect one plug wire at a time to see what one is not firing. Look at all your plug wires for the obvious, burns and split boots. Also look at the distributor cap inside and out for arcing and cracks. It doesn't sound like a dead miss though, so I doubt you have an internal problem.

Bob

I had that problem once, I checked 2 things, one the front wheel balance and second, the chassis bushings, because of use they get cracked and car starts to vibrate, when you go faster vibration stops, is the same with wheels.
Hope this could help.

I was driving one day in my caprice classic, retired police car, and i noticed that when im driving somtimes at steady speed at like 40Mph it shakes. I thought it needed an alightment for the tires but thats not it. it turns out to be the engine. Its a 4.3 265 V8, i asked around why the car did that and some guy said it might be a Sticky Valve. It might be skiping a cyinder... not sure,, Please help me!!
Ive owned atleast 8 caprice /impalas, sounds like a warped break rotor to me, also the caliper can get sticky and the pad not always retract, lots of times you can feel the 40-45mph vibration also at 70-75 and at low speed like a luggy feeling at 3-5mph.