A weak starter will sound ok, but won't spin the engine fast enough to start. I once had a '84 300SD that I wore the battery down three times trying to start after it sat for three years. Finally, after using a booster pack and a fresh battery, it started and blew a mushroom cloud of smoke that was so large I was afraid the fire department was going to arrive soon. A new starter had the old girl starting with one glow like a new car. It was a rusty beach car that I traded to the guy that reroofed my house. This was a couple of years ago, I saw it yesterday with three ladders strapped to the roof pulling a trailer chugging down the road. It had low compression also.