how does the MDS on a Ram 1500 with a 5.7 Hemi work?

mds works by closing all the valves to a cylinder and shutting off the fuel to it as well. if they did a leakdown test, you will have to ask them what they mean by bad cylinder. It could be rings, or either intake or exhaust valves, there is even a possibility of a head gasket failure. I am guessing that they think it is rings. if you recently had a rebuild, it is possible that they didn't seat in that cyl. I am sorry that I dont know for sure how mds is tested, but I would guess that the dealer would do some kind of a Live data scan while driving. hope this helps.

The dealer told me it could be the rings or I spun a valve. I never heard the term spun valve, I thought the valves turn, that's why you seat your valves. .At an idle it feels like I have a miss, but when I accelerate it runs fine until I hit about 50 mph then I get a hard vibration out of the motor. When I hit the passing gear it starts to go like a bat out of hell then I lose power

I think its time to take it back wherever it got rebuilt. bummer. hope it is under warranty. btw, i have never heard of a spun valve either. spun bearing, but not valve. oh well. live and learn. sounds expensive. really sorry.